Thread: another gear ratio ?

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I need to change gears in the chunk.

My 69 chevelle nomad has a 700 R4 overdrive, and I can't really use overdrive under 70 mph. Also, the current gears are horrible for towing. I don't have a tach, sorry.

A quick conversation today yielded that "most" guys whom upgrade to a 700 R4 go with a 3.73

I'm getting new wheels, to eliminate the rub issue on the rear. The new wheels will be 8" rims all around. This will allow me to rotate tires front to back, and I can also drop the lift, improving pinion angle. Also, it means the tire diameter will be about 26". ( I lifted it because of wheel rub )

To get new gears, I must identify the rear end. It's the number of bolts on the gear carrier, right? While I have the cover off, I could also count teeth I suppose, to determine exactly what I have now.

So: recommendations on new gears, and is there anything else I need to find out to order them?

Yes, the number of bolts dictates whether it is a 10 or 12 bolt. The year of the rear end could dictate a different ring gear size as well. The diff cover shape will be a good clue on what it is. I've had really good luck with yukon gears as well as richmond. Here's a link to Randy's that might help you out. It says your car has a 12 bolt or a 8.2" 10 bolt if your rear differential is stock to that car.

Diff Wizard

With a 26" tall tire and 3.73 gears you'll turn 2362rpm at 70mph in OD, which is a little bit high for my taste, but may be OK if you're using it for towing. Going up to a 3.55 only drops you to 2250. Those short tires really boost the rpm's. With the 3.06 to 1 first gear I wouldn't think that towing would be a big problem, provided you have the HP & torque you need.

You can play with the numbers:
Cruise RPM = (mph x Gear Ratio x 336 x OD Ratio)/Tire Diameter in inches

This lets you manipulate the formula to solve for what you want. For example, knowing that your OD ratio is 0.70 in the 700r4 you can simplify to:
Cruise RPM = (mph x Gear Ratio x 235.2)/Tire Diameter in inches, and then if you want to solve for speed at a certain RPM it's simply:

MPH = (RPM x Tire Diameter)/(Gear Ratio x 235.2)

The Yukon site is wonderful, it IDs my diff exactly.

Now to decide between the 3.5 and 3.7

Originally Posted by firebird77clone

The Yukon site is wonderful, it IDs my diff exactly.

Now to decide between the 3.5 and 3.7

I'd suggest that before you go much further you get closer than

Originally Posted by firebird77clone

....it means the tire diameter will be about 26".

It is the CIRCUMFERENCE of the tire that runs down the road, not the diameter or loaded radius. When you mount your new tires, air them to spec, then use a cloth or fiberglas tape to find the circumference of the tire, which you can then convert mathematically to the diameter to use in the formulas that you find online. D=C/pi.

You'll want to watch your cruise rpm's if you have anything but a stock cam in the motor. If you get the R's too low on cruise with an aftermarket cam, the motor won't like it much, running at an inefficient rpm. Power and mileage will suffer.

Here are a few examples offered by Crane Cams....
192/204 1600-2200 minimum cruise rpm's
204/216 2200-2600 " " "
210/216 2400-2800 " " "
218/230 2600-3000 " " "


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Originally Posted by techinspector1

It is the CIRCUMFERENCE of the tire that runs down the road, not the diameter or loaded radius. When you mount your new tires, air them to spec, then use a cloth or fiberglas tape to find the circumference of the tire, which you can then convert mathematically to the diameter to use in the formulas that you find online. D=C/pi.

100% correct on the circumference point, Tech. I always use the published diameter spec for the tire I'm running, which is a calculated value at recommended inflation as opposed to measured, but it's probably better to say:

Cruise RPM = (mph x Gear Ratio x 107 x OD Ratio)/Tire Circumference in inches (at normal inflation)

The "336" fudge factor that keeps the units right was divided by Pi - the Devil's in the details......