Transmission

[ParadigmCrime]

My bus doesnt have overdrive, is it possible to put a new-used tranny in with overdrive?

My Bus is an International Harvester 1986 S1700 "short bus". Overdrive would increase my mileage on long drives wouldnt it? (its an automatic trans)

[frank-id]

It is my belief that all auto trans have a 1 to 1 final drive with no overdrive. Some manual trans have a .8 or .7 final drive ratio. There are huge overdrive auxillary trans that connect to the rear of manual trans. To get better MPG the rear axle gears are the changes to be made. There is a compromise between high speed MPH or climbing a long steep hill. The mountains are in the west and the hills are east of the rockys. Frank

[the_experience03]

The new Allison 2500 series has overdrive IIRC from when I was buying my bus from the dealership. The odd thing is though the guy said they get the same mileage and same top speed with basically the same rear axle gears as the older buses. They just get there quicker. Now I understand how 5 gears could help with acceleration, but not how the overdrive wouldn't help top end unless you were actually starting to run short on power. He said the 2500's are often found behind the school bus duty C7 Cats and they run a 4.56 gear usually with a top speed around 70 on the new computer controlled buses. Now I know I would be happy to get the 8mpg at 70 mph the new buses are getting, but he said they were doing the same thing with the last of the AT545'ss Any thoughts on that one? I still think a bus would benefit from a 2500 series Allison in terms of strength, and the gear options, but the cost is high, they need a computer, etc. Was this guy just blowing smoke up my backside or what?

[wtd]

8mpg at 70 mph

I wouldn't mind having either one of those ....

[Jarlaxle]

The 1000 & 2500 series Allisons are OD trannies (5-speed autos). They are able to run deep axle gears for around-town use, but still run at normal highway speeds without revving the engine on the governor. The F-650 box truck I drive at work has a Allison behind a C7 ACERT Cat engine, & spins ~2200RPM at 65. 60 is <2000. Pushing a 12'6" high box with plenty of city use, I get ~10MPG.

[the_experience03]

Originally Posted by wtd

I wouldn't mind having either one of those ....

Hehe...I have the 8 mpg thing atleast. Actually, I found that I get over 10 if I drive 45 everywhere.

The quasi 1000 series Allison GM is using in the HD pickups is actually a 6 speed now. I used to know a lot more about that pickup drivetrain, but they've changed it up a little the last couple of years. I'm not sure how much of a real Allison 1000 series that tranny is anymore. I do know that the transfercase is a New Venture 263 which surprised me since the Duramax has the most torque of the Big Three's diesel offerings yet that's the lightest duty transfer case in a heavy duty pick up today....