Speedometer Calibration

What do I need to do to calibrate my speedo for my 31" tires??? I believe the originals (H78/15) were about 28" diameter.

 

I went from the original size tires to 31's as well. There was really no serious difference between the two. My speedo is still fairly accurate, or at least as accurate as it ever was. R)

 

about all you can do is check the speedo driven gear where the cable attaches; count the teeth on the gear for reference; (if still stock, would be 16 tooth with 3.73 axle ratio; 18 tooth with 4.27 ratio)
since the speedo would now be reading slightly slower, you would need a gear with fewer teeth for faster rotation.
So....this means a 15 or 14 tooth gear
and they are very hard to find, not available new anymore
we used to figure one tooth for every 6-8 mph difference
hope this helps

 

heres an formula to find the new mph
speedo reading (mph) x new tire height/old tire height = actual speed (mph)
there u go. i have 32's and i am goin five mph faster than what my speedo says of course there is a more accurate way and alls u need is a gps and that is about as accurate as ur gonna get