TECH: What gears are in your rearend?

This will be my second contruibution to Tech Week. Some of you may have already seen my first exciting contribution, Driveshafts, How Long?

Sorry I can't show you how to chop a top or paint flames or something incredibly cool like that. But when you're building a ride, some of this boring mechanical stuff might just help ya out a bit.

This is a question that I see asked fairly often on the HAMB. A lot of folks want to know how to tell what rear gear is in their car, but they aren't sure how to figure it out. Using the following method you can tell what you have without removing the rear cover or dropping the pumpkin. Even most owners of cars with an early banjo rear can find their gear ratio without too much fuss.

You can find out what's out back without too much trouble. All you need is the ability to jack up the rear end of your car, and knowing how to count helps too.

If you have a car with a posi or limited-slip rear, jack up the car and get both rear tires off the ground. Get the car up high enough so that you can get your hands on the driveshaft to turn it. Make a mark across the pinion yoke and onto the rear end with chalk or a grease pencil. Do the same to the tire on the car, so you can see when the tire has rotated exactly one turn.

Now count the number of turns of the driveshaft that it takes (use your chalk marks) to make the rear tire rotate ONE time exactly. If it takes about 3 and a quarter turns of the shaft, you have 3.23 or 3.25 gears. A bit more than 3, you have 3.08s. Very close to 3 and three-quarter turns, 3.70 or 3.73 gears. This method won't work very well on a posi that's worn-out, one that won't spin both tires equally.

If your car has a one-legger open rear, jack up only one rear tire and leave the other on the ground. Turn the driveshaft and again count the number of turns, but this time you must make the tire do TWO full rotations. This is due to the 2:1 ratio of the differential spider gears.

If you have a car with a closed-drive torque tube and a manual trans, put the trans in high gear, assuming your high gear is a straight-through 1:1 like most 3 and 4-speed trannys. Then turn your crank with a wrench on the pulley nut, or remove the spark plugs and turn the pulley by hand if there isn't any nut (early SBC is one). Again count the turns of the crank and use whatever method applies to the type of rear, posi or open. Sorry, folks with auto-trans torque tube cars are kind of out of luck on this one.

This method is very useful when doing boneyard or swap meet rear end shopping. Never again will you get ripped off by the guy supposedly selling you that 4.11 rear that turns out to have 2.73s when you finally get it apart to check it at home.

Yeah, you might not be able to tell EXACTLY what gear you have, but you'll be VERY close. If you have knowledge of what ratios are available for your particular rear, you'll know for sure. It's handy to be able to figure what gears you have without tearing anything apart.

 

Nicely done.

 

Or bug Rocky......or the other Pontiac gurus on the HAMB.

This is really cool, I've always wanted to know the secret, but didn't want to lose any cool points with the in crowd.
xxx
Brandy

 

Baaaack to the top for the weekend crowd.