What tire measurment are you guys with 285/70/17 KM2's putting into the AEV procal?

 

It varies by psi, mfg, wheel width, vehicle weight, etc. Grab a tape measure and go measure...

My 295/75/16's measured out at 32" even.

 

+1 on this:

My 35" km2's came out to 33.5" when I put a tape measure up to it with 30 psi in them. I looked at the little instruction sheet that came with the procal and set the module accordingly. Speedo has been accurate ever since.

 

It seems that the best "first guess" is to measure from the ground, vertically to the top of the tire where the shoulder first begins (where it makes the hard angle into the tread). Don't measure to the bulge of the tire, or across the tire horizontally (especially bulge-to-bulge). It's pretty typical for this measurement to be an inch or almost two smaller than the rated tire size. 33.25" works on my 35" Duratracs to give accurate speedometer (as compared to GPS).

 

Put a chalk mark on the side of the tire at 6 o'clock, and a mark on the ground, lined up with it.
Roll the Jeep forward until the tire chalk mark comes around to 6 o'clock again, and mark the ground there, too.

Measure between the lines on the ground, and divide by 3.14159. The result will be your tire diameter, without the confusion of where to measure the tire.

 

...i took a 'level' and laid it across the full width of the tire tread on top and measured.

...i believe i'll try your method to see the difference.

 

Thanks guys, to the nearest quarter inch, I measured the tire out to be 31.75.

 

My Toyo 285/75/17 MT's measured 33.25" at street pressure (32 psi). That seems fairly accurate for my Procal numbers. Compared to GPS unit my speedo is within 1-2 mph.