Thread: rear end ratio

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I've been looking at transmission specs latly, and they always say "If your rear axel is"..."then at 2000 rpms your speed will be..." But I don't know what rear end ratio I have. It's on my 73 El Camino 10 bolt. It's the stock rear end and I don't think its been rebuilt or modified.
thankz
weylin-

do you have a th350? if you do, my best guess is that its a 2.73, but im not sure. i could be way off

try elcaminocentral.com for the options avalible for that year

Best way to be sure is to jack it up and count the revolutions of the drive shaft in one full revolution of rear wheels.
Example: approx. 2 and 3/4 turns of the driveshaft for every one turn of the wheels = 2:73.

Pro70z28 is right. That is the fastest way to determine your rear end ratio.

Another way to determine the ratio is to "back" your way into the number. There are only five variables to know. If you know any 4 out of 5, four then the fifth is easy to determine by simple math.

1. Transmission
2. Tire Size
3. Speed
4. RPMs
5. Rear End Ratio

So, If you know:

1. Tire diameter...(measure your tire height to determine the diameter).

2. Your transmission Gear Ratio. (If you know your transmission, we can determine the transmission gear ratio).

3. Your Speed...[If your speedometer works (or have a GPS...or someone can drive next to you at speed), then your speed is identifiable].

4. Your RPMs (no explanation needed).

We can determine your rear end ratio using the following mathmatics if you find math easier than jacking up the car and recording the rotations of the rear wheels versus the rotations of the driveshaft...

(RPM)=168 x (TR) x (MPH) / Divided by 1/2 of Tire Diameter

(MPH) = (RPM) x 1/2 of Tire Diameter / Divided by 168, then Divided by (TR)

(Ratio) = (RPM) x1/2 of Tire Diameter / Divided by 168, then Divided by (MPH)

(TR) = Rear Gear X Final Transmission Gear


I created an Excel spreadsheet that calculates this for you if you know the above input information. Let me know if you need it and I will email it to you.

Well I took the math aproach first, and I got 2.73, I looked it up to and got 2.73. So I've got a rear end I don't like lol oh well.
Thanks guys, it'll help me when looking at transmissions now

All it takes is Horsepower to overcome the tall gear. Then on top end yer' "OFF TO THE RACES"

yeah, dont feel bad, i have the optional 2.52 (it might be 2.56) in my 86, but an 8.5 swap with 3.73 isnt far off