I have a 87 yota 22r with 31x10.5 MT's. How much have you guys seemed to be off on speedo?

Girlfriend was behind me and she was doing 5 over and said she wasnt catchin me...but seemed like i was doing 1 under, cuz truck wasnt legal at time..

31's throw it off about 6 mph? Anyone have a clue to how far off it is...

 

Prolly not 6 mph, but a bit. Depends on if you have stock gearing or not too.

 

let's say that your stock tire size was 245/75/15, this means the following:

your tire circumference = 92.5", for a 31X10.5 tire diameter is ideally 31 but more likely 30ish.

let's say it is a real 31" tire. that means tire circumference = ~97.34"

97.34/92.5 = 1.052 thus the 31 is 5.2% "longer" than the stock tire.

This means that assuming you have the same gears as before, every rotation of the tire will take you 5.2% or 4.84 inches farther than stock...


we know that 60 mph is 88 ft/sec so... this means

assuming 5th gear on your tranny is oh let's say 0.8:1 and you have 4.10 diffs, you will be revving at 2188 rpm's with stock tires and 2082 rpm's with 31's. As you can see, the roughly 5.2% difference comes back.

in order for their to be a 6 mph difference in speed you would have to be doing like 100mph, so 6mph is unlikely. At highway speed (70mph indicated) the 31's will put you at about 73.5-74 mph. Basically if you cut down your speed by about 4 mph you will be okay as far as the cops are concerned.

Hope this helps,

Jon

 

Are speedo gears available to correct for oversize tires (like on Fords)?