Front to rear gear ratios...What is maximum variance allowed?

[RSchwarzli]

Looking for guidance. I am aware front to rear gear ratios do not have to be 100% the same. Many new fords even come with a slight difference in ratio (example 4.10 to 4.11).

The 90 has 3.08 axles. I want to stick a Dana 80 into the truck, but dont want to lose my lovely highway gears. And as the best highway gear I can get for an 80 is 3.31.....

Talking to Randys Ring and Pinion they say 4% is acceptable difference on off road non pavement applications. The internet in general seems to feel the same.

So the tire I run currently is a 265/70/17 aka a 31.6" tall tire.
A 285/70/17 is 32.7" tall.

My proposed theory I need someone with experience to verify:

If I run a 3.08 front on 265/70/17, and run 285/70/17 on the 3.31 Dana 80.... in theory that means via on line calculators that at 2000 rpm in OD, that would be 84 mph front, and 81 mph rear. Never would be that fast in 4wd so thats kinda pointless calculation. Worded better is the at that speed and rpm the front covers 3769 ft/min while the rear does 3618. Thats 4.01% difference.

Anyone really good with these numbers and can verify or kill this idea? I feel that I am in the 4% and for the one emergency