I am changing out rearends in my car and at the same time I'm going from a 3.00 to a 3.73 gear. Does anybody know what kind of speedo gear I should get for my C4 so that speedo isn't WAY off?

 

A sure fire was is to buy a few gears and test them against a buddies car at 70. If you are off by more than 10% change it. I once saw a calculator that you entered your tire diameter, rearend ratio, and drive gear and it spit our the tooth count you needed. I think the formula is:

Driven teeth = ((# of drive gear teeth)* (axle ratio) * (Tire revolutions per mile)) / (1000)

The # of drive gear teeth is going to be 6,7, or 8. That is going to depend on your tranny, and I cannot tell you how to find out. Typical Mustang transmissions used 7 tooth for 2.70-3.50 cars and 6 tooth for 3.50-4.30 cars. I would guess 7 tooth and go from there. Now you need to know if you need a RH or LH driven gear. LH is typically used in 4 speeds and RH are used in everything else. If the speed-o gear is on the driver side of the tranny it is a good bet it needs a RH gear.

 

I have a C4 and it is goes in on the drivers side. Im thinking I need a 20 or 21 tooth gear. Gun jam went with a 21 tooth for his 3.80 gear, but he has a T5 trans now. I wonder how many drive teeth a T5 has??? Also, my back tires are about 28" in diameter, so I would need a smaller gear to compensate for that as well. I think I am going to leave it alone for now, and see how much it is off and then go from there. That way I can get a percert difference and then determine how many teeth the new gear should have. What do you all think?

 

One change in tooth will move your speedo about 10% or 5-7mph at 70, best I can figure. My 4 speed toploader with 3.50 rearend and 26in tires takes a 19 tooth driven gear with a 7 tooth drive gear.

 

So... say I am going down the interstate and my speedo says I am going 70, but I am actually going 63, that would be a 10% difference. So I would need a gear with one more tooth. Correct? If my speedo says I am going 70, but I am only doing 54, that would be a 23% difference, so I would need a gear with 2 more teeth? Do I have this concept correct?

 

Yes sir.Your example is correct, from my experience. I got my wife to set the cruise control on the Honda at 70, which has a super accurate speedo, and then drove up next to her. That way I could really dial in what the difference is. Following someone is ok but leaves some room for error.

 

Thank you sir! I will check it out once I get it all together!