3.31:1 ???

[EJ10]

my M20 rear has BB stamped on it so it should have 3.31:1 gears right? Had the one rear tire off today and thought just for kicks I would spin it and count the drive shaft rotations. With one full rotation of the axle I got roughly 1.75 turns of drive shaft. I thought that was how you could check rear ratio, or am I supposed to double it? Or should I go back to school and learn how to count!

[Brown Bear]

Well, you're right on the BB, that does indicate 3.31. To count, it's probably best to have someone under it to count the driveshaft revolutions. I like to make a mark with chalk so it's easy to read, reference it to a position on the housing or somewhere. Lift the tire up, and start spinning, another reference mark on the tire helps as well. Start turning the tire and counting until your helper sees you getting to 1 full rotation. How in the heck you got 1:1.75, I have no idea....

[Crazy_Jeepman]

you will get a bad count spinning a tire with one on the ground, as the driveshaft rev's are getting divided by the spider gears. Double is close. Tag on D44 front will give you the ratio as well

[EJ10]

would it make a difference if I only had one tire up?

[Al Johnson]

You need to turn two revs of the wheel and count revs of the driveshaft, if the other wheel is on the ground. This is due to the effect of the spider gears doing their thing, "differentiating" when one wheel is not turning. If you were able to turn both wheels exactly the same, then you would count the driveshaft revs for one rev of both wheels.

[Crazy_Jeepman]

no diferance with one or both. When both are in the air one will turn backwards anyhow and the driveshaft may not turn at all. Closest you can get is to have both wheels on the air, and turn the driveshaft one turn and count the rev's of the wheel. Sometimes this is dead on and other times it is off, depends if both wheels have the same resistance and the spider gears are not differentiating power from one wheel to the other.

[EJ10]

I had a feeling that might be the case. Boy you guys respond fast! Thank you all.

E.

[joe]

The double letter codes means it also came with the tracloc option. B = 3.31 gears open diff, BB = 3.31's w/tracloc. If you can just spin one rear wheel with the other on the ground the tracloc isn't working anymore though. Not a big surprise there since they don't last long anyway.
And as everyone above said you do need to double the shaft revs unless you have a working LSD or locker in which case you would need to lift both rear wheels to be able to turn one wheel and then count shaft revs directly.