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Author Topic: Tire inflation: MPG versus comfort  (Read 4085 times)
belfert
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« on: April 21, 2009, 09:07:14 AM »

How much difference does tire inflation make for MPG?

I am running 90 PSI front and 80 PSI rears as required for my axle weights.  I had been running them at the maximum 105 PSI.  Am I killing my MPG by lowering the PSI?
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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« Reply #1 on: April 21, 2009, 09:47:05 AM »

Brian,

Generally, the higher the tire pressure, the better the fuel mileage.  Opinion (and testing) varies on how much mileage changes with pressure, but you will find lots of it if you Google around.

All is not roses, however.  In addition to ride comfort, you will also be sacrificing safety by running your tires over the recommended pressure, and the tires will also wear out faster.

The safety aspect comes from the fact that there is less rubber in contact with the road.  That means increased braking distance on all surfaces, and also less cornering traction in emergency handling situations.

The tire wear has to do with the fact that the smaller contact patch will mean the center of the tread will wear faster than normal, and the shoulders will wear more slowly.  The uneven wear will get you to sub-legal tread in the center in fewer miles.  For many with bus conversions, this is a red herring, because the tires need to be replaced from age (7 years maximum, generally) long before the tread wears down.  (We wear our tires out every four years, give or take.)  It becomes very difficult to calculate whether the improved mileage offsets the increased tire wear in lifetime cost-of-operation; obviously, the more expensive fuel is, the more lopsided the equation gets.

Personally, I would not trade the safety off for minor fuel savings.  YMMV, as they say.

-Sean
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« Reply #2 on: April 21, 2009, 12:20:06 PM »

Over-inflated steer tires have been known to contribute to wandering.

Proper contact patch is pretty important for lateral traction in the rain.

Pounding the crap out of the coach suspension and frame with harder tires might have some issues down the road.

Adjusting one's driving style and terminal velocity are the surest way of moderating fuel use.

Stop putting the pedal all the way to the floor.

Best to count anything over 5 mpg as a bonus and be done with it?

happy coaching!
buswarrior

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Lee Bradley
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« Reply #3 on: April 21, 2009, 01:03:00 PM »

The conditions that influence hydroplaning include speed, tire design, tread depth, water depth on the road, load on the tires, and inflation pressure. At low speeds (less than about 50 mph), if your tires are under-inflated, you actually have more tire touching the road. However, hydroplaning does not occur very often at speeds below 50 mph, unless there is deep water (usually standing water) on the road. As you get to about 55 mph and the water pressure going under the tire increases, an under-inflated tire has less pressure in it pushing down on the road and you have less tire-to-road contact than a properly inflated tire as the center portion of the tread gets lifted out of contact with the road. As speed increases to 70 mph and above and water depth increases due to a severe local storm with poor drainage, the under-inflated tire could lose 40 percent of the tire-to-road contact area compared to a properly inflated tire. The higher the speed (above 50 mph) and the more under-inflated the tire is, then the lower the tire-to-road contact and the higher is the chance of hydroplaning.

Tread depth has a substantial impact on the probability of hydroplaning. If you make a simplifying assumption that the water depth exceeds the capability of the tread design to remove water (which most likely would occur with very worn tires), then an approximation of the speed at which hydroplaning can occur can be estimated by the following formula:

Hydroplaning speed? = 10.35 x  inflation pressure [25]

Under this assumption of water depth exceeding the capability of the tread design to remove water:

At 30 psi, hydroplaning could occur at 56.7 mph

At 25 psi, hydroplaning could occur at 51.8 mph

At 20 psi, hydroplaning could occur at 46.3 mph.

This is presented to show the relative effect of inflation pressure on the possibility of hydroplaning.


Skidding and Loss of Control

Table of Contents

For loss of control crashes, speed is the most critical factor. Excessive speed alone can cause a loss of control in a curve or in a lane change maneuver. Tread depth, inflation pressure of the tires, and road surface condition are the most notable of a long list of factors including vehicle steering characteristics and tire cornering capabilities that affect the vehicle/tire interface with the road. In the Indiana Tri-Level Study, under-inflation was not considered a contributing factor to a crash when there was high speed involved. It was only considered when the tires were significantly under-inflated (an undefined term generally taken by the investigators to mean at least 10 to 15 psi below recommended pressure). Still, it is hard to know whether correcting this one problem area could result in the collision being avoided or reduced in severity. That is one reason why under-inflation was never cited as the definite cause of a crash. We tried to consider this by comparing under-inflation as a percentage of all of the probable causes in crashes. Certainly, reducing under-inflation is an important area and a move in the right direction. However, it is difficult to determine what the effectiveness of increasing tire pressure would be on these crashes. The following discussions describe how inflation pressure affects these crash types to the extent known.

