I have a 79 vette with a 3-speed automatic. 235/60 x 15 tires. at 65 mph@2,800 rpm, 70mph@3,100 rpms. Can anyone tell me whats my rear ratio.
Thanks

 

First, you can't be running exactly 2800 at 65 and exactly 3100 at 70, the resulting ratios of RPM to speed are close but not exact (if you're running exactly 2800 at 65 you're doing 3015.4 RPM at 70). Its hard to read a tach properly and they aren't perfectly accurate anyway.

Regardless, I expect you have a 3.23 axle ratio. Using a 25 inch diamter tire (about right for that size), 3100 RPM at 70 computes to a 3.294 ratio, and 2800 at 65 computes to 3.203, neither of which Chevy offers, but which bracket the ratio 3.23:1, which was a popular ratio for the automatic in the Corvette.

 

Yeah, I did the math and get the same #s, most likely a 3.23.

 

I checked one of my books, the 79 w/auto was standard w/355's, 336 and 370 were the only optional gears.

 

Then, he has a 3.36.

60 mph = 88 ft/sec.

Thus 65 = 65/.60x88 =95.33 ft/sec
x60 = 5720 ft/minute

A 235/60x15 tire is 26.1 inchs in diameter (I got this buy going to Tirerack website and checking "specs" on tires of that size. The first five I checked in this size, Pirelli, BFG, Goodyear, Firestone and Bridgestone, were all this diamter).

26.1 inchs = 2.175 feet

which means circumference around the tire is 3.14159x2.175 ft ==6.833 ft.
So, at 65 mph, the tires and rear axles are turning at 5720ft/6.833ft=837.15 RPM

He says the tach reads 2800 RPM ==> 2800RPM/837.15RPM=>3.34

 

Based on your math I'd say he's got the optional 3:36 rear.

 

You can't be sure, until you know if the tach and speedo are really right, but its hard to believe they are so far off that that isn't it.

 

There's also the possibility the gearing has been changed sometime in the past. I'd still have to say it's better than even oddsthat it's a 3:36-good math by the way-not to mention tire circumference varies by brand and wear.