gear ration-rpm-mph question

[Viperwolf1]

I can think of a few more things. None are worth mentioning until the OP provides some additional facts.

 

[OneTonGoldfish]

i am going to test drive it asap to confirm it is shifting to 3rd. I will get rpm/mph stats for 1st and 2nd as well. I would have done it tonight but it is about 35 deg out...not fun without top or doors. it may be a couple days..looks like rain ugh!

 

[DirtDonk]

Unimog portal ends on his 9" givin' him that extra 2-to-1 underdrive?
Just kidding. That'd be pretty cool though.
I was thinking a slipping converter too OneTon, but I'm not an automatic expert so don't know what characteristics would show. Or just how much a TC can slip.

When you get the chance, or just don't have anything more pressing, see if you can verify just what rear-end ratio you do have.
Like was said, you're not likely to find an 11 to 1 gear set, but I'm still curious to see just what you have.

Paul

 

[OneTonGoldfish]

Unimog portal ends on his 9" givin' him that extra 2-to-1 underdrive?
Just kidding. That'd be pretty cool though.
I was thinking a slipping converter too OneTon, but I'm not an automatic expert so don't know what characteristics would show. Or just how much a TC can slip.

When you get the chance, or just don't have anything more pressing, see if you can verify just what rear-end ratio you do have.
Like was said, you're not likely to find an 11 to 1 gear set, but I'm still curious to see just what you have.

Paul

i jacked up one rear tire and put the tcase in N. Then i put a peice of tape on the tire and another on the drive shaft. For every 1 rotation of the tire the drive shaft turns 2.5 rotations

 

[DirtDonk]

Sounds like your 5.13 then. Not sure why it did the normal half-turn thing though, since you only had one tire in the air, but that's what they do sometimes. Sounds like an open diff with 5.13 ratio then.
Now, I know you said they're 38" tires (that right?), but I wonder if they're super-short and worn out? Not that this would explain the huge discrepancy of course, but if the rolling radius is only, say, 17 inches, instead of the expected 18.5 or 19 (half of a roughly 38" tire), that would make up for some of it at least.

Paul

 

[OneTonGoldfish]

Sounds like your 5.13 then. Not sure why it did the normal half-turn thing though, since you only had one tire in the air, but that's what they do sometimes. Sounds like an open diff with 5.13 ratio then.
Now, I know you said they're 38" tires (that right?), but I wonder if they're super-short and worn out? Not that this would explain the huge discrepancy of course, but if the rolling radius is only, say, 17 inches, instead of the expected 18.5 or 19 (half of a roughly 38" tire), that would make up for some of it at least.

Paul

yes, open rear diff. how do you get the 5.13 gear ratio? what is the math formula?
the tires are 38.5 and are brand spanking new. under 100 miles. they still have the little whiskers on the middle of the tread...fully inflated too

 

[OneTonGoldfish]

also, according to TCI website http://www.tciauto.com/Products/TechInfo/gear_ratios.asp the C4 has a 2nd gear of 1.46 or 1.57

if my c4 was not shifting out of 2nd then my rpm at 40mph should be around 2615 (1.46) or 2812 (1.57)

so it still doesnt add up...

 

[av bronco]

5.13 is pretty low for the street, and a auto will add a couple of hundred rpms to crusing speed, but 4500 is kinda hi- possible bad tach?

 

[DirtDonk]

Yeah, he said he checked that though, so we're still huntin'.

yes, open rear diff. how do you get the 5.13 gear ratio? what is the math formula?

Not a real formula for that final .13, from a 5.0 to a 5.13. Just used some fuzzy-logic to arrive at the (expected) 5.13 ratio.

You might normally expect to directly compare the number of turns of the shaft to one turn of the wheel and get a direct readout. Well, you will get that with a spool, and usually a locker, and sometimes with a limited-slip, where rotation of the ring gear moves the piinion gear around consistently. But an open diff allows the spider gears to rotate around the side gears without turning them. When that happens you can end up getting half the number of turns on your shaft.
So, knowing you couldn't possibly have a 2.50 ratio, I assumed an open diff, doubled the 2.5 you came up with and just presumed to add the .13 because I didn't know of a 5.0 gear set.

See? Simple. And it could be completely wrong, but that's what I figured.

Paul

fuzzy-logic train of thought; first draft:
What I got from your "2.5 turns" was that you have something close to a 5.0 gear ratio. And since 5.13 is pretty darn close to 5.0, you would have to have measured your driveshaft rotations to a gnat's pituty to see it. Which I assumed you didn't bother with because you were naturally in a hurry to get it over with. So, with the assumption that you didn't get a super-precise measurement when you said "2.5 turns" and by extrapolation, I assumed that the additional .13 to get a 5.13 out of your 2.5 x 2 = 5.00 spin-test would equal the pre-expected 5.13 that the PO said it had would make it so. But only after induction of alcohol and inhalation of the essence of friction-modifier.
And all would be right with the universe.

 

[OneTonGoldfish]

Yeah, he said he checked that though, so we're still huntin'.



Not a real formula for that final .13, from a 5.0 to a 5.13. Just used some fuzzy-logic to arrive at the (expected) 5.13 ratio.

You might normally expect to directly compare the number of turns of the shaft to one turn of the wheel and get a direct readout. Well, you will get that with a spool, and usually a locker, and sometimes with a limited-slip, where rotation of the ring gear moves the piinion gear around consistently. But an open diff allows the spider gears to rotate around the side gears without turning them. When that happens you can end up getting half the number of turns on your shaft.
So, knowing you couldn't possibly have a 2.50 ratio, I assumed an open diff, doubled the 2.5 you came up with and just presumed to add the .13 because I didn't know of a 5.0 gear set.

See? Simple. And it could be completely wrong, but that's what I figured.

Paul

fuzzy-logic train of thought; first draft:
What I got from your "2.5 turns" was that you have something close to a 5.0 gear ratio. And since 5.13 is pretty darn close to 5.0, you would have to have measured your driveshaft rotations to a gnat's pituty to see it. Which I assumed you didn't bother with because you were naturally in a hurry to get it over with. So, with the assumption that you didn't get a super-precise measurement when you said "2.5 turns" and by extrapolation, I assumed that the additional .13 to get a 5.13 out of your 2.5 x 2 = 5.00 spin-test would equal the pre-expected 5.13 that the PO said it had would make it so. But only after induction of alcohol and inhalation of the essence of friction-modifier.
And all would be right with the universe.

absolutely right. glad we got to the bottom of that ;D

 

[canrel]

While you are checking rpm and mph use a common rpm.

Put it in what we hope is 2H and 1st gear run it up to 2000rpm and note the mph. put it in 2nd and again hold 2000 rpm and note the mph. Do the same for 3rd.
Then put it in 4L and 1st gear at 2000 rpm, 2nd at 2000rpm and 3rd at 2000. Note the mph in all gears, this may help sort out your troubles.

 

[OneTonGoldfish]

While you are checking rpm and mph use a common rpm.

Put it in what we hope is 2H and 1st gear run it up to 2000rpm and note the mph. put it in 2nd and again hold 2000 rpm and note the mph. Do the same for 3rd.
Then put it in 4L and 1st gear at 2000 rpm, 2nd at 2000rpm and 3rd at 2000. Note the mph in all gears, this may help sort out your troubles.

this is exactly what I plan on doing as soon as the rain and freezing weather pass