Calculating Speedometer Gears

If you have just swapped rear end gears, then the next thing you'll want to do 
is calibrate your speedometer to compensate for your new gears.

There are essentially two ways to do this.

You can install a speedometer ratio adapter available from many sources, 
including transmission shops, or you can change the speedometer gears.

Since changing the speedometer gears is the preferred way to go, due to cost 
and originality, we'll describe how to make the needed calculations. 

What you probably should do first is to remove the speedometer cable from the 
transmission, count the teeth on the driven gear and apply this formula;

We'll assume you had a 3.00 gear ratio, and you've changed them to a 3.50 ratio.

Here's the formula;

So, if you had a 3.00 and you now have a 3.50, and you have an 18 tooth driven 

3.00     3.50
----- = ------
 18       X

if you cross multiply (18 x 3.50)/3 = 21

So you need a 21 tooth gear.

{Special thanks to Chris Hunter from [email protected] for correcting my math}

If you need to change your drive gear here's the formula;

Driven gear teeth=Drive gear teeth(axle ratio)(tire rev/mi)/1000 

Now, I'll assume you have a set of P235/70R-14 tires.

They are 27.0" in diameter, they have a circumference of 84.7" and so they 
revolve 748 times per mile. {Phew!} 

Driven gear teeth=Drive gear teeth [axle ratio][tire rev/mi]/1000

21 = X[3.50x748]/1000

21 = X[2.618] 

X = 8.02

So, to use your 21 tooth driven gear, you'll need a 8 tooth drive gear.

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