Been checking my speedo with a GPS after recalibrating with the flashpaq.

Set to 33in = 4mph high at 65
Set to 32in = 1-2 mph high at 65
Set to 31.75in = 1-2 mph at 65 and above

Should I keep reducing tire size on the flashpaq till its dead on or just live with 1-2mph high as closer is impossible??

 

For my similarly sized tires, I had to set my Hypertech at 31.25" to get the speedo to read correctly. Verified with two GPSs, and my son's F-150 pick-up speedometer. The best way to get an accurate estimate is to measure from the ground vertically to the sharp shoulder where the tire bends at almost 90 degrees. Don't include any of the bulge of the tire tread.

 

I'd go one more size down. I'm going through the same thing right now with 285/70-17. Measured 32 1/4 but I'm down to 31.75 and still 1-2 over at 70. I'm going to try 31.50 and see what happens but that will probably be it for adjustments, any further and I imagine it's going to start going under the actual speed.

 

I finally quit and settled on 31.50. Its 1 mph high but that's what my M3 was. Shoot my YJ was worse than that and no way to fix it unless I played with speedo gears. Done.