Skidding and/or loss of control in a curve

Low tire pressure generates lower cornering stiffness because of reduced tire stiffness. When the tire pressure is low, the vehicle wants to go straight and requires a greater steering angle to generate the same cornering force in a curve. The maximum speed at which an off-ramp can be driven while staying in the lane is reduced by a few mph as tire inflation pressure is decreased. An example provided by Goodyear shows that when all four tires are at 30 psi the maximum speed on the ramp was 38 mph, at 27 psi the maximum speed was 37 mph, and at 20 psi the maximum speed was 35 mph while staying in the lane. Having only one front tire under-inflated by the same amount resulted in about the same impact on maximum speed. But, the influence of having only one rear tire under-inflated by the same amount was only about one-half of the impact on maximum speed (a 1.5 mph difference from 30 psi to 20 psi).

The agency also has run a series of tests to examine the issue of decreases in tire pressure on vehicle handling. A 2001 Toyota 4-Runner was run through 50 mph constant speed/decreasing radius circles to see the effects of inflation pressure on lateral road holding. We examined lefthand turns from 0 to 90 degrees handwheel angle for tire inflation pressures varied from 15 to 35 psi. The data indicate to us that in on-ramps/off ramps, tire inflation pressure is a critical factor in vehicle handling. The data show how much friction the vehicle can utilize, in terms of lateral acceleration (g?s), before it slides off the road. The more lateral g?s the vehicle can utilize, the better it stays on the road. So, if you are going around an off-ramp and need to turn the wheel 50 degrees at 50 mph, you can utilize 0.27 g?s at 15 psi, or you can utilize 0.35 g?s at 30 psi.





« Last Edit: April 21, 2009, 01:04:56 PM by Lee Bradley » Logged
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« Reply #4 on: April 21, 2009, 01:11:57 PM »

Lee,

That ought to do it!

Nice write up.

John
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« Reply #5 on: April 21, 2009, 02:07:16 PM »

Best to count anything over 5 mpg as a bonus and be done with it?

I must be getting a huge bonus as I've been getting right at 8 MPG on long trips.  That 8 MPG was with the tires at 105 PSI.  I'm hoping not to lose a lot of MPG by going to lower PSI in line with the actual axle weights.

I already keep the MPH at 65 MPH to decrease fuel usage.  I discussed perhaps even going to 60 MPH last summer, but everybody else on the trip didn't want to spend an extra 3 hours in the bus each way, plus being 15 to 20 MPH slower than other traffic out west gets dangerous.  (Speed limits on I80 from Nebraska to California are mostly 75 MPH.)
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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« Reply #6 on: April 21, 2009, 04:30:41 PM »

So, for your 2340 mile trip:

36 hours at 65 mph
39 hours at 60 mph

Question needs to be: How much fuel did it cost extra to save the 3 hours?

Perhaps the single biggest issue that lets the fuel companies empty our wallets, car companies sell us powerful SUV's and who knows how many collisions happen... is the inability of the human mind to get its head around the relationship between time, speed and distance.

If just one vehicle passes us, we "feel" we are being left behind. Which bothers that other head that periodically thinks it is in charge....

We are the most educated masses in recorded history, yet we still can't stand it when others pass us on the highway, and feel we are being left behind. And we let ourselves get fleeced at every automotive related turn because of it. anyhow....

My coach gets 6 mpg at 70 mph and gets 7 mpg at 60 mph.

MC8, 8V71 at the 270 hp settings, HT740, 3.7:1 diff gear.

To travel down for Jack's party:

For fuel costs, we'll use my preferred cheapest-fuel-on-the-trip stopping point, the Flying J at Wytheville VA, where fuel this afternoon is said to be selling for the low price of $2.019 a gallon....
(where did $4.50 go?)

1500 miles

70 mph = 21.5 hours = 250 gallons = $504.75
60 mph = 25 hours = 214.3 gallons = $432.67

so, saving 3.5 hours costs us 35.7 gallons/ $72.08 one way.

(At $4.50 a gallon diesel, the savings would be $160.65)

If there was a way to earn more than $20.60 per hour for those saved 3.5 hours, then the economics of driving faster and burning more fuel works.

I can choose to spend some of those savings on "back home beverages" for the ex-patriots and snowbirds.

