Mr Fufu

Anyone know how I can determine the rear axle ratio installed in my '67?

All I can tell you is that in high gear at the following speeds my car's tach shows the following engine speeds:

60mph - 2250rpm
70mph - 2750rpm

Can I tell my axle ratio from this information?

The car is running 215/70R15 Bridgestone radial tires.

Kerrmudgeon

You can tell what the rear end carrier WAS from the factory by the markings on the housing, BUT, if anyone has changed out the gear over the years the only way to correctly get the ratio is to count how many times the driveshaft rotates in one revolution of the tire, 3.08, 3.36, 3.55 etc, right up to 4.56. That way also takes into account the height/series of your tire, also important in actual ratio.

65GGvert

It's most likely a 3:08. Certainly no more than 3:36. I have 205-75 tires with a 3:08 and I turn 2650 at 70 mph in 4th gear. The estimate above for 3:70 or 3:91 would be closer to 3700 rpm

65GGvert

Plug your numbers in here.

http://www.coastdriveline.com/htmlfo...lculator2.html

Kerrmudgeon

I used to run 4.11s years ago and the speed was roughly half of what the tach read, 60 was 3K, 70 was 3500, etc. so YA, I misread the opening post, thought the 2 was a 3. Oops!

BobT59

I currently run 4:11 in my '59 and have the same readings as you posted....

Larry21556

If your tires are 26 inches in diameter you are solidly at 3.08

65GGvert

I looked up your tire height and completed the calculator. Given a small tolerance for tire wear, tach accuracy and speedometer accuracy, I'm saying you run a 3:08, just as I do.

Mr Fufu

[QUOTE=65GGvert;1589487202]I looked up your tire height and completed the calculator. Given a small tolerance for tire wear, tach accuracy and speedometer accuracy, I'm saying you run a 3:08, just as I do.

Thanks for checking this out for me, 65GGvert!

I recently bought my car and the seller wasn't sure what the rear ratio was. So it's nice to know.

Now I wonder what difference it would make for me to switch to a 3.36 or 3.55 ratio?

I'm running a 300hp 327 with Powerglide.

She takes off pretty good off the line, and I suppose the torque multiplication of the torque converter helps here. If I went up on the rear ratio, what trade-off would I experience?

Just curious...

ghostrider20

[QUOTE=Mr Fufu;1589492920]What type of driving do you mostly do? Around town, short drives in the country? Then I would go with 3:73

Mostly highway, touring, daily driver, 3:08 -3:36

Mostly in town, out for dinner, cruise night, then 4:11

Your fun factor in town with the 4:11 will transform your car.

65GGvert

[QUOTE=Mr Fufu;1589492920]If it "takes off pretty good from the line", you probably should leave it alone. You've only got one more gear. If you go to lower gearing, you'll turn more revs at all speeds, less gas mileage, more wear, etc. If you're not happy with the take-off, you can go with more, but I definitely wouldn't want 4:11 in a powerglide.

Solid Axle Guy

I agree. If you are happy with "takeoff," leave it alone.

Railroadman

As said, the trade-off is more kick in the rear when you plant your foot, but lower top end and lower gas mileage. If you want to race it, or burn a lot of rubber, 3:08's are the worst choice. If you do a lot of cruising and are happy with the low-end "zip" now, they are the best option. And you DO have several options on between your 3:08's and the 4:11's someone suggested.

JohnZ

3.36 was standard with the 300hp Powerglide, with 3.08 as the "Economy" option; no other ratios were available. There were good Engineering reasons for that.

427435

A 2750-2800 rpm tach reading gave a 70 mph speed in my 3:36 factory equipped car.

Mr Fufu

Good info here JohnZ.

So, theoretically, what difference might I notice from going up to 3.36 from 3.08. Would the improvement in acceleration be noticeable?

What would have been the reason they didn't offer higher axle ratios with Powerglide? Is it something to do with the extra torque mutiplication afforded by the automatic?

JohnZ

The difference between 3.08 and 3.36 wouldn't be noticeable, either in apparent performance or fuel economy. The Powerglide transmission (in production form) wasn't designed for regular WOT high-rpm shifting - it was a "cruiser" powertrain.