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Old 09-21-2016, 10:08 AM   #371
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Location: Houston, Texas
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Year: 1946
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Chassis: 1- 1/2 ton
Engine: Cummins 4BT
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Yes...Thanks...my bad. I should have said both rear wheels.
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Old 09-22-2016, 09:25 AM   #372
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So, the only way to do this without lifting both rear wheels off the ground it to roll it.
So, find a SAFE place to do this like a parking lot or very quite dead end road.
Mark a line on your tire where it contacts the ground.
Put a large Zip Tie on the drive shaft pointing in the 6'O clock position.
Now have someone drive the bus VERY SLOWLY and have someone count the times the Zip Tie passes the 6"O Clock position. This will be 4 times plus some.
When the mark on the tire returns to the bottom where it started, stop the bus.
Now look to see where the zip tie is pointing in relation to the start position(6'O Clock), this will give you your ratio.
Keep in mind you need to drive in a straight line, and you need to be careful, the driver can't see you.
A quiet area, or walkie-Talkies help.
This is the only way you can do it if you have no tags and no way to lift the bus.
Also, it is not going to tell you if you have a 4:11 vs. a 4:10, you just be able to see that you have about a 4:10 and will need to look up what was available and match it to what you found.
Hope this helps.
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Old 09-22-2016, 06:09 PM   #373
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Hey thanks Oldmopars, that makes sense and I could probably actually pull that off (!).

So, getting back to Jolly Roger's point, once I've done that and I know my diff gears, what next? How do I know if it's safe/advisable to change or bypass the governor and/or the gears to give me more speed and most importantly better milage? Or am I most likely stuck with what I have?
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Old 09-22-2016, 07:19 PM   #374
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That is where you start to get into Black Magic and it's hard to answer. However if you find out you have, say 4.78:1, going to 4.11:1 would be an improvement. But too big of jump and you could end up with a gutless bus that won't climb a hill.
You will also need to recalibrate your speedometer.
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Old 09-23-2016, 10:09 AM   #375
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Ok, I'm going to do this this weekend and will report findings.

Assuming there is room for "reasonable" improvement in swapping out the gears that won't diminish its overall abilities, how much improvement in performance and milage can I reasonably expect...e.g., is it really worth it and can I measure the expected gain at least roughly?
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Old 09-23-2016, 10:16 AM   #376
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Chassis: 1- 1/2 ton
Engine: Cummins 4BT
Rated Cap: 15
Really no way to precisely calculate MPG, but you can nail the numbers as far as RPM at a given speed. Use the calculator below...

Engine RPM CalculatorEngine RPM Calculator
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Old 09-23-2016, 10:30 AM   #377
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Very interesting, thanks Tango! And I do know my question doesn't really have an answer, I'm just trying to get some idea of a benefit I MAY get if I decide to make the change.
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Old 09-23-2016, 10:44 AM   #378
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It is worth running the numbers. It doesn't take much of a reduction in RPM's to create pretty substantial fuel savings. Like backing off from 70mph to 65, if you can get your RPM's down near the sweet spot by cutting down on revs, the difference can be surprising.

That said, unless you spend a fair amount of time at highway speeds...the difference may not be worth the effort.
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Old 09-23-2016, 01:24 PM   #379
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Right...I do expect many miles will be highway miles, not all but a fair amount but of course I won't know that until I do it. Until then I'll make whatever study of it I can and see if something significant reveals itself enough to push me to do it.
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Old 09-23-2016, 01:38 PM   #380
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Year: 1991
Coachwork: Carpenter
Chassis: International S3800
Engine: DT360
Rated Cap: 7 Row Handicap
heres my experience..

7 row carpenter, 27500 GVWR, high top
DT-360 AT545 trans flat ground

running 65 - 68 engine pegs on rev limiter 2700-2800 constantly. MPG 6-6.5
running 60 - engine occasionally hits the rev limiter.. usually stays at 2500-2600. MPG 7.5 or so

running 55 - engine never hits rev limiter.. usually at 2200-2400 RPM on average. 10-11 MPG. (I got 11.5 on a couple tanks of fuel)

my bus is running 11R22.5 tires and a 4.78 rear.. my RPMs vary because of no lockup in the AT545 trans...

these numbers were measured making Long rides of 500 miles or more.. i typically took the MPG numbers when i had been on flat ground.. up and down hills lowered it.. im guessing because of the AT545...

-Christopher
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