I have 305/65/17 extreme at procomps [33s]. Mounted on the jeep they are exactly 31 1/2 inches tall which is fine. Calibrated speedo with superchips to the measured height and my speedo is off according to garmin gps.


started entering different tire sizes with flashpack. My speedo is dead on with 30 1/2 entered.

So should I use the actual height of 31 1/2 or 30 1/2 which puts my speedo deadon with gps.

Any ideas appreciated.

 

Use whatever works with GPS' reading. My 35s are 'calibrated' to 34.25 on my s/c.

 

my 35's had to be set to 34" in order to read correct on my gps.

 

I too found the Superchip speed calc. is off. running 37 KM2's and had to set superchips 34.25

Go with the GPS they are on the money

 

Try a different route to get to the answer - measure the (effective) circumference then calculate the diameter from that.
How I did it was to mark the tire and ground with chalk, roll one full tire rotation, mark the ground again, then measure the distance between marks. This is the circumference. Divide that by pie (3.14) and you'll have the diameter to input into the programmer.

This technique takes into account the give that a tire under load experiences. GPS is OK but I found it to be somewhat trial and error.