According to the door plate, my 66 F100 came with a 3.73. I suspect that the rear axle, like the engine, has been changed. I tried jacking up the back, rotating the tire, and counting the number of times that the driveshaft turns, but the ratio I get that way is 1.37:1 and obviously wrong.
Is this method not reliable? If not, how much trouble is it to pull the front plate for a look at the ring and pinion? The thing looks heavy.
Find gear ratio?
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DanSanDiego2000
- Posts: 275
- Joined: August 14, 2006, 8:59 pm
- Location: San Diego
Re: Find gear ratio?
Jack up only ONE tire, rotate the wheel, and have someone count the driveshaft rotations while you count the wheel rotations. When they get to 10 driveshaft rotations, stop counting the wheel rotations.jfm wrote:According to the door plate, my 66 F100 came with a 3.73. I suspect that the rear axle, like the engine, has been changed. I tried jacking up the back, rotating the tire, and counting the number of times that the driveshaft turns, but the ratio I get that way is 1.37:1 and obviously wrong.
Is this method not reliable? If not, how much trouble is it to pull the front plate for a look at the ring and pinion? The thing looks heavy.
You then must DOUBLE the tire rotation count, and that gives you the ratio.
This only works with an open differential. If it is a locking diff, you will need to jack up both wheels, and NOT double the final number. With an open diff, the ring and pinion and the spiders react differently, and only 50% of true ratio is observed.
Assuming an open differential, for every 10 driveshaft rotations, you might count 17-1/2 tire rotations, or 15, or 20.
17-1/2 x 2 = 35, or 3.50 ratio
15 x 2 = 30, or 3.0 ratio
Etc.
I have tried this with a locking differential (without doubling the final count), and it's difficult to get an accurate idea.
Re: Find gear ratio?
Thanks for the input, but this is not clicking for me. The driveshaft is supposed to rotate faster than the tire; otherwise at, say, 2500 engine rpms, the tire would be rotating more than 2500 rpms, or, in other words, faster than the engine. Not the way it works.DanSanDiego2000 wrote:Jack up only ONE tire, rotate the wheel, and have someone count the driveshaft rotations while you count the wheel rotations. When they get to 10 driveshaft rotations, stop counting the wheel rotations.
You then must DOUBLE the tire rotation count, and that gives you the ratio.
This only works with an open differential. If it is a locking diff, you will need to jack up both wheels, and NOT double the final number. With an open diff, the ring and pinion and the spiders react differently, and only 50% of true ratio is observed.
Assuming an open differential, for every 10 driveshaft rotations, you might count 17-1/2 tire rotations, or 15, or 20.
17-1/2 x 2 = 35, or 3.50 ratio
15 x 2 = 30, or 3.0 ratio
Etc.
I have tried this with a locking differential (without doubling the final count), and it's difficult to get an accurate idea.
The way I did it was to rotate the tire 10 turns while counting the driveshaft revs. That's the way some guy on Youtube did it. He got 44 revs of the driveshaft for 10 revs of the tire, for a 4.40 ratio. He didn't double anything. If he had, it would have been a ratio of 8.80 - obviously wrong. FYI, he had both wheels elevated, as did I.
Still confused.
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DanSanDiego2000
- Posts: 275
- Joined: August 14, 2006, 8:59 pm
- Location: San Diego
Silly me.
I had it backwards.
Jack up one wheel, and leave transmission in neutral.
Have someone turn the wheel 10 full rotations forward, slowly, as you count driveshaft rotations.
You are looking for something like 4.10:1 or 3.73:1, so it makes sense to turn the wheel 10 times for a more accurate reading.
If you cannot turn the wheel, it's because you have a locking diff. If the wheel turns, turn wheel 10 times, count the driveshaft rotations multiply by 2.
If you cannot turn wheel, you'll have to jack up both wheels, rotate the wheels 10 times, and count driveshaft rotations. 41 driveshaft rotations = 4.10 ratio, 35 rotations = 3.50 ratio.
Reading the open diff ratios has always been accurate for me, but I have not had much success reading the ratio on a locking diff. It might have to do with the centrifugal nature of some diffs, or the clutches or torque.
If I had to do a locking diff, I would likely have someone rotate the second wheel at the same time, rotate both forward 10 turns, and read the driveshaft rotations.
I had it backwards.
Jack up one wheel, and leave transmission in neutral.
Have someone turn the wheel 10 full rotations forward, slowly, as you count driveshaft rotations.
You are looking for something like 4.10:1 or 3.73:1, so it makes sense to turn the wheel 10 times for a more accurate reading.
If you cannot turn the wheel, it's because you have a locking diff. If the wheel turns, turn wheel 10 times, count the driveshaft rotations multiply by 2.
If you cannot turn wheel, you'll have to jack up both wheels, rotate the wheels 10 times, and count driveshaft rotations. 41 driveshaft rotations = 4.10 ratio, 35 rotations = 3.50 ratio.
Reading the open diff ratios has always been accurate for me, but I have not had much success reading the ratio on a locking diff. It might have to do with the centrifugal nature of some diffs, or the clutches or torque.
If I had to do a locking diff, I would likely have someone rotate the second wheel at the same time, rotate both forward 10 turns, and read the driveshaft rotations.
Okay, then, the guy in the video must have a locker, which makes sense if he is running 4.40 or higher. With an open dif, though, you would multiply by 2? So my 1.37 then turns into a 2.74? - more reasonable, but definitely not original.
And, if the ratio is < 3.00, then I probably don't need to be thinking about going to a 5 speed tranny. Something with a granny low and 1:1 on the top might be preferable.
And, if the ratio is < 3.00, then I probably don't need to be thinking about going to a 5 speed tranny. Something with a granny low and 1:1 on the top might be preferable.
If you have an open diff... get one tire off the ground, rotate the tire TWO turns while counting the driveshaft revolutions. That will be the ratio.
If locking differential... get both tires off of the ground and turn the axle ONE turn while counting the driveshaft revolutions. That will be the ratio.
It's best to mark the tire and driveshaft with tape or something so you get the rotations exact.
If locking differential... get both tires off of the ground and turn the axle ONE turn while counting the driveshaft revolutions. That will be the ratio.
It's best to mark the tire and driveshaft with tape or something so you get the rotations exact.
jerry
Is that pretty reliable? The ratio determines what kind of transmission I need to look for.jmf66 wrote:If you have an open diff... get one tire off the ground, rotate the tire TWO turns while counting the driveshaft revolutions. That will be the ratio.
If locking differential... get both tires off of the ground and turn the axle ONE turn while counting the driveshaft revolutions. That will be the ratio.
It's best to mark the tire and driveshaft with tape or something so you get the rotations exact.
I've been dealing with this stuff for 35 years. That's what the rear end ratio is... it's number of turns of the driveshaft to one turn of the axle. The reason for 2 turns of the tire is in the way that a differential works.. if it can't turn both tires, it'll turn the one at twice the rate. Give it a try an let us know what you find..
jerry
I did it with both wheels up. I'll try it again with one up and will have much more confidence in the result. I hadn't heard the 'open dif vs locker' procedures before. Thanks.jmf66 wrote:I've been dealing with this stuff for 35 years. That's what the rear end ratio is... it's number of turns of the driveshaft to one turn of the axle. The reason for 2 turns of the tire is in the way that a differential works.. if it can't turn both tires, it'll turn the one at twice the rate. Give it a try an let us know what you find..
