How to recalibrate your speedometer after changing tire sizes or gears

Reading faster on the GPS than the speedo after I replaced the part.

So going 65 on the dash, 70 on the GPS?

If that’s the case, you need a 29 tooth gear.

65 * 31 / 70 = 28.8

I never recommend anybody buy their gear based on a chart or nominal tire size. Math is always the best way to get it accurate. Too many incorrect charts out there and too many fluctuations in real world tire diameters.
 
So going 65 on the dash, 70 on the GPS?

If that’s the case, you need a 29 tooth gear.

65 * 31 / 70 = 28.8

I never recommend anybody buy their gear based on a chart or nominal tire size. Math is always the best way to get it accurate. Too many incorrect charts out there and too many fluctuations in real world tire diameters.

Thanks so much for the response!

We actually had it flipped- 65 on speedometer, and 62 on the GPS.
 
So going 65 on the dash, 70 on the GPS?

If that’s the case, you need a 29 tooth gear.

65 * 3 / 70 = 28.8

I never recommend anybody buy their gear based on a chart or nominal tire size. Math is always the best way to get it accurate. Too many incorrect charts out there and too many fluctuations in real world tire diameters.
 
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So I replaced the speedometer gear with a 31 tooth gear, per the graph (3.73 gears and 33x12.50/15 tires). Originally my speedometer was 10% slow against my GPS Speed App. Now, however, I am 3 mph fast at all speeds, 20, 40, 50??? This doesn’t make any sense? Any suggestions?
 
I replaced my speedo gear with a 35 tooth, and it runs fairly close to the GPS UNTIL i get up to 60+ MPH, then it’s like 5 MPH low now on the speedometer.
 
I'm confused. I have 99, 4.0, 3 speed auto and 33s 4:10 gears. What speedo gear do I need?
 
Usually 35 or 36. What tooth do you have now and how much are you off by at a set speed?

Thanks. I'll have to check it's original for 3:07 gears and 32rh. I'm off like 15-20 mph. Been like that for a couple years since my regear. Really should get around to correcting it.
 
My TJ had the original 36 tooth gear (3.73). 50MPH on the speedo was 57MPH on my GPS unit.

50/57*36 = 31.57 = 32

I installed a 32 tooth gear and now 50MPH on the speedo is 51MPH on my GPS unit. (y)

Andy
 
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