mesmith60

I recently purchased a used 2006 F150 FXR, it has 35x12.5x20 tires (door sticker says stock were 275x55x20) . Is there a way to tell if the gears have been changed based on speed/rpm? What is approximate difference in size of stock to aftermarket tires?

MGD

Any number is online calculators will give you the answer:

Example:

http://www.miata.net/garage/tirecalc.html

Stock 275/55/20's = 31.9".

Vlad

mesmith60

Is there a way to determine what gears I have?

CobraJeff27

If you know what tire size you have and you know what rpms your are at at a given speed you can out that in the speed rpm calculators that come up on Google. It will give you the gear ratio. It will be your transmission ratio in that gear tones your differential ratio.

The alternative is jack up the rear end and spin the tire. One revolution of the tire should be x turns of the Driveshaft. X being your gear ratio.