Gearing

[KANTANKRUS]

Alright y'all, I have a question.

 

I have an 87 Comanche with a 4.0 Liter motor, that sits on a 6.5 inch lift, rides on 33x12.50-15 rubber, it has a fully rebuilt AX15 transmission, and if my memory serves me right, a NP231 t-case with a SYE and a new spicer rear driveshaft. I want to install a Dana 30 HP front axle under it and a Chrysler 8.25 rear axle, out back. With Yukon 4.56 gearing. My question is, how many RPM's will be produced, while driving at highway speeds (65 to 70 mph)?

 

Please advise!

[Blaine.D]

This might help you out, here's a link to an engine rpm calculator that would give you a rough idea of what you should expect.https://spicerparts.com/calculators/engine-rpm-calculator

[gogmorgo]

Wheel circumference at 35" diameter is roughly 109" (in reality probably a hair less because tire diameters are nominal)

1 mile is 63,360", divided by 109" is  581 revolutions per mile.

Divide mph by 60 for miles per minute, 60 mph is 1 mile/minute for easy math.

581 revolutions per mile times 1 mile per minute is 581 revolution-miles/mile-minute, which cancels to 581 rpm at the tires. (I did say easy math)

A gear ratio of 4.56:1 means the driveshaft (input) turns 4.56 times for every turn of the axle (output). 581 rpm times 4.56 is 2650rpm. 

2wd (and 4hi) is 1:1 on an np231 so we'll ignore the transfer case. 

5th gear ratio according to Novak on an ax15 is 0.79:1. Like the axle ratio, this means the input turns 0.79 times for every turn of the output. (As an aside, this is an overdrive gear because the output turns faster than the input)

2650rpm x 0.79 is about 2090rpm. This is again at 60mph. You can swap out gear ratios and tire diameters and speeds as you will to see how the affect results. 

 

Again because tire diameters are nominal you'll probably see more rpm than that. Some manufacturers will publish actual diameters and revolutions per mile of your specific tire, but these things will change with load and tire pressure; increased load means the tire squishes outwards, reducing effective diameter, and increasing tire pressure unsquishes it, increasing effective diameter. Either way, most tires end up with effective diameters that are smaller than their nominal diameters, which will lead to increased engine rpm at the same speed over a theoretical tire where effective and nominal diameters are the same. 

[KANTANKRUS]

Many thanks! I may want to bump up to a 35 inch tire then... but until then, I will drive accordingly.

[Pete M]

the answer is, you'll be just fine like that.    tons of guys out there running 4.56s and 33s.  my 4.10s aren't quite as deep as I'd like.  

[Eagle]

https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/pub?hl=en&hl=en&output=html

[75sv1]

http://www.grimmjeeper.com/gears.html

[Eagle]

The problem with all those on-line calculators is that they are based on the theoretical (mathematical) diameter of the tire. In the real world, tires have some deflection of the sidewall and the effective radius (what the tire manufacturers call "static" radius) is less than the theoretical radius. That affects the actual number of revolutions per mile. The tables I linked to represent a spreadsheet that I created using actual tire manufacturer's revolutions per mile numbers, from published sales literature and tire company web sites. Since it's based on actual revolutions per mile, it should be more accurate than any numbers based on nominal tire size.

 

The revolutions per mile I used are shown for each tire size. Note that the overdrive ratio used is 0.75:1. If your overdrive is 0.72 or 0.78, just adjust accordingly.