Rear end ratio

[sincro]

How can I determine what ratio rear end I have in my 65 coupe with a M-21 4 speed transmission? I just drove it on the first road trip and the engine ran at 2800 RPM to push the car at 56 MPH. I plan to use the car for touring. I am not interested in racing but I don't want a mouse either. I can't find any tags on the rear end and haven't been able to find any stampings either. Where do I look to find the stampings?

Second question, If I change the ring gear and pinion, what ratio should I install to replace what I have? 308 or 273?

 

[BarryK]

Jack up the rear so both rear wheels are off the ground. Rotate the tire one full revolution for posi rear and or and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

What rear gears you want are best based on how you plan on enjoying the car more and what motor you have in it. If you do a lot of long highway cruises a 3.08 or 3.36 would be good choices. If you do more around-town driving, or like more power off the line a lower gear such as a 3.55, 3.73, or even a 4.11 are better.
the trade-offs are that a lower gear (higher numerical number) results in higher cruising rpm's so highway driving is less pleasant. A taller gearing (lower numerical number) giver lower rpm's for highway cruising but is harder to get the car off the line and requires more clutch feathering to get started.
Everything is a trade-off.

As an example, my '65 L76 (327/365hp) has the 30-30 cam which makes a lot of power but the power range is from 3500 on up. Below that the torque is very low. To compensate my car has 4.11 rear gears to help it get off the line better but the trade-off is that during highway cruising at 60mph I'm at almost 4,000rpm. The sidepipes are screaming loud and everything in the interior is vibrating. Not the best highway cruiser car but around town it's a blast to drive.

 

[Tom Bryant]

Also give this calculator a try. You will find it and other calculators in the toolbox in the Tech Center here on the CAC. Check the top header of this page. All you have to do is enter 3 of the 4 variables and it will calculate the fourth for you. For example: enter your speed, rpm and tire diameter and it will calculate the rear gear ratio.

Rear End ratio Calculator

Tom

 

[BarryK]

geez Tom, a calculator? I suppose if you want to make it easy for people that would work.......

 

[sincro]

The calculator tells me that I have a 3.914 : 1 ratio rear end. I have the following data points that I plugged into the calculator. 2800 RPM, 56 MPH, tires 205 X 70 - 15 @ 26.3" diameter.(accuracy of the tachometer or speedometer is unknown but I will verify both tomorrow)

I have a 300 HP engine in the car with the M-21 transmission with the following gear ratios 1st.-2.56, 2nd.-1.91, 3rd-1.48, 4th-1.00.

I will jack it up tomorrow and rotate the tires and count the turns of the drive shaft. Doing 2 revolutions sounds like a good idea. I will try it that way and post what I find. Thanks

 

[Tom Bryant]

I keep off the concrete as much as possible anymore. :L

I wonder if your tach or speedo is off a little. Barry, what do you turn at 55 mph?

Tom

 

[BarryK]

Tom
don't remember and I only drove the car once this year back in March taking it to the paint shop where it's still at.
I know at 60mph I'm at almost 4,000rpm. Probably closer to maybe 3700rpm but I don't know what I'm turning at 55mph, I'd guess between 3200-3400rpm maybe??