Of course, in any road trip, and making these kinds of calculations, we need to consider that the whole trip is not made at the calculated speeds, any city driving will negatively effect fuel economy as will all the accelerating to highway cruise speed. The actual driving time is longer than our math.

If the family is inspired to get in and get out when we stop, the "tortoise and the hare" story can easily be told again. It would be pretty easy for the people on the "faster" coach to burn through 3.5 hours of sitting still during two long days of road travel. If you haven't done it, try running a log book like the commercial drivers do, you'd be amazed at how time can get burned up sitting still while you are on the road, and, relatively how little the slower moving traffic through an urban setting slows your pace across the nation.

On your bus, the differences in tire pressures you are considering will not make a difference you can accurately measure at the fuel pump with your odometer. (Hot fuel will do more to you, I might suggest) Too many other variables in operation and driver behaviour, especially when you are thinking harder about fuel consumption now.

THINK versus FEEL....

only a few will get it, the rest will subsidize?

happy coaching!
buswarrior



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« Reply #7 on: April 21, 2009, 04:33:17 PM »

I must be getting a huge bonus as I've been getting right at 8 MPG on long trips.


That's the advantage your Series-60 and B500 has over those of us with the older powerplants and trannys.  Also, the Marcopolo Viaggio is a more aerodynamic body than many.

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I already keep the MPH at 65 MPH to decrease fuel usage.  I discussed perhaps even going to 60 MPH last summer, but everybody else on the trip didn't want to spend an extra 3 hours in the bus each way, plus being 15 to 20 MPH slower than other traffic out west gets dangerous. 


FWIW, That is an oft-quoted but completely unsupported statement.  AFAIK, no traffic safety studies have ever found that vehicles traveling below the posted limits (within reason, also usually posted) are less safe.

BTW, we normally drive 55 for safety and fuel economy, but we also stay off the Interstates as much as possible.  When forced to be on an Interstate, we drive 60, unless on our way to a disaster, when we will push it to 67-68, just a couple MPH below our top speed.

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(Speed limits on I80 from Nebraska to California are mostly 75 MPH.)


The top speed limit for all trucks and any vehicle towing anything in California is 55 (the limit for cars tops out at 70).  Nevada, Utah, Wyoming, and Nebraska are, as you say, 75.  Illinois, BTW, is 55 again for RVs.

We have found that the US highways are a much more relaxing drive.  It's hard to say for certain, but my sense is that we save way more fuel driving at these speeds than we use due to grade, alignment, and traffic controls.  YMMV, of course.

If you really want to feel like the slow guy on the road, parts of Texas have 80mph limits, and even "farm to market" roads are often posted at 70.

-Sean
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« Reply #8 on: April 21, 2009, 05:09:30 PM »


If just one vehicle passes us, we "feel" we are being left behind. Which bothers that other head that periodically thinks it is in charge....

We are the most educated masses in recorded history, yet we still can't stand it when others pass us on the highway, and feel we are being left behind. And we let ourselves get fleeced at every automotive related turn because of it. anyhow....

At 65 MPH on a highway signed at 75 MPH we get passed pretty regularly.  It doesn't bother me if vehicles pass me.  It is less stressful as a driver of a large vehicle to have others pass me than for me to regularly pass other vehicles.  I just stay in the right hand lane and let others pass if they want to go faster than me.  It was quite the rare event to pass a vehicle on a 4,000 mile round trip last year.

We always drive straight through to our destination because we have limited time for our trips.  We are always going to an event so we have no choice on how far to drive if we want to attend the event.  We can't choose to drive to the next state if the event is 2,000 miles away.
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« Reply #9 on: April 21, 2009, 07:46:41 PM »

Brian, there is a safe operating range for your tires, as there is for ours. We found that we had more than 1 mpg difference in operating at the low pressure end from the high pressure end.

For us, it was worth running the higher pressure.

For what it's worth.

Tom Caffrey
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« Reply #10 on: June 03, 2009, 09:07:08 PM »

Not to step on any toes here being a noob, however you guys are missing one very very important factor with Tire Inflation and that's LOAD! Every tire manufacture has a chart for there various product that will give you Tire pressure in reference to axle weight! Most you can google for those you can't hit your local truck shop. As Sean said under / over inflation will end up leading to premature tire failure and possibly even a blowout! OK I'm done rambling!
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« Reply #11 on: June 04, 2009, 01:33:42 AM »

Not to step on any toes here being a noob, however you guys are missing one very very important factor with Tire Inflation and that's LOAD! Every tire manufacture has a chart for there various product that will give you Tire pressure in reference to axle weight!


