ive installed the procal in my jk and still my speedo has different speed from my gps!!!!

 

Did you measure your tires to set the ProCal?

After the initial measurement, I adjusted my ProCal by 1/4" increments to reach the corrected speed by the GPS.

It took about 4 times to get it right.

Still off by 1 mph at 70....

 

You can't measure just the overall height of he tire. You need to measure from center to floor and double that. The rolling distance is affected by the compressed tire on th ground. I did it this way (being a stupid math guy) and my input to the procal is spot on.

 

Huh that's wierd. I understand what your saying but mine is spot on using overall height on jeep. I think every jeep is a little different cause my bros is like 1/2-3/4 mph off

 

I was too impatient to go home and measure. I guessed in the parking lot, then just did the trial and error method, stopping several times along the way to finally match the gps.

 

thanks a lot!! ive set it to 33,5" and now i am only 2kph different!!!

 

I'll readjust mine, I think I'm like 3 MPH off at 60... I'm set for 35s, but the tires are really 34.5, but when I set for 34.5 it was WAY off... so maybe 34.75 lol.

I'll just keep playing when I get some free time this winter. inches aren't what I would call accurate enough for this. I can use a speedo GPS app on the iphone and set it to that.