Axle Gearing Decision Guide

[RamblerRyGuy]

There are a few charts out there showing the RPM’s that you run with certain axle gears with certain tires at a given speed. I used one of these such charts from one of the posts I stumbled across on this website a long while back. Sorry, I don’t remember where I found it on here. I got tired of referencing it during my diesel Ranger build, since the ideal RPM range highlighted for the stock Ranger gas engines is not at all the ideal economy and power RPM ranges of the VW TDI I’m installing. So I determined that I was going to make formula for calculating what RPM’s your engine will turn with any setting of tires, axle gear ratio, and/or transmission gear ratio (also including low or high range if you want).

Sure you can collect anecdotes from other people with certain combinations, but isn’t it better to be self sufficient? Plus, if you have an off-road rig, you can figure out your crawling RPM’s as long as you plug in the MPH number.

This calculation turned out much easier than I originally thought.
So here it is.


Pi • Diameter of tire / 12 = X

5280 / X = Revs of tire per mi.

Revolutions of tire per mile • Speed \ 60 • Axle ratio • Trans gear ratio (multiply by low range ratio if desired for final number)

Feel free to post your numbers from your particular setup at highway speeds or any other speed you want to specify.

 

[scotts90ranger]

I find it easier to look at a table in Excel or open office Calc (don't have Office at home) to decide on things, basically using that equation but with different engine speeds in the rows and the transmission gear ratio's calculated in columns to give the vehicle speed in the table, those calculations are referencing axle gear ratios, tire diameter and transmission gear ratios at the top of the screen so they can be changed independently. I apparently made it at work, I was going to attach it, I'll share it on Monday, if I feel fruity I might even make it graph the speeds in different gears and might throw in low range...

 

[RamblerRyGuy]

I find it easier to look at a table in Excel or open office Calc (don't have Office at home) to decide on things, basically using that equation but with different engine speeds in the rows and the transmission gear ratio's calculated in columns to give the vehicle speed in the table, those calculations are referencing axle gear ratios, tire diameter and transmission gear ratios at the top of the screen so they can be changed independently. I apparently made it at work, I was going to attach it, I'll share it on Monday, if I feel fruity I might even make it graph the speeds in different gears and might throw in low range...

Thats pretty awesome! I’d love to have a file like that around for reference. Certainly easier than working out the calculation every time.

 

[scotts90ranger]

I spent some time working on the excel file, but it's not an allowed file type so I can't attach it... anyone interested send me an email through the forum thing (I enabled the button, don't remember where it is...). I have the new Excel at work where I added graphs and stuff but opened it up in Apache Open Office at home and made sure it worked on the free software for those like me that don't have Excel at home... It has data in it for the 4R70W with BW 4406 T case and M5OD with 1350 or 1354 T case for all gears including reverse and low range graphed for vehicle speed in MPH... capped at 100mph so you can see the lower ones... all ya gotta do is change the tire diameter and axle ratios and it will recalculate. Should be easy to figure out...

 

[RamblerRyGuy]

I would be interested to do so, but I've never emailed someone through here. Where would I find the option to email you?

 

[racsan]

You could always mention what you have, (engine, transmission, chassis type, gearing /tire size you have now) and what your plans are as far as usage, tire size ,ect. Theres a good chance someone has been there/done that and can advise on a certain ratio, or other helpful advise.

 

[sgtsandman]

I would be interested to do so, but I've never emailed someone through here. Where would I find the option to email you?

Just send him a PM with your email address.

 

[scotts90ranger]

Dangit, that button seemed to dissapear? No more personal messages just conversations? Whatever, that works... I'll get you the file in a couple minutes sgt...

It really doesn't matter what you have for a vehicle, it's a simple file, there's a spot at the top to enter tire diameter in inches and gear ratio, I already have the ratio's for the 2.3/3.0/2.9 M5OD but it's easy to change the ratio's at the top. It also has the 4R70W so there's a 4 speed and 5 speed page and graph...

 

[hat_man]

This is the formula I use for rpm range in any transmission gear with any rear end and any tire diameter at any speed.

RPM = (Speed x Rear End Gear x Transmission Gear x Tire revolution per mile) / 60