There was no need to bring it up, because the OP's very first post indicated that he had already consulted the load and inflation tables for his tires:
I am running 90 PSI front and 80 PSI rears as required for my axle weights.

(emphasis mine).

Also, we've discussed this topic here many times before, and so most of us took the question at face value.

-Sean
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« Reply #12 on: June 04, 2009, 06:11:41 AM »


.....hydroplaning can occur can be estimated by the following formula:

Hydroplaning speed? = 10.35 x  inflation pressure [25]

Under this assumption of water depth exceeding the capability of the tread design to remove water:

At 30 psi, hydroplaning could occur at 56.7 mph

At 25 psi, hydroplaning could occur at 51.8 mph

At 20 psi, hydroplaning could occur at 46.3 mph.

This is presented to show the relative effect of inflation pressure on the possibility of hydroplaning.


Sorry, but I need to point out that the above formula contains an error.   The hydroplaning point is not 10.35 times the tire inflation, but rather it is 10.35 times the square root of the tire inflation.   You can see that 10.35 x 30psi obviously does not equal 56.7 mph.

I believe this equation originated from Samual K. Clark's work entitled "The Mechanics of Pneumatic Tires".
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« Reply #13 on: June 04, 2009, 07:59:00 AM »

Well that certainly invalids that study.

I knew there was a reason I was posting less and less here. Now I think zero.
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« Reply #14 on: June 04, 2009, 10:34:11 AM »

Oh, the study itself is just fine. I am simply pointing out that it appears the special character for the square root symbol did not survive the cut & paste process.  It might be important to someone who wonders why the rest of the math in the document doesn't work, or for anyone who wants to do the calculations on higher psi figures.
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« Reply #15 on: June 04, 2009, 03:18:43 PM »

Elkhedge,

I for one had not caught the Axel weight issue, Which reminds me I need to weigh mine to be sure my pressures are right.

Thanks

John
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« Reply #16 on: June 04, 2009, 04:02:50 PM »

We'll mark that one for a "special" moment on my part Wink
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« Reply #17 on: June 06, 2009, 08:04:44 PM »

.



Quote
I already keep the MPH at 65 MPH to decrease fuel usage.  I discussed perhaps even going to 60 MPH last summer, but everybody else on the trip didn't want to spend an extra 3 hours in the bus each way, plus being 15 to 20 MPH slower than other traffic out west gets dangerous.


FWIW, That is an oft-quoted but completely unsupported statement.  AFAIK, no traffic safety studies have ever found that vehicles traveling below the posted limits (within reason, also usually posted) are less safe.


Quote
(Speed limits on I80 from Nebraska to California are mostly 75 MPH.)
-Sean
http://http://OurOdyssey.BlogSpot.com





https://secure.truckflix.com/news_article.php?newsid=1460

http://www.landlinemag.com/Special_Reports/2009/Apr09/040209_ohio_split.htm

The only speed limit policy that makes sense is to have all vehicles traveling at the same speed. It is a welcome change in Ohio that is long overdue,” said Todd Spencer, executive vice-president of the Owner-Operator Independent Drivers Association.

“We are appreciative of the lawmakers recognizing the importance of this issue and once and for all resolving it.”

After days of intense negotiations, Senate Republicans and House Democrats meeting in a conference committee were able to work out their differences on the transportation plan. The full House and Senate approved the budget earlier in the day, setting the stage for Strickland to sign it into law.

Among the changes endorsed by lawmakers is the elimination of the provision in Ohio law that set up a slower speed on interstates for vehicles with a gross weight of more than 8,000 pounds. Speed limits on other roadways will remain unchanged.

Currently, large vehicles are required to travel 55 mph – 10 mph below the 65 mph limit for other vehicles. With the bill’s passage, all vehicles traveling on interstates soon will be cleared to drive 65 mph.

Owner-operator and OOIDA member Lewie Pugh of Freeport, OH, was pleased to hear about the action of lawmakers. He said that truckers in the state have touted the safety benefits of all vehicles traveling the same speed for years.
 


Not to be contrary, Sean, but I think there might be some evidence to the contrary. Smiley I realize the references are about speed limits,but you get the point. Smiley Mitch
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« Reply #18 on: June 07, 2009, 03:51:18 PM »

...  AFAIK, no traffic safety studies have ever found that vehicles traveling below the posted limits (within reason, also usually posted) are less safe.


