I used my Pulsar to change mine. 275/65/20 Kenda Klever M/T. Set it to that size to start. Speedo read 2mph faster than GPS speed. Tried 265/65/20 and noticed no change in speedo error. Ended up at 275/60/20 and speedo is within 1mph with cruise set at 70mph. Whe initially changed to actual tire size, the 2mph error started at 40mph.
Easier to just do trial and error starting with the manufacturer tire size, than trying to measure a tire. I messed with mine over several days. Wasn't something I felt the need to stop and change immediately until I got it right. Set it, noticed the error. Next time I started the truck, I adjusted it. Still noticed error. Then kept forgetting to try a different size. Pretty simple process with the Pulsar other than you use the gas pedal to change the numbers. Idle is 0 and full throttle is 9.