Getting a little more serious about going to the 5 speed and was just wondering what my rear end might be. (ON the car that is)

At 50 mph the rpm is 2200
At 60 mph the rpm is 2500.

Can any one of the masters give me an idea ??

Thanks,
Roger

 

It sounds like 3:42 gear ratio. Mine spins around 3000 at 70 with 3:42 gears.

 

I'd guess 3.08.

At 70 I'm at 2800 with 3.23.

Tire size can play a part here.

I"d suggest to pull the cover. The only way to know for sure with out a doubt. Might as well change the diff fluid while your there.

 

Lift the rear tires and spin the drive shaft one rotation by hand and count the rotation of the wheel and you'll be able to tell. A chalk mark on the tire and shaft will be accurate enough to tell what you have now.

 

Tell us the rear tire size on your car.

 

Agreed. Engine RPM at a given MPH can vary significantly due to different rear tire heights.

 

Sorry I should have mentioned that. Rear tires are 15x7 255 60
I have checked speedo and it is fairly accurate.

 

The BFG site shows 27.1" tire height.
Using the Waller Racing calculator and 2500 RPM @ 60 MPH shows a 3.36:1 ratio. 2200 RPM @ 50 MPH shows a 3.55:1 ratio.

I don't think Olds ever made those ratios but Chevy did. If you have an Olds rear, my guess is 3.42:1 ratio. I found my tires were a bit shorter than what the BFG site said so the calculator didn't give exact numbers.

 

Thanks guys.

Your bang on Fun 71 when I measure my tire diameter I am a bit over 26" which calculates to 3.42

 

Olds did offer 3.55's in 65-67, and in '69, Canadian built cars, which were Chevy rears.

 

just put in a legend 5spd in my 71 442, 308 rears. it was spinning around 3600 rpm at 70 mph.
new 5spd dropped it 1000 rpm at 70. now 2600 rpm.

still comes out of hole great nice second gear rubber and third gear chirp.

love new trans, but not cheep. trans, clutch, drive shaft, droped abit over 5k
but worth it.....

 

Wow, you must have really short rear tires. My car with 3.23s and 245-60-14s (25" tall) turned 3400 RPM @ 70 MPH.

 

You're right. 3600 RPM @ 70 mph with 3.08 gears equates to 20" diameter tires. (stock is about 26.5")

Or maybe his tach is just a wee bit off ......

 

Sure you have 3.08 gears??? I would dig into the rear more...

Or like stated tach is set wrong cylinder count or something (4 6 8)

 

I calculate about 3.23...??
With all the RPMs given and it being a stick car it will have no slip in the calculations like a Auto trans would

At 50 mph the rpm is 2200
At 60 mph the rpm is 2500.

Could still be 3.08 with trans slippage factored. ALways best way is to open it up and read the ring gear edge for the numbers of the tooth count
13 -40
or 13 -42
or 12 -41

Jim
J D

 

The standard rear end ratio for a 71 442 with a TH400 is 3:23. There is a code on the passenger side axle tube, facing forward, which may be hard to see if the area is too rusty. It should be "SR" for an open 3:23. The letter after the code is the division which built it and then the day of year. If you have "anti-spin" the code is "SS". I have attached a picture of my 442's axle tube for reference purposes.

 

So ummm... I can't believe we are still trying to figure this out with online calculators and tape measures Unless you have some off the wall crazy custom gearing what you have is limited to a few choices. Spin the damn shaft once already!