... The only speed limit policy that makes sense is to have all vehicles traveling at the same speed. It is a welcome change in Ohio that is long overdue,” said Todd Spencer, executive vice-president of the Owner-Operator Independent Drivers Association.
...
Not to be contrary, Sean, but I think there might be some evidence to the contrary. Smiley I realize the references are about speed limits,but you get the point. Smiley Mitch


Actually, you are being contrary.

Note, please, that I said (qualified by "AFAIK") that no studies have found that vehicles traveling below posted limits are less safe. I have added the bold emphasis to highlight the difference between this statement and the articles you cite, which
  • Do not cite or even mention any formal scientific studies.
  • Do not discuss self-selected travel speeds but only deal with posted limits being different for different types of vehicles.
  • Are clearly talking about a legislative change brought about by political action from the powerful trucking industry lobby.

So, yes, I get the point.  The point has nothing whatsoever to do with my assertion that choosing to drive at a speed below the posted limit makes you less safe.

Now, if you would like to cite evidence to the contrary (as opposed to speculation, or the political machinations of a special interest group), I would be happy to review it and revise my opinion.  But I spent too many years in the highway safety business for these sorts of unsupported arguments to be persuasive.

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #19 on: June 07, 2009, 04:10:04 PM »

I stand corrected. Mitch
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« Reply #20 on: June 08, 2009, 09:00:33 AM »

Hey All,

At the risk of sounding incredibly stupid I have always had an issue with the MPG issue.

My thought is this:

Using the earlier description of the difference of 7 and 8 mpg translated into three extra hours of running time between his destinations.

Does the fact that Brian's series 60 was running three extra hours to accomplish this negate the saved fuel costs?

I mean on one hand he conceivably drives there at the higher speed which lowers his fuel economy but has him arriving 3 hours earlier, on the other hand he slows down and gets better fuel economy but his engine has to run an extra three hours to reach his destination.

I have never been able to get my brain around the concept in my head that says, hey I'm getting better fuel mileage but the extra time my engine spends running is time it wouldn't have been running if I was already there. Fuel costs cease when our engines aren't running...

I started this with the right phrase: "at the risk of sounding stupid". I am quite sure I am missing something here...

Rick
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« Reply #21 on: June 08, 2009, 09:23:36 AM »

The engine is using enough less fuel per hour over the entire trip that running three extra hours is still more efficient.  It is all about the amount of fuel required to move the bus forward one mile rather than how much fuel is burned per hour.  Fuel usage measured in hours only really makes sense for stationary applications.
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« Reply #22 on: June 08, 2009, 12:02:34 PM »

Exactly!

Without the experience/learning it is counter intuitive! And that's why we, the great unwashed masses, continue to spend WAY too much money on fuel, having been fooled about the sociological meaning, or the flawed assumption about changed arrival times, of the speed we choose.

One of the less well known measures for wearing out an engine is how much fuel has been burned in it.

Not how long it has run or how far it has gone.

As for fuel efficiency, the fuel use charts for our engines are somewhat "U" shaped, there is a "sweet spot" in the middle revs somewhere that the engine uses the least amount of fuel per time interval, lower or higher than that, the engine burns more fuel per time interval. Some of the more recent engines have a more pronounced "V" shape to that graph, operate the engine outside of a fairly narrow sweet spot, the fuel economy will be VERY poor in relation.

So, running the engine closer to the sweet spot rather than wide open, will produce better fuel economy on two fronts, less work in pushing wind, and running the engine at an RPM which makes best use of the fuel.

happy coaching!
buswarrior

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« Reply #23 on: June 08, 2009, 04:28:03 PM »

Rick - Mills are usually designed around "cycles" or revolutions - technically a mill that is designed and operated at one speed or RPM using a CVT (think hydrostatic drive) will be more efficient and outlast others - You don't have the problems associated with lugging, piston slap, heat, etc. - A mill designed to last 2 million cycles at an average 2 thousand RPM will should last 1 thousand minutes - there are many other ways to describe it, as BW did in fuel consumed, but they all assume an ideal operating range is being maintained - Fall outside that "ideal" and the life of the mill is shortened - HTH
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« Reply #24 on: June 08, 2009, 05:11:32 PM »

One of the less well known measures for wearing out an engine is how much fuel has been burned in it.
happy coaching!
buswarrior
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many years ago at an FMCE convention, a speaker (I don't remember his background) was asked about oil consumption. His answer was that oil consumption should be based on fuel used, not miles traveled.  His reasoning was that heavier fuel usage= harder engine usage and this will cause an increase in oil consumption.  I o not know if this is true, but at the time it sounded reasonable. Jack
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