RPM

[Big J]

What rpm will I run at 70mph using 33's and 4.56 gears shurly sombody is using this combonation that can tell me. I am running the factory automatic transmission.

 

[Robert]

What RPMs do you run now at 70 MPH and what gears do you currently have and what is your current tire diameter? Knowing this information, you can calculate your new RPMs.

 

[JOSEPHOUYSON]

gears

I am running 3.73's with 30x9.50 tires. I have used a math equastion from an offroad mag. But I was hopeing to get a number from some body that is actual running 4.56 w/33's so that there is no guesing or math error. My math says at 70 mph I will be turning around 2500 rpm's w/4.46 and 33's currently with my 3.73's and 30in. tires I turn around 2200. I realy would like to here a real number from somebody running that ratio and tire size, so that I can putt my mind at eas.

 

[Robert]

Any math errors will be less than the variances you can get from vehicle to vehicle. If the Explorer tachometer is only accurate to +/- 5% then you can be up to 10% off from someone else that has the exact same setup that you want to go to. I have rounded the calculations off to within +/- .002%. The only errors greater that you will run into is the size difference between 30" and your tire's true diameter and 33" and that tires true diameter. The gear ratios will not vary so there won't be any error with them.

Going from a 3.73 to a 4.56 will make your RPMs 1.22252 times higher at every given speed. Going from 30" tires to 33" tires will make your RPMs 0.90909 times lower at every given speed. Multiply the two figures together and you get 1.11138 times your current RPM for your new RPM with 33" tires and 4.56 gears. At 70 MPH you will be turning 2445 RPM.