C3 REAR END RATIO?

[Larry Daigle]

I read on here one time there was a method to calculate rear ratios from speed,rpm and maybe the gear your in? I have an original 1971, with M21 close ratio tranny. Can I roughly figure my ratio? Thanks in advance,I just love this site! Learn a lot just checking out this site!

[Duke W.]

Re: C3 REAR END RATIO?

Axle ratio = top gear revs@60 / 775.

The above assumes that you have the correct speedometer gearing for the installed axle ratio.

775 is the revs per mile figure for the OEM F70-15 tires. Actual installed tire does not impact the formula - just the accuracy of your speedometer.

Duke

[Dale P.]

Re: C3 REAR END RATIO?

want a down a dirty method .. jack up the car .. so you can rotate one of the rear wheels .. put a chalk mark at the 6 oclock position of the rear tire and one of the side of the drive shaft .. with the car in "N" have some rotate the tire one full rev .. count the number of times the chaulk mark on the drive shaft passes its orginal position ... with the last rev being only partial convert that to a "." add to the whole number and that is your approx. ration

EX..... three full revs plus about 1/2 .. convert to 0.50 add to the 3 and your ratio is approx 3.50 ...

By the way there is a way to do it with RPM, tire size, speed and a known transmission gear ...

[Jim T.]

Re: C3 REAR END RATIO?

Another way to determine your ratio is to look on the rear differential and find the code for the ratio and post the code and someone will be able to tell you what ratio matches the code for your 71.

[Dale P.]

Re: C3 REAR END RATIO?

the code on the rear axle can only tell you what was OR could be there ... Trust me it does not tell you for 100% certainty what is there

[Gary S.]

Re: C3 REAR END RATIO?

http://www.idavette.net/tech/ratioc.htm

Would this be it?
Gary

[Dale P.]

Re: C3 REAR END RATIO .. great link?

Gary ...... what a great link ..... beats running the numbers with a calculator !! thanks for sharing

[Gary S.]

Re: C3 REAR END RATIO .. great link?

As others will point out, there are allowances for error in this program but it is a nice FYI page. It make not be as exact as rocket science but it does give you something to compare against, despite any inherent errors.

Gary