51al: 5/31/2000 - 7:00:09 AM
Anyone know how to tell what ratio rearend one might have without tearing it apart?,
Thanks Al

JimC: 5/31/2000 - 8:16:04 AM
RE: ratio
Jacking one wheel up and turning the engine over by hand will let you determine the ratio. Make a chalk mark on the bottom of the tire and crank the engine over exactly 2 revolutions. Also, make sure all the slack is out of the system when turning the engine over. Then measure the rotation on the tire. It will be plus or minus one turn, but accuracy with these measurements is critical. Take this tire rotation value, divide it into 1 (in other words, find its reciprocal), then multiply it by 4. This will give you a pretty good ball park of the ratio, which can be compared with known ratios (4.11, 3.78, 3.54) to see which one comes closest. For example, I recently went through this with my car and found the tire rotated slightly less than one turn for 2 engine turns, so I used a tire rotation value of 0.95. Dividing this value into 1.0 gives you 1.05, and multiplying 1.05 by 4 gives you 4.20. So I figured the rear end ratio was 4.11. I then used this method on a friend's car before he pulled the rear end out. I got about 1.05 tire rotations for 2 engine rotations, which figures out to a rear end ratio of 3.8. When the gears came out, sure enough, they were 3.78.

1) Jack up one rear wheel and chalk-mark the 12 o'clock position on the tire. (Make sure the slack is out of the drive train)
2) Hand crank engine 2 complete turns.
3) See where chalk mark on tire ends up.
A) Between 11 and 12 o'clock, ratio is 4.11
B) Between 12 and 1 o'clock, ratio is 3.78
C) Between 1 and 2 o'clock, ratio is 4.54

Brian G: : 5/31/2000 - 8:17:06 AM
RE: ratio
If it's not a 'posi', jack up one wheel, count the number of revolutions of the wheel to make the input shaft turn one revolution and divide by two. eg: 6 turns = 3.00:1 ratio. If it's a 'posi', Jack both wheels off ground and do the same thing. Since both wheels turn in this case, the ratio is whatever number of turns of the wheel it takes to turn the input shaft once. eg: 3 turns = 3.00:1 ratio.

rodnut: 5/31/2000 - 8:31:38 AM
RE: ratio
Don't forget to do this in HIGH (1 to 1) gear!

Mart: 5/31/2000 - 5:45:46 PM
RE: ratio
As above but read wheel, not input shaft. for wheel read engine. If you have open drive, you can turn the prop. (driveshaft) If you turn the engine till the wheel has made two full turns, the number of turns of the engine equals the ratio. As already mentioned, the gearbox must be in top. (direct)

rodnut: 6/1/2000 - 10:25:45 AM
RE: ratio
PS: Brians' formula doesn't make sense to me, or I'm missing something. I used Jim C.s' - works good!

5/31/2000 - 8:56:52 AM
Right,! High Gear
Very important detail supplied by Rodnut. Otherwise the math becomes wicked complicated!

MFOPMA: 5/31/2000 - 10:04:41 AM
RE: Right,! High Gear
Look for a metal tag covered in crap on the back cover that lists the ratio. Or is this only on the truck rear axles?

V8CHRIS: 5/31/2000 - 4:10:29 PM
RE: Right,! High Gear
On most of the Flathead rearends from 1932 to 1948 there are 2 numbers stamped on the bottom of the centersection. These represent the number of teeth on the pinion and the ring gear. Dividing one into the other gives the ratio. Hence 37 and 9 represent 4:11. This assumes of course that the gears have not been changed.

51al: 5/31/2000 - 8:30:12 PM
RE: Right,! High Gear
Thanks guys guess I should have mention this is a 51 and I have a fordomatic how do I get it high gear can I simply turn the shaft ???????

rodnut: 5/31/2000 - 8:56:49 PM
RE: Right,! High Gear
Yes, as a manual trans in high gear will turn the driveline at engine speed, just mark the driveline and go by it instead of the motor. You can work the other direction and observe how many revolutions you have to turn the tire in order to turn the driveline two revolutions - same formula....rodnut

JWL: 5/31/2000 - 10:38:33 PM
RE: Right,! High Gear
Turning the tire, with only one elevated, while counting driveshaft revolutions, will give a result you won't (and shouldn't) believe.

MFOPMA: 6/1/2000 - 9:32:30 AM
RE: Right,! High Gear
That's what I've found.

rodnut: 6/1/2000 - 10:19:46 AM
RE: Right,! High Gear
Just tried it on my '40 rear. One wheel on the ground. Turned the raised wheel to rotate the driveline two revolutions. The wheel rotated right at 1.06 revolutions. 1 divided by 1.06 = .9434 x 4 = 3.774. My ring and pinion is a 3.78! Looks like it works pretty darned good, to me!

JWL: 6/1/2000 - 11:12:48 AM
RE: Right,! High Gear
Yes, I see Jim has it right. Brian had the right idea, but didn't express it correctly. Things got turned around a little.

JimC: 6/1/2000 - 12:31:33 PM
Even MORE Simple
I checked the algebra and all you really need to do is take the tire rotation value and divide it into 4. For example, Rodnut's tire rotation value of 1.06 divided into 4 gives the answer he got (3.774). Sorry I made the formula more complicated than it needed to be -- I was thinking out what was going on with the gears and the relation between tire and engine rotation when I was figuring it out.

Mart: 6/1/2000 - 12:48:09 PM
Even simpler still
Jack up 1 wheel, mark it and the driveshaft. Turn the driveshaft until the wheel has turned exactly 2 revs. The number of turns of the shaft equals the axle ratio. No math required. It must be simple, I've done it loads of times.

Jason: 6/1/2000 - 1:53:46 PM
Open drive shaft
This would be the way to do it for the open drive shaft. But on the closed drive shaft cars the other way seems pretty good to use.

BrianG: 6/2/2000 - 7:23:51 AM
RE: open drive shaft.
Sorry, I wasn't thinking that many of your flathead cars are pre-'49. However, on cars with open driveshafts (or closed driveshaft cars with tranny in high) I stand by my method. Jack up one wheel, turn it till the driveline has made 1 revolution, divide the number of times you turned the wheel by 2 and you have your ratio. I guess we have beat this one to death!

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