I have input my info into 6-7 different calculators and they all come up with different ****. In about a 400rpm range. Im trying to figure out what my cruise rpm is going to be at 75mph with 35" tires and the .75 OD ratio with the 80e. Basically Im trying to hit as close to 2K as possible so its either 3.73s or 4.10s. I havent driven my truck in a year so I have no idea what it was turning at 75mph. I need a link to a calculator that you KNOW is accurate, Im tired of guessing and its alot of work doing a gear swap in a 4wd

 

with the combo listed above a 3.73 gear will put you at 74mph at 2000rpm according to my math which is pretty accurate most of the time.

 

74.5 mph with 3.73s @ 2000rpm
67.8 mph with 4.10s @ 2000rpm

 

Try this.

 

http://www.bgsoflex.com/auto.html
I like that calculator. It uses more info than most of the others I have seen for it's calcs. It's pretty close to what those guys are saying if you turn the Transmission Slip Factors off.

 

You don't need website calculators to figure this out.

Start with pi times your tire diameter = 3.14 * 35 = 109.9
Divide that by 12 = 109.9 / 12 = 9.158
Then divide 88 by that = 88 / 9.518 = 9.609 (this is your tire factor)

Multiply mph times trans ratio times rear end ratio times tire factor =
75 * .75 * 3.73 * 9.609 = 2016RPM

The only numbers here you have to remember are:
12 because there are 12 inches in a foot.
88 to convert miles per hour to miles per minute (5280 / 60)
3.14, that's pi.

There's no guesswork here and if you want to be very precise, measure the exact circumference of your tire and use that number instead of 109.9. Also use more decimal places and use the pi key on the calculator.

Enjoy.