MPH Chart

Geoff · July 27, 2016, 05:53:18 PM

Does anyone have a link for a speed a mph chart using tranny gear ratio, engine rpms, and tire size?? I used to use Mallie's chart but that was quite a while ago.

-- Geoff

luvrbus · July 27, 2016, 06:00:34 PM

I use one of the speed shop calculators like www.wallaceracing.com

TomC · July 27, 2016, 08:01:01 PM

One of the fun things I used to do figuring proper gearing, tire size, transmission for trucks. All done with a calculator.

luvrbus · July 27, 2016, 08:37:03 PM

Some of the internet calculators you can enter the ratios of the transmission,rear gear with the speed and rpm and it will tell you the diameter and rpms of the tire neat huh

HB of CJ · July 28, 2016, 08:36:25 AM

This goes back to high school 50 years ago from the auto shop classes. From feeble memory: ...

RPM divided by rear end gear ratio, (net, after OD figured in, if present) times the tire circumference in inches, divided by 12, divided by 528, times 6 equals the MPH at the aforementioned RPM. Easy as pie. Can almost do it in ones head.

Example. 1974 Crown Supercoach I owned. 2100 rpm divided by 3.9 times .82 equals 3.20 times 130 inches divided by 12 divided by 528 times 6 equals about 81 mph in 10th gear at 2100. Obtained on flat level long straights.

Oonrahnjay · July 28, 2016, 09:05:43 AM

Geoff, here is a calculator that I used. At first, I was sort of baffled by all the boxes but once you figure out which ones are necessary to fill in, it is very helpful. It answers a lot of questions.

www.tremec.com/calculadora.php

Geoff · July 28, 2016, 12:55:19 PM

Thanks everyone. I have tried all the charts as suggested, but I am not having a lot of luck with my dumb smart phone.

Here is what I have -- 315 80r 22.5 tires, 2900 egine rpms, .88 tranny ratio, 4.10 rear axle.

I know that at 2350 rpms I do around 78+ mphs.

I installed military Jakes along with the buffer/Jake spring, the high/low governor setup with the fast idle as part of the high/low spring. The problem is in order to get the military setup to work the Jake and fast idle you have to run the high speed spring up to 2900-3000 engine rpms. I don't plan on going 2900 rpms,. I just wonder what the military was doing running their engines so high

--GEOFF

Ed Hackenbruch · July 28, 2016, 08:21:49 PM

Dodging bullets!

TomC · July 28, 2016, 10:12:41 PM

315/80R-22.5=485rpm (revs per mile). 485 x 4.1 x .88 = 1749.88rpm @ 60mph. 60 ÷ 1749.88 x 2,900 = 99.435mph. Good Luck, TomC

luvrbus · July 29, 2016, 06:30:36 AM

That is interesting I never saw 12361124 issue (automotive) 8V92 series set at 2900 rpm,the 53 series are set at 2800-3000 rpm.I have one of the military 8v92 12361124 here with the same setup as yours from a Oshkosh M977 it is 450 hp @ 2100 rpm with Jakes and fast idle

TomC · July 29, 2016, 07:17:50 AM

I agree-most 92 series are not very happy past 2,300rpm. I remember some truckers in the 60's and 70's twisting 71 series up to 2,800. A 92 series would not last very long

Oonrahnjay · July 29, 2016, 07:41:14 AM

Quote from: TomC on July 29, 2016, 07:17:50 AMI agree-most 92 series are not very happy past 2,300rpm. I remember some truckers in the 60's and 70's twisting 71 series up to 2,800. A 92 series would not last very long

Geoff, how long does this particular engine have to be run at 2900 Rpm? Is it just a short time to "initialize" the Jake setup? Something about this just doesn't seem right.

Geoff · July 29, 2016, 07:57:40 AM

Quote from: TomC on July 29, 2016, 07:17:50 AM
I agree-most 92 series are not very happy past 2,300rpm. I remember some truckers in the 60's and 70's twisting 71 series up to 2,800. A 92 series would not last very long

Thanks, Tom. I don't think I will be seeing 99 mph anytime soon!! The story is the military buffer/Jake activator is shorter than the usual buffer screw and the high speed/idle is longer. I had to use the longer screw to reach the Jake screw and it would not reach at 2300 rpms. Only at 2900 rpms. Plus the fast idle would only have room to work with the longer screw. So the governor might be set at 2900rpms no load, but my shifting rpms remain the same. I just have to watch the rpms and not overspeed down the road. The Jakes work normally as they would with a 2350 rpm top no load.

--Geoff

luvrbus · July 29, 2016, 08:22:15 AM

Most military 8v92's use a variable speed governor not the LS (limiting speed) governor

Geoff · July 29, 2016, 09:29:53 AM

Quote from: luvrbus on July 29, 2016, 08:22:15 AM
Most military 8v92's use a variable speed governor not the LS (limiting speed) governor

Cliff--. The 92s I took the Jakes off had DWLS governors. I have no idea what military vehicles they came off of.

--Geoff