When recalibrating speedo use tire spec diameter or actual tire height

[Payton34]

I have my intune programmer but not sure if I should use the 34.8 diameter that is listed on my tire manufacturer specs or the measured height 34" with the weight of the truck?

What method has everyone used?

[nadrojcote]

Measured height will be lower because of the vehicle weight on it. I'd enter 34.8. Or else enter in the actual metric size. I enter mine in as 305/45/r22.

 

Once you get up to speed on the highway your diameter will expand to 34.8.

[Bayou 6.0]

When I plugged in the tire size for my ProComp 35x12.50R20LT M+S tires, my actual speed was 2-mph slower than what the speedometer displayed. When I measured actual tire height by placing a level on the tire to ensure I had an accurate & flat line, then measured from garage floor up to the level. 35.5-inches.

Plugged that into the Diablew Custom tuner. Speedometer is dead accurate now. Tested speed with three different GPS units to verify.

[crafferty]

I enter in my new tire size and then see how far off my gauge is. Currently for my 35x13.5R20 Toyo R/T's my Diablo Intune is set at 343/51/20. Now my speedometer is dead nuts at any speed.

[nadrojcote]

I enter in my new tire size and then see how far off my gauge is. Currently for my 35x13.5R20 Toyo R/T's my Diablo Intune is set at 343/51/20. Now my speedometer is dead nuts at any speed.

 

Dead nuts.

[TJay74]

The stock calibration uses rolling circumference via rev's per mile.

 

Tire height example

 

275/55/20

275 x .55 = 151.25"

151.25 / 25.4 = 5.95" (to convert from mm to inches)

5.95 + 5.95 + 20 = 31.9" tire height

 

You could also measure with the tires on the truck at the cold pressure, then warm the tires up. Then use a level across the top of the tire in the center of the tire and measure to the floor for the actual under weight height.