bojangles

I am trying to figure out the gears in my rear end. Cruising at 65 mph the tac shows 2600 rpm's. Is it a 3.08, 3.36 or lower.?
Thanks

cardo0

You need to measure the tire diameter and verify both your tach and speedo are accurate. Or else put the car on jacks and place a readable mark on the driveshaft. Then you can count the number of turns plus partial turns of the driveshaft makes as you turn the rear wheel one full turn.

SWCDuke

Assuming the installed speedo gears are correct for the installed axle:

Revs @ 60 = OE tire revs/mile x axle ratio...

Rearranging: Axle ratio = revs @ 60/tire revs/mile and the relationship is linear, so if revs at 65 = 2600, revs @ 60 = 60/65(2600) = 2400.

The speedo gears are set up for 760 rev/mile tires. The data and calculations say 3.08. Verify by reading the tach at 60 to the nearest 20-25 revs and do the calculation.

Different than OE tire revs/mile does not affect the speedo-tach relationship. It just affects the accuracy of the speedo.

Duke

buns

Calculator here:


https://www.summitracing.com/experta...tio-calculator

cardo0

I hope to someday observe at stock mechanical tach that doesn't drag and lag.

-sinner-

It is pretty easy to get the rear of the car up on stands, mark the driveshaft and tires, then turn the tire ONE full turn and count the number of times the driveshaft turns. You are looking for the ratio of turns of the driveshaft to turns of the pinion in your rear-end. Like 3.56 to 1 or even 4.1 to 1. These would be 3.56 gears or 4.10 gears, respectively.

Using a calculator with RPMs and speed gets pretty tricky, because you are putting the tranny's ratios and tire sizes into the mix.

bojangles

Thanks for the quick responses. I'll put it up on jacks this weekend.

bcwaller

If you have side pipes, be careful that the tire is not less than a finger width from the exhaust tip when up on jack stands. If you don't pay attention, you can make a nasty slice with dirty metal while turning the tire and looking at the driveshaft.

Voice of experience here from checking my ratio when I bought my first Corvette. It's been about 30 years and the scar is still black with carbon from the exhaust!

bluzrocker

here is a handy guide to your gear on the rear
http://www.angelfire.com/fl/procrastination/rear.html

be sure and hit the COMPUTE tab for results...and try and get the tire size
27" 27.5"etc it's tire height there looking for.

65GGvert

My 65 with stock size tires and a 3.08 rear turned just over 2600 rpm's at 70 mph. Verified tach and speedo.

ejboyd5

If using the tire spin method, you will improve the accuracy of your findings by increasing the number of wheel rotations. Ten spins is ideal if you can find anyone patient enough to count that many drive shaft turns.