Rear end gear ratio - Printable Version

Rear end gear ratio - Printable Version

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Rear end gear ratio - Mageeacres - 05-09-2021

I read a thread where determining rear end ratio by knowing the rpm divided by revolution of the tires = gear ratio. So I do the math (3100/470=6.59) and the answer is definitely wrong. So obviously my tachometer is not programmed correctly... (electronic with dip switches) I have a Cat 3208T with an Allison 750 tranny. The PO repowered from a 555 and left the existing radiator and transmission with 11r22.5 tires ... I know lockup in 5th gear is 1:1 and the engine should be kept at about 2800 or below. If I do the math it seems to be reporting about 75% above actual (measured speed with gps).

My question is does anyone know what manufacturer and rear end ratio they put in the late 70s early 80s single axle classic coaches build date Oct/79? Thanks in advance.

RE: Rear end gear ratio - HoosierDaddy - 05-09-2021

I don’t know the answer to your question. Most rear ends have a stamp or tag on them indicating gear ratio. A strobe light tachometer could determine the tachometer accuracy. There’s even an app for that!!

RE: Rear end gear ratio - Richard - 05-10-2021

Mike,

Very easy to determine. Chock the coach tires, and release the Park Brake. Jack up one rear tire. Put visible marks on the tire and driveshaft.

And what Dean said about strobing the engine to determine true RPM. I do not think you are turning 3100 going down the road.

RE: Rear end gear ratio - Mageeacres - 05-10-2021

I Spoke to mike at newell and he says the differential is a 2.83:1 ratio. So I should be able to set the rpm dip switches to accommodate the 1400 rpms at 60 mph. I’ll be trying the app too!!
Thanks for the info

RE: Rear end gear ratio - MrE - 05-10-2021

Mike, load a GPS speedometer app on your smart phone instead of trusting your mechanical speedometer. My 2 cents...

RE: Rear end gear ratio - HoosierDaddy - 05-10-2021

GPS speedometer is great idea!!. Most GPS routing apps will display speed as well. I run three different ones on my phone including WAZE. I like WAZE the best when I have to make good time with my car.

RE: Rear end gear ratio - Mageeacres - 05-11-2021

Awesome idea, that’s how I tracked the speedometer because it wobbled from 50 to 70 at highway speeds. Does anyone have a recommendation for an optimal rearend ratio for this 3208T engine? I’m getting conflicting information. Newell says they probably put a 2.82 but I found a Rockwell tag that says 5.29. Now I’m not sure which one is in it. Spoke to the company that installed the repower and they said they kept the original rearend. If I need to replace the pumpkin, what ratio would you use? Thanks in advance.

RE: Rear end gear ratio - MrE - 05-11-2021

Mike, I'll probably have to do the same thing with my re-power. What I plan on doing is gathering some data to determine what speed and RPM I want to travel at and calculate from there. I have the advantage of an electronic drive train that will display my mileage as I'm traveling. Your best bet would be to look at torque/hp graphs for your 3208T hp rating. You might check the Wanderlodge forum for other 3208T owners recommendations.

RE: Rear end gear ratio - Mageeacres - 05-12-2021

Jon, you were reading my mind, looking for the sweet spot in the power curve and then adjust accordingly. When these coaches were made it was when 55 mph was the federal highway speed limit. So naturally 50 to 55 was normal. I found a speed ratio chart in with the original paperwork from Newell. I’ll post the photo. It lists all available ratios and their rpms for both 22.5 and 24.5 wheels. It looks like they (Newell) had marked which ratio to use. It was the 3.70. Now to verify... also I’m trying to get a response from caterpillar technical division to verify what rpm they recommend for the current configuration at 65 mph. I’ll definitely post the results for you also. Also waiting on a callback from a transmission expert and a OTR supplier for input into this equation. ....

RE: Rear end gear ratio - TGDianics - 05-12-2021

If you have to change the ratio (ring and pinion gears) consider changing the carrier assy. to get a switchable locking differential.