Different tires sizes and rear ratio's on a AWD

I'm running a D44 front axle and Ford 9" rear axle. (For example) One axle I can get a 3.70 gear and the other I can only find a 3.70 gear. Is that a large enough difference to cause issues? (No transfer case just a gear box).

I have looked into running different tire diameter's with different gear ratio's but I can't get the numbers to match exactly. Do I have any allowance for a rpm spread? If it's ok to have 5-15rpm's I can make it work.

3.70 in both ends would be good as it gets. am I missing something?

cool40 wrote:3.70 in both ends would be good as it gets. am I missing something?

I can only find a 3.73 for the Dana 44 (reverse cut) and 3.70 for a Ford 9".

I want to run a 29x10.5 drag radial at all four corners.

3.70 @ 50mph & 29" tall tire=2,143.45rpm
3.73 @ 50mph & 29" tall tire=2,160.83rpm

That difference in rpm. I can get the numbers closer together if I use different size tires and gear ratio's.

Dana 44 & Ford 8.8 gears


Available Ford 9" gears

jones wrote:

cool40 wrote:3.70 in both ends would be good as it gets. am I missing something?

I can only find a 3.73 for the Dana 44 (reverse cut) and 3.70 for a Ford 9".

I want to run a 29x10.5 drag radial at all four corners.

Look to the OD and different manufacturers, example 3.73x29.5 = 110.035   3.7x29.7=109.89 get it as close as you can the circumference dimension will be even more accurate, but with the above example it will eat the difference from the 3.73x29.5 tire which is not much.

FalconEh wrote:

jones wrote:

cool40 wrote:3.70 in both ends would be good as it gets. am I missing something?

I can only find a 3.73 for the Dana 44 (reverse cut) and 3.70 for a Ford 9".

I want to run a 29x10.5 drag radial at all four corners.

Look to the OD and different manufacturers, example 3.73x29.5 = 110.035   3.7x29.7=109.89 get it as close as you can the circumference dimension will be even more accurate, but with the above example it will eat the difference from the 3.73x29.5 tire which is not much.

Thats what I'm wondering, how much is too much? I can get closer if I run 29" tires in the front and 31" tires in the rear with different gear ratio's. (I wouldn't worry about it if this was a mud truck but it's not.)

Looks like I will need to create a spread sheet to compare ratio's vs tire diameter's

jones wrote:

FalconEh wrote:

jones wrote:

cool40 wrote:3.70 in both ends would be good as it gets. am I missing something?

I can only find a 3.73 for the Dana 44 (reverse cut) and 3.70 for a Ford 9".

I want to run a 29x10.5 drag radial at all four corners.

Look to the OD and different manufacturers, example 3.73x29.5 = 110.035   3.7x29.7=109.89 get it as close as you can the circumference dimension will be even more accurate, but with the above example it will eat the difference from the 3.73x29.5 tire which is not much.

Thats what I'm wondering, how much is too much? I can get closer if I run 29" tires in the front and 31" tires in the rear with different gear ratio's. (I wouldn't worry about it if this was a mud truck but it's not.)

Looks like I will need to create a spread sheet to compare ratio's vs tire diameter's

No, The example I gave you was much closer ...multiply the circumference by the gear ratio if they do not list it call them. Your example is 3.73 x 29 = 108.17 and 3.7x31 = 114.7

Mine = 110.035 - 109.89 = .145 difference slightly more than a 1/4"
Yours = 114.7-108.17 = 6.53" difference

Ask for the circumference measurement the numbers will be much larger but more accurate eg 84.5" circumference x 3.73 gear = 315.185 which would be the same as 85.19 circumference x 3.70 gear (close) 315.203 for a difference of .018 or 18/1000

FWIW you could probably make up the difference between the gears with an air pressure adjustment from front to rear with the 29.5's

I thought about tire pressure but I don't think it would make a difference because the roll-out would stay the same wouldn't it? Isn't roll-out what they measure the make sure slicks will run correctly?

I understand what you are saying about the tire OD but it would be nice to run the same brand on both axles. But,,, that might be the easiest way to go.

If I stay with a Dana 44 and 8.8 it wouldn't be a problem because I can get matching gears. (basically)

jones wrote:I thought about tire pressure but I don't think it would make a difference because the roll-out would stay the same wouldn't it? Isn't roll-out what they measure the make sure slicks will run correctly?

I understand what you are saying about the tire OD but it would be nice to run the same brand on both axles. But,,, that might be the easiest way to go.

Correct, hence why i wrote a book earlier, however the air pressure adjustment will give enough sag and drag that it will not hurt the gear it will wear the tire instead, ^^^ the circumference and the roll out are the same, run the numbers and you will be where you want to be

FalconEh wrote:

jones wrote:I thought about tire pressure but I don't think it would make a difference because the roll-out would stay the same wouldn't it? Isn't roll-out what they measure the make sure slicks will run correctly?

I understand what you are saying about the tire OD but it would be nice to run the same brand on both axles. But,,, that might be the easiest way to go.

Correct, hence why i wrote a book earlier, however the air pressure adjustment will give enough sag and drag that it will not hurt the gear it will wear the tire instead, ^^^ the circumference and the roll out are the same, run the numbers and you will be where you want to be

I was asking how much difference in rpm's can I get away with before drivability and mechanical parts become dangerous? Of course I want to run them as close as possible. Say, I want to run 29.5 on the front axle and 31" on the rear and the closest possible rpm's by changing gear ratios. Is 10 rpm noticeable between front and rear? At what point in rpm difference is it noticeable.