The Stovebolt Site Forums The Shop Area General Truck Talk GEAR RATIO QUESTION

whats the easiest way to tell what gear ratio is in a rear end. i saw someone do it wiith the cover off and counted the ring gears and then the pinion,did some quick math and within a minute or so,,"its a 3:73". theres gotta be a simple way to do it..............
thanks all,Marty

compare driveshaft revs to wheel when jacked up.

The Camaro rearend I put in my '49 had the numbers stamped on the edge of the ring gear. Didn't realize that's what the numbers were until I counted the teeth!

gazim,

So are you saying that in the previous example of 3:73, there would be three rotations of the wheel to 73 rotations of the driveshaft? Or vise versa? Just curious how you get the ratio that way and not understanding...

What can I say? I'm dense.

Nope. The driveshaft would turn 3 3/4 times to one turn of the wheel. The ratio is 3.73 - 1 (almost 3.75 - 1). If you counted the teeth on the crown gear (the big one) and divided that number by the number of teeth on the pinion gear, you'd have your differential gear ratio of 3.73 - 1. Hope that helps.

You can count the driveline rotations just as the post above stated. You have to have both wheels off the ground if its a positrac. If its just a open differential where only one wheel drives or spins, then you have to jack only one side up. Mark the tire and ground or fender with chalk so you can tell one full revolution. Mark the driveshaft the same way. Turn the driveshaft till the tire goes (two) full turns and count the driveshaft turns. Since there are side gears in the differential and one wheel is heald solid on the ground the wheel rotating will go twice as far with each turn of the driveshaft. You still count the the two turns of the tire as one, so yes to your question, 1 tire to 3.73 of driveshaft will be a 3.73 to 1 gear ratio. 4.11 will be, one tire to 4.11 driveshaft. There are a lot of choices in gears ratios depending on what axle you have, 4.56, 5.13, 3.08, 3.42, 3.55, 2.76, etc.......

Of course none of this will count on a inclosed torque tube set up! Joe

As much a hassle as it sounds,you can do the same trick on a torque tube set up,only you pull the spark plugs,put the transmission in HIGH gear,and crank the engine until the wheel turns a full turn,and count how many full rotations the engine makes. Needless to say,this one's a ton easier with two people. I've also used a mirror propped up so I could see the mark on the wheel,and cranked the engine with a big ratchet and socket.
Speed

thanks Hotshoe...thats the way he did it! exactly what i was trying to confirm.

think nothing of it.

Hotshoe is correct. The only way to get the precise ratio is to count the teeth and divide.