TimmyG Posted August 10, 2019 Report Share Posted August 10, 2019 Trying to figure out what my rear gear ratio is on this 84 720 KC 2wd (11/83). The engine bay ID plate inside of the pass fender is missing. Besides jacking it up, marking the driveshaft and tire, and counting the turns...how else can I figure out the gear ratio? Any markings on the axle housing, etc? Quote Link to comment
wayno Posted August 10, 2019 Report Share Posted August 10, 2019 (edited) There is no other way but removing the drop out pumpkin/gearset and reading the numbers on the ring gear. Block the front tires so it cannot roll away, lift one rear tire off the ground, release the e-brake, put it in neutral, put the valve stem at the top, mark the drive line with a bright colored chalk, then turn the lifted tire 2 complete turns and count how many times the drive line turns. If it has a limited slip gearset, then both tires will need to be lifted off the ground and you would turn the tire one complete turn and count how many times the drive line turns. Four and one third times are 4.37s, slightly more than four times would be 4.1s, and so on. Edited August 10, 2019 by wayno Quote Link to comment
TimmyG Posted August 10, 2019 Author Report Share Posted August 10, 2019 21 minutes ago, wayno said: There is no other way but removing the drop out pumpkin/gearset and reading the numbers on the ring gear. Block the front tires so it cannot roll away, lift one rear tire off the ground, release the e-brake, put it in neutral, put the valve stem at the top, mark the drive line with a bright colored chalk, then turn the lifted tire 2 complete turns and count how many times the drive line turns. If it has a limited slip gearset, then both tires will need to be lifted off the ground and you would turn the tire one complete turn and count how many times the drive line turns. Four and one third times are 4.37s, slightly more than four times would be 4.1s, and so on. 2 full turns for the open diff with one tire up? If both tires were off the ground would this be the same? I'm familiar with diffs, but I guess this isn't something I remember. Quote Link to comment
wayno Posted August 10, 2019 Report Share Posted August 10, 2019 Sadly, if you lift both tires off the ground and the other side moves it messes things up, normally if the brake drums didn't drag at all the other side would turn the opposite way and the drive line would not turn at all. If you have both sides off the ground have someone hold the other side so it cannot move, what counts is that the other side don't move. Quote Link to comment
TimmyG Posted August 10, 2019 Author Report Share Posted August 10, 2019 7 minutes ago, wayno said: Sadly, if you lift both tires off the ground and the other side moves it messes things up, normally if the brake drums didn't drag at all the other side would turn the opposite way and the drive line would not turn at all. If you have both sides off the ground have someone hold the other side so it cannot move, what counts is that the other side don't move. Yep now that makes sense. Its been a while. Lol. So based on your method with one tire off the ground and open diff, I came up with about 3.75 axle ratio. Quote Link to comment
wayno Posted August 10, 2019 Report Share Posted August 10, 2019 That would be 3.70 gears, that is a common gear ratio for the 2wd 720 in the mid 80s. 1 Quote Link to comment
G-Duax Posted August 10, 2019 Report Share Posted August 10, 2019 (edited) Both tires off the ground (on jack stands), turn the drive shaft, and unless you have one brake dragging, both wheels will turn the same. And a white paint pen is your friend. Or buy a LSD.... No, wait, that just makes adjusting the rear brakes a pain then........ Edited August 10, 2019 by G-Duax 1 Quote Link to comment
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