Determining rear axle ratio
I wanted to know what my ‘68 Tempest had for a rear axle ratio, I’m thinking about using it for commuting and am wondering what I could do to improve mileage. First I jacked up one side of the rear axle (if you have positraction you’ll need both wheels off the ground), then put a chalk mark on the drive shaft and the differential.
Then I put a mark on the tire where it lined up with a crack in pavement.
Next you just rotate the tire, and count how many times the driveshaft rotates and the tire rotates until both marks line up similtaneously. It takes a lot of turns, I kept tally on the sidewalk with a piece of chalk.
It took 32 turns of the driveshaft and 25 turns of the wheel before the marks lined up again. 32/25=1.28, but since only one wheel was turning I have to multiply by 2 to get 2.56 for the actual ratio. I guess I’ll have to look elsewhere for gas mileage improvements.
What are you looking at in that tempest, a straight 6 or v-8?
With a 2.56 gear ratio (and remember, tire/rim size affects this as well) if you build an engine with a shallower power curve, you could get really great mileage. I had a 71 Chevelle with a 2.73 rear gear ratio, threw on a 2500 rpm stall converter, a cam with decent lobe separation and a set of heads with 74cc compression chambers.
With a Carter AFB and a stealth intake, I got 26 MPG from a 4000 lb car with a v-8.
Comment by Huckleberry — July 17, 2007 @ 4:36 pm
Here’s the right way to do this:
Jack one of your rear tires off the road.
Turn that tire one full turn and see how many times your driveshaft turns.
That’s your rearend gear ratio.
Comment by DaVe — September 14, 2007 @ 12:27 am