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Author Topic: what is the mileage difference between a 4 speed man. and a auto. transmission  (Read 2748 times)
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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buswarrior
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'75 MC8 8V71 HT740




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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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Frozen North, Greater Toronto Area
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
PD4106-2784 No More
Fresno CA
andy
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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
PD4106-2784 No More
Fresno CA
hargreaves
1987 MCI 102A3
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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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