Ok, I am trying to program in the correct tire size on my Hypertech. I have 285/75/16 Uniroyal Laredo AWT tires with maybe 10,000 miles on them. I know that the tires are supposed to be 32.8 inches in diameter. How come when I measure the tire from the ground to the top, it only reads 31 inches? I want to make sure I put in the right size on the tuner.

tires vary by brand, inflation etc. put in 31 and do a 60mph test on the highway for 2-3 miles. if the speedo is right it will be exactly 60 sec per mile between the mile markers.

Thanks, I'll give it a try.

Try measuring the tire without your trucks weight sitting on it... bet its much closer to 33".

along with the fact the tire size is never exactly what it says, for example the toyo mud terrains are actually bigger than they say they are, so you have to factor that in, you also have to factor in tread wear, and like he said above the weight of the truck squishing/flattening out the tire where it touches the pavement. and don't forget about your muffler bearings

Didn't the Hypertech come with directions? Or did you get it used? They tell you how to measure the tire size. If you don't have the directions, I'll get mine out later and tell you how they say to do it.

Here you go.

If the tire height is not known, ask a tire dealer or use the
following steps below:
1. Place a chalk mark on the tire where it contacts the pavement
and also mark the pavement. These marks should be at the center
of the tire footprint pointing straight down to the pavement.
2. Roll the vehicle in a straight line until the chalk mark makes
one revolution and is pointing straight down at the pavement
again. Mark the pavement again at this new spot.
3. Measure (in inches) the distance between the two (2) marks
on the pavement. Divide the measurement by 3.1416. This
will give you the tire height in inches.

Thanks, I did not get directions with the used Hypertech. Does the tire size adjustment only calibrate the speedo? Or does it do something else also?

Speedometer and odometer. I think it helps with shift points too. I know everytime I changed mine to correct for tire size, the truck shifted better and at the right time.

i think speedo only. it might/might not have some influence over the electronic transmission control.
edit - yep, odometer too

^zman's post of tire size measurement is technically accurate, but be sure and confirm it with a road test or gps test or other. i tweaked mine a couple of times to meet the 60mph = 60 second test on the highway.

^^Will do, thanks.