1stTimeBuilder

Hey folks,
I know when you change the size of wheels/tires, it can throw off your speedometer. What is the trick to adjusting it for the variances in wheel size? Also, what parts do you have to change out? Thanks.

Robert
Clarksville, TN

mdvaldosta

changing the speedo gear on the tranny

1stTimeBuilder

Thanks for the quick answer. One more question, though. How do you know which gear to buy? For example, say I am going to put 245/45/17 tires on the car (just picking numbers out of the air here). When you buy the gears, are they built for a certain tire diameter?

Robert
Clarksville, TN

mdvaldosta

I know their matched to gear size, Im sure theirs somewhere you can find out the correct gear for tire size but I don't know

69mach1377

Go to the 5.0 section for a recent thread called 'gear?'

roundman

small changes in tires sizes can't really be adjusted for by changing the speedometer driven gear but the best answer I can give you is take it to a speedometer calibration shop, the one where people go when the get speeding tickets, to have them run it on the dyno and show you how far it's off, if any, and they may be able to recommend a gear that will get it closer to reading correctly than where it is now. years ago I had an old Torino and the speedo was off about 6 mph slower than actual speed at 60 mph, so knowing that, I just drove accordingly.

1stTimeBuilder

I found a formula to figure out how many teeth the gear should have. It is:

(Drive gear teeth) x (Axle ratio) x (Tire revolutions per mile)
---------------------------------------------------------------------- = 1000
(Driven gear teeth)

Hope that helps someone else. I got the forumal from the K.A.R. Mustang parts catalog, by the way.

69mach1377

Too many unknowns in this one...
Who knows how many teeth are inside their tranny's?
Tire revs per mile should be expressed in tire circumference or diameter.