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Author Topic: what is the mileage difference between a 4 speed man. and a auto. transmission  (Read 2841 times)
mike davis
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« on: January 26, 2008, 10:49:59 AM »



 my tdm got 9-11 mpg  with a 671/4 speed What is the mileage with an 8V71/Auto.

 oh the TDM converted weighed 29,000

                     thanks

                                      mike
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Dallas
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« Reply #1 on: January 26, 2008, 10:58:18 AM »

on a good day.... 8 on a normal day about 5.5 to 6.5 I won't tell you what a bad day of climbing into a headwind pulling a 18' flatbed trailer is.
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mike davis
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« Reply #2 on: January 26, 2008, 11:55:34 AM »

on a good day.... 8 on a normal day about 5.5 to 6.5 I won't tell you what a bad day of climbing into a headwind pulling a 18' flatbed trailer is.


Dallas,

Lets say one of my best friends uncle owns one of the largest diesel salvage yards in the North East.  Lets say on the weekend said friends uncle likes to line his pockets with a little bar money his wife doesn't know about and it not on the books.  And, the currency conversion rate is very favorable. 

Would it be worth switching one of these more modern coaches Like the one I'm looking at to a four speed?  They have every machine known to mankind to fabricate linkages, etc. etc.  There might even be a coach that I could yank them off of.  They have the big smasher.  It smashes buses.  But they can't sell buses 

What I'm looking at is half the fuel millage from an auto.  Is it the 8v71 vs the 671 or is it the tranny. 

Thanks,
Mike

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Utahclaimjumper
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« Reply #3 on: January 26, 2008, 12:20:22 PM »

The size of your foot has to be factored in..>>>Dan
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« Reply #4 on: January 26, 2008, 12:36:43 PM »


What I'm looking at is half the fuel millage from an auto.  Is it the 8v71 vs the 671 or is it the tranny. 


An automatic transmission is a little less fuel efficient, but I've never seen, or even heard of, it cutting fuel economy in half.  I've always understood that, all other things being equal, a manual transmission will get 1-1.5mpg better mileage.   If one is getting half the economy of the other, I would beleive it to be a combination of several factors.

  • Engine
  • Transmission
  • Engine and transmission health
  • Injector size and tuning
  • Total operating weight of the buses
  • Driver habits (even if it's the same driver, the extra power can go to ones head and from there the foot)
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mike davis
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« Reply #5 on: January 26, 2008, 02:00:08 PM »



so It's the 8V71 vs the 671 at 30,000 lbs also a steel bus


         mike
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Jerry Liebler
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« Reply #6 on: January 26, 2008, 03:09:45 PM »

Mike,
    The fact that there is more horspower available with the 871 means it can burn more fuel, However if the 871 is driven so no more power than the 671 could give is used it is possible to get the same mileage if gear ratios, weights, etc. are the same.  But most of us use all the power we've got accelerating and climbing hills.  In the case of 'V' drives the popular V730 has a lower gear ratio than the typical 4 speed in addition to being less efficient.  More modern fuel controls, turbochargers and 4 stroke engines all are more fuel efficient than the WWII designed 2 cycle Detroits.
Regards
Jerry 4107 1120
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Beatenbo
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« Reply #7 on: January 26, 2008, 04:47:29 PM »

I have owned 4 MCIs

MC8 8V71 Auto 7mpg
MC9 8V71 4sp 7mpg
96A3 6v92T 5spd 7-8mpg
102C3 6V92T Auto 7mpg

I run highway miles @ 70-80mph if limit and traffic permits 30-40 K a year. 65 mph will drag every coach I ever owned down on the grades. I keep mine wound up when safely possible
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mike davis
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« Reply #8 on: January 26, 2008, 06:46:18 PM »

Mike,
    The fact that there is more horspower available with the 871 means it can burn more fuel, However if the 871 is driven so no more power than the 671 could give is used it is possible to get the same mileage if gear ratios, weights, etc. are the same.  But most of us use all the power we've got accelerating and climbing hills.  In the case of 'V' drives the popular V730 has a lower gear ratio than the typical 4 speed in addition to being less efficient.  More modern fuel controls, turbochargers and 4 stroke engines all are more fuel efficient than the WWII designed 2 cycle Detroits.
Regards
Jerry 4107 1120

 Jerry,

So I guess besides the TDM aluminum exoskeleton construction and the newer coaches steel rib construction, the addition of ? 35% ? more power, then adding an automatic transmission are what reduces the gas mileage by 35-40%?   

