what speedo gear are you guys with c4's and 4.11 gears running? tire size will be 245 45 17.i dont know how many teeth are on the gear in the trans.thanks.
Do you know what the tooth count on the output shaft is where the speedo gear rides, it can be 7, 8 or 9 tooth..... You need to know this to properly calculate what speedo gear you will need..... The formula is:
The ouput shaft toothe count makes a big difference in the calculation..... You either need to pull the tailshaft housing and count them or you could do the calculation using the 8 tooth count get a gear and try it..... If it's correct your fine and depending which way it's off yo could figure out which way you need to go.....
Not many options for gears. I run the white gear with my 4:11's. I read about 5mph high at 60 mph. So its out around 10% too high. The white gear doesnt last very long but it doesnt get many miles anyhow. http://www.tciauto.com/Products/Techinfo/speedo_gears.asp
From what I have read, most c4s and manuals have the 7 tooth gear on the output shaft. Here is some formulas I got out of Mustang and Ford magazine a couple of years ago. First you need to know the diameter of your tires. The formula is 2 x ( width x aspect ratio / 2450) + rim diameter. In your case it would be 2 x ( 245 x 45 / 2450 ) + 17 2 x ( 11025 / 2540 ) + 17 2 x ( 4.34 ) + 17 8.68 + 17 25.68 inces tire diameter Next is how many revolutions it takes to cover 1 mile. The formula is 20,000 / tire diameter. So yours is 778.81 revolutions. The final formula is Drive Gear Teeth x Axle Ratio x Tire Rotations per mile / 1000 7 x 4.11 x 778.81 / 1000 22406 / 1000 = 22.4 So you would need a 22 or 23 tooth gear. Not sure if they make them with that many teeth.
21 tooth is the highest tooth count Ford made. You need to run a reducer that was common with the 3.89-4.30 gear ratios. Any speedometer shop can hook you up with one or maybe a Mustang Vendor will have them
