rear end ratio

[Ray G.]

I have a 65 convertable small block, running 215/70 radials which I believe are close in size to the original. The rear end says "AM" which would be 3:36 gears
but the tach reads 2500 rpm at 60 mph, and about 3000 rpm at 65 mph. Sounds like 3:08's to me? Any thoughts? Thanks, Ray

[Craig S.]

Re: rear end ratio

Ray - 2500rpm on my 3.55's is about 57-58mph on my 67 with GR70-15 radials, so I think you may be in the ballpark for your 3:36's. Doesn't sound like 3.08's to me....Craig

[Dave McDufford]

Re: rear end ratio

A program I downloaded from gearzone says with those tires 3.08 should be about 2300 at 60mph. Per the program the 3.36 would be 2521 rpm at 60mph.

Dave

[Craig S.]

Re: rear end ratio

Sounds like a good program, I guess my guessing per my car was close!...Craig

[Duke W.]

Re: rear end ratio

Revs at 60 = axle ratio times OEM tire revs per mile.

The revs per mile for 215/70R-15 and the OEM 7.75-15s are identical - 775, however, installed tire revs per mile do NOT affect the rev/speed relationship - only the accuracy of the speedometer.

775 x 3.36 = 2604

Duke

[Ray G.]

Re: rear end ratio

Thanks to all of you! I guess they are 3:36's after all!
Ray