When calibrating for a new tire size. Do you input the tire manufacturers size ( unloaded) or do you input the measured tire size (loaded).

 

Measured. (10char)

 

Here is a quick and accurate way to find your tire diameter:

Stick a piece of chalk in the tread.
Move the Jeep until there are two chalk marks on the pavement.
Measure the distance between the chalk marks.
Divide the measurement by 3.1416.
That's your tire diameter.
It removes the variables encountered with other methods.

 

I was given this tip by H&S Performance. Measure from the ground to the center of the wheel. Multiply this by two and use it as your starting point. I got it right on the money first time when calibrating my Ram with it's 37's.

 

Measure horizontally or wrap a tape around your spare and divide by pi.

 

In short measured diameter. Thanks for all the replies. Stay classy jk-forumers

 

I just used AEV's Procal Module and the instructions that come with it say to measure. Never use the manufacturers dimensions. To the nearest quarter of an inch is as accurate as you need to be. Works great!

 

I used a GPS to dial in the speedo calibration.

 

This seems like it could be inaccurate, the bottom half has load on it (the tire should have a little bulge) and the top half doesn't. I can't imagine it would be off all that far, but not the true diameter.

 

This is true. The distance fron the center to the ground is less than the distance from the center to the top of the tire, yet for each axle revolution, the same length of tire circumference still has to contact the road.

This is why the method in post #3 above is good. It removes such variables.