I think the 8V71 was introduced in '56 or '57? but it wasn't the total factor in the less miles per gallon.  I thought it would have been a shocker to greyhound as an example to absorb 35-40% loss of economy at one shot.  Even though diesel fuel was 10-12 cents a gallon then. 

Just trying to wrap my head around the loss of 40% of the fuel mileage.  Also, thinking that there might be a single cause, but apparently I was mistaken. 

My TDM was governored out at 73MPH.  I usually drove it between 65 and 70 because that was what the laws were at the time.  I did notice that when I kept in wound up it got better fuel mileage.

Thanks everyone for all of your help and insight

Mike
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« Reply #9 on: January 26, 2008, 07:51:19 PM »


Just trying to wrap my head around the loss of 40% of the fuel mileage.  Also, thinking that there might be a single cause, but apparently I was mistaken. 


A 40% drop (assuming that weight and driving terrain/habits are similar) still sounds like there might be something wrong.

Going back to the driving habits though.  Especially if it is driven in an areas with a lot of grades or even just rolling hills.  Keep in mind that 8V71 will climb a grade faster than a 671. Likewise, a 6V92TA will climb it still faster and a 8V92TA will climb even faster.  But each will burn progressively more fuel to accomplish it.  All other things remaining equal, faster climbing takes more horsepower and that takes more fuel being burned.

Likewise, if both are driven in a city or other environment where frequent stops/starts or at least accelerating/decelerating is common, the fuel economy difference of larger engines to smaller ones will be more pronounced.  The larger engine will get rolling/accelerate faster, but use progressively more fuel to do it.  Once it is up to speed and on average terrain, driven at similar speed and with all other variables equal, fuel consumption should be comparable.

MC8 8V71 Auto 7mpg
MC9 8V71 4sp 7mpg
96A3 6v92T 5spd 7-8mpg
102C3 6V92T Auto 7mpg

40' RTS II 6V92T V730 Auto 7mpg (30,000 pounds plus towing my pickup)

Maybe some folks here with 671 experience could post their numbers to give a good comparison across the full range.
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TomC
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« Reply #10 on: January 26, 2008, 10:17:53 PM »

With my AMGeneral transit pulling my MB 300 turbo Diesel (34,750lb total) cruising at 58mph and 1850rpm I averaged 5.15mpg on the round trip to Las Vegas.  But with the 8V-71TATAAC engine I have, it is such a pleasure to drive it, not having to down shift but once going up the big hill at Baker and Cajon, and that it doesn't smoke at all anymore, I don't mind 5.15mpg.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #11 on: January 27, 2008, 07:49:59 AM »

88 102A3 8V92T 6.5 to 7.5 at 65 to 75 MPH 740 auto Jerry
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« Reply #12 on: January 28, 2008, 07:37:44 AM »



   What you also might keep in mind, the quality of fuel (cetane rating) that the engine is running will make all the differance. The national advarage is 42.5.numbers.  Higher the number, the better the preformance.  2-EYTEL-HEXEL NITRAIT is the componet they put in diesel fuel for combustion.  The more BTUs , and less pre- engine knock with this product will produce unbelieveable results.  I say this because I'm in that type of business!


MY 2cents worth.

