I have been looking for some sort of formula of how my speedometer changes with my bigger tires I went from 32's stock on a rubicon with i think 16 inch rims. to 37's on 20's If i work the math out on only the tires i would think i would be going about 6.5 % faster than the speedometer (32 (old tires) divided by 5 (difference between new and old tire size) = 6.5%) But does my calculations take in account the different rim size. the only way i know to fix this and please correct me if i am wrong is to get a chip like hypertech or something. in the meantime I was just trying to be close

thanks
Mike

 

The only thing that matters is the overall outside diameter of the tire.
Your numbers are off. I believe going from 32 to 37 is a 16% difference.

 

All you need to recalibrate your speedo is the AEV ProCal...much cheaper than Hypertech and other chips, but keep in mind the Hypertech, Superchips, and the other performance programmers have a lot more adjustability options than the ProCal. For my money, the ProCal is all I need, probably all you will need as well.

 

Oops forgot the 1

thank yiou

 

X2 to on the AEV pro-cal

 

if you can, borrow a GPS from someone and check the speed vs your speedo.

Also, don't forget that most tires are different than advertised. The only way to get an accurate measurement is to do the chalk method.

I used a procomp accupro to recalibrate mine along with the chalk measurement. I verified it with my wife's GPS and all is good

 

The formula is calculated in a percentage...

1-(2*pie*R of the larger tire)/ (2*pie* R of the smaller tire)
Where R = raduis of tire or half of diameter. i.e. 32" tire has an R of 16"

That should do it for you. You can use 3.14 for pie if its not on your calculator

 

check out the 1010 tires tire size calculator, you can plug in the factory size and then what you are putting on and it will give you the speedo readings.

http://1010tires.com/TireSizeCalcula...?action=submit

 

No need to get pie involved, you can simplify it from the equation. Just need to divide the diameters.

 

i have 35s on my rubicon and by GPS the speedo is 4 mph slower at 40 mph