C-3 REAR END RATIOS?

[Larry Daigle]

I have a 71 454/365 4speed corvette! I rotated the drive shaft about 3 times to get one rev. of the rear tire, does this mean I most likely have 308's? Also is the rear a posi if both wheels turn in the same direction, or are there limited slip ones offered back then? and last but not least, with a 370 rear app. what rpm would my engine be at if I was going 60 mph in 4th gear? I would like a little more low pickup, but don't want to be over reving if I want to hit the highway once in a while, like 411,s would be! Thanks so much for any help! Larry!

[Joe L.]

Re: C-3 REAR END RATIOS?

Larry-----

If you have about a 3:1 turn ratio between driveshaft and wheels, then that would be consistent with a 3.36:1 rear ratio. Of course, using the "wheel turning" method there's enough "imprecision" that it could also be a 3.08:1. I would recommend turning the wheel about exactly 5 times. Then, have someone count the revolutions of the driveshaft. Divide the number of turns of the driveshaft by 5 (the exact number of wheel revolutions) and you will have a better idea of the ratio.

For 1971 positraction was standard, so, if you have the original rear, it should be posi.

On that score, you should check the bottom of your differential carrier case, just forward of where the cover mounts. There should be a stamping there that may identify your rear end ratio. If it's "AW", then you have 3.08; if it's "AX" or "LR", then you likely have 3.36. Of course, someone could have changed the gearset, but based upon your wheel turning/rev counting, I doubt it.

[Larry Daigle]

Re: C-3 REAR END RATIOS?

Thanks Joe, the driveshaft actually turned about 2 and 7/8! Know I know where to look and with those letters I'm sure I will be able to figure it out!

[Joe L.]

Re: C-3 REAR END RATIOS?

Larry-----

My brain went into some kind of "reversion mode" when I first responded to your question; 3 turns of the driveshaft to 1 turn of the wheel is consistent with a 3.08:1 ratio and not 3.36:1. Still, there is the "imprecision" that I mentioned using this methodology unless you use multiple turns of the wheel to help reduce the error. In any event, if you actually got 2-7/8 turns of the driveshaft to 1 turn of the wheel, it's much more likely that you have 3.08. Lower (numerical) ratios were not used for 71 and it's unlikely that anyone replaced with lower. But, it's possible.