Steve 5B.....
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« Reply #13 on: January 28, 2008, 11:25:09 AM »

Steve 5B,

A while back there was a post about adding "acetone" to your fuel.  Gas or diesel both were claimed to benefit significantly.  The theory, as I vaguely understood it, was that the acetone aided in the atomization.  I said vaguely!  The amounts added were very small and I thought almost any increase in power/efficiency would have justified the expense.  This was claimed to be based on some real science.  I think cetane rating was something it boosted.

You said you were in the business and I thought you might be able to comment with authority.  Anything that adds power or decreases cost in of interest to me.

Thank you,

John
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« Reply #14 on: January 28, 2008, 12:30:36 PM »

Hi Mike,

The 8V71 was first put into buses in 1961 (GM PD4106)  The fuel hit was probably worth the extra power.  As good an engine as the 6-71 was and even the 6V71, they are both underpowered in some circumstances.  Are you still considering the 4905 or have you moved on?


-- Seaton
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4905 doc
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« Reply #15 on: January 28, 2008, 02:00:17 PM »

I'm with Beatenbo. My 4905 fully converted pulling a 20' enclosed trailer w/camaro conv., motorcycle, and electric scooters always averaged 7mpg. Oh and thanks to Fred Hobe for one super strong trailer hitch.
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« Reply #16 on: January 28, 2008, 08:35:43 PM »

And lets not forget frontal area...

Larger the brick you are trying to punch through the air, the more energy required.

Just drive what you have, and be wise about how hard you push the throttle, and the cruise speed you choose.

You'll not get a return on the investment for a tranny swap.

happy coaching!
buswarrior
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mike davis
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« Reply #17 on: January 28, 2008, 09:41:19 PM »

Hi Mike,

The 8V71 was first put into buses in 1961 (GM PD4106)  The fuel hit was probably worth the extra power.  As good an engine as the 6-71 was and even the 6V71, they are both underpowered in some circumstances.  Are you still considering the 4905 or have you moved on?


-- Seaton

seaton

  I'm still checking the 4905 Went over and started it today I have not found any better yet


                            mike
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RJ
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« Reply #18 on: January 29, 2008, 09:16:53 AM »

Mike -

Here are some numbers for you to roll around in your head awhile concerning GMC V-drive powertrains:


All Models -

Bevel gear ratio in the 4-spd manual gearbox is 0.808:1 for all 8V71-powered units, starting with the 1961 PD4106.

Bevel gear ratio in the V-730 is 0.87:1

Bevel gear ratio in the 6-71-powered models was 1:1, such as your old TDM.


35-foot Coaches -

All the Parlors in this size used a 4.125:1 rear axle.  They were basically geared to run 60 mph @ 1650 rpm using tires that turn 495 revs/mile with the manual gearbox.

The overall final drive ratio with the OEM powertrain is figured as follows:  4th gear x rear axle ratio x bevel gear ratio = final drive ratio.  Plugging in the numbers gives you:

([1:1] x [4.125:1] x [0.808:1]) = 3.333:1

If you install a V-730, you get:

([1:1] x [4.125:1] x [0.87:1]) = 3.588:1


40-foot Coaches -

This size Parlor used a 4.375:1 rear axle ratio.  This works out to about 100 rpm higher at any given road speed than the 35-foot models.  Makes sense - the 40-footer's heavier. 

Plugging the numbers into the above formula gives you a final drive of 3.535:1 for the stick shift, and 3.806:1 for the V-730.


Real World Mileage -

After 25+ years in the bus industy, IBME that most stick-shift, 8V71-powered 35-foot GMs get 8-9 mpg overall.  Some owners, running in the flat mid-west and keeping the speed down to around 60, have reported 10, but that's rare.  V-730-equipped models return 1 - 1.5 mpg less. 

Most 40-foot stick GMs get 7-8 mpg, and again, the V-730s about 1 - 1.5 mpg less.

It really does depend on the routing, winds, weight of right shoe, highway vs city, overall weight of coach, etc.

Because of the limited amount of powertrain options for a V-drive GMC, about the only way to improve mileage (besides parking it and never driving) is to install 24.5" wheels with the tallest 11R24.5 tires you can find.  Bridgestone, for example, makes a drive tire that turns 470 revs/mile.  This tire will just about bring a V-730-equipped coach back to similar mileage as a stick-shift unit.

The heavier 40-foot MCIs will return 1 - 1.5 mpg less on average than a comparable GMC.  Simple physics.

Where a bus really shines compared to a stick 'n staple is in range.  A PD4106 with the OEM stick shift powertrain, for example, will go over 1,000 miles on a tank of fuel, and still leave you enough to find a truck stop.  So will the 4905, for that matter, but the reserve margin's a little tighter.

Finally, you don't buy a bus for fuel mileage.  It's just like the old joke about the Mercedes salesman telling the prospective customer after being asked about a model's fuel consumption: "If you cannot afford the fuel mileage, you cannot afford the car."  You buy a bus for the safety and convenience, not mileage.

FWIW & HTH. . .

 Wink
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RJ Long
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Fresno CA
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« Reply #19 on: January 30, 2008, 03:40:02 PM »

Mike, I tried the # you left the other day no luck call me at 765 362 3877
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mike davis
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« Reply #20 on: January 30, 2008, 08:51:57 PM »

Mike, I tried the # you left the other day no luck call me at 765 362 3877
andy
        I'll try on Thurs. If my wife does not get mandated at work. It going to Snow. She is in health care and they can make her stay at work till more people can get in I'll have my 2 kids At 3 & 5 they don't let me talk on the phone


                    thanks
                              mike
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mike davis
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« Reply #21 on: January 30, 2008, 09:04:22 PM »

russ

             thank you

I do have some more questions about the numbers But my wife will haft to type them

 I love hard numbers Do you know off hand. The surface contact area of the 11R24.5 vr a 10.00/20

                             thanks

                                   mike     
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RJ
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« Reply #22 on: January 30, 2008, 09:43:02 PM »

Mike -

10.00x20s are about 10" wide, 11R22.5 or 24.5 are about 11", and 12R22.5s run about 12" wide, give or take a little.


But wait. . . there's more:


The trucking industry is converting over from the above measurements to metric sizing, similar to your car.  12R22.5s now equal 315/80R22.5 and so forth.  You have to go to the charts to find out what metric size equals the old sizing, which is not that difficult, as most manufacturers have that info online.  Of course, dealers have it too. . .

The 20" tube-type are becoming more rare, nearly everything's tubeless nowadays.  That and many tire shops are refusing to mount split rims like these, primarily for liability reasons.

A GM coach, and it's limited powertrain options, requires you to concentrate on "tire revolutions per mile" when shopping for wheels and tires, as well as load range, speed rating, etc.  The magic number is 495.  That's the revs/mile GM designed the powertrain around, and the closer you stay to that number, the closer you'll come to OEM performance.

Simple rule of thumb: 

Tires that turn MORE than 495 will increase fuel consumption and reduce top speed.

Tires that turn LESS will have the opposite effect.


Be careful shopping for "bus tires."  For example, there are two different versions of the common 315/80R22.5 bus tire.  One is for transit buses, and one is for highway models.  The difference?  The transit tires have much heavier sidewalls, to cope with the fact transit drivers think curbs are part of the bus's braking system.  They're often limited to 50 or 55 mph, and can easily overheat and self-destruct when subjected to sustained 70 mph speeds.  So speed rating as well as load range are two additional concerns when buying tires, in addition to revs/mile.

Clear as mud?


FWIW & HTH. . .

 Wink
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RJ Long
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Fresno CA
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« Reply #23 on: January 30, 2008, 09:43:53 PM »

I got a 72  Flyer D700 6v71 647 auto trans, 4.11  gears 7.5 - 9 mpg U.S  towing a jeep driving in British Columbia where it is hard to get over 65 mph with all the hills we have. It's all pretty simple H/P X rpm means fuel consumption.  Gerry









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now as of Feb 2012 series 50 B400  . Sunshine Coast British Columbia
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