Someone told me that there is a formula out there to determine what number of teeth the speedometer gear that goes into the transmission should have to match modifications. For example, the formula takes into account wheel/tire diameter, rear gear ratio and trans type. Anyone know what it is? Or can anyone tell me how many teeth I should use if I have a 3.55 posi on a C4 with 15 inch wheels (tire unknown) so I can get a realistic speed reading?
Take the tire diameter(in inches) multiply by Pi (3.1416) to get the circumference. Divide that into 63,360 (inches per mile) to get the tire revs per mile. Then Multiply the drive gear tooth count (C-4 is 7 teeth) X axle ratio X tire revs per mile, then divide by 1000 to get the driven gear tooth count. The tire diameter can be calculated with the sizing on the sidewall. First number is the section width in millimeters, second number is the aspect ratio. Multiply the section width X aspect ratio ( we'll do a 275/60/15) 275 X .60 =165 This is the sidewall height. Take this times 2(top and bottom of the tire) and divide by 25.4 to get the sidewall height in inches, add that to the wheel diameter to ge the total diameter. (165 x 2 = 330, 330/25.4 = 12.99, 12.99 + 15 = 27.99, round that off to 28.
Thanks- In your example you dont take into account the rear gear ratio then? So the 28 you got means a 28 gear tooth count? Wait I see you did take the ratio in account now. Thanks a ton. I have 2 college degrees, but being a lawyer doesnt help me on this. It may help with tickets.
Ain't Algebra fun ? Bet you thought you'd never use it after high school, huh? I didn't. :16suspect Now that I've figured out all those letters in the formulas were supposed to repesent numbers, it all came clear to me.
My son is in the most advanced math class for an 11th grader. Pre Calc. He looked at the formula and will run it for me. Finally I get a return on my 17 year old investment in him.
I'm looking at the same problem right now. I just switched to 3.55 gears and I'm now running 26 inch tall rear tires. At 50 mph my speedo says 55 or so. I was told by my tranny guy that one tooth is worth about 3 miles per hour. The speedo gear in my tranny now has 20 teeth which would mean I would need a 22 tooth gear. Does that sound right?
I've got 3.50's with a 25.5 tire and I worked it out as needing a 19 tooth gear. The speedo vs tach checks out close with this, till it hits about 80+ mph. (7 teeth X 3.5 X 792 = 19404. 19404/1000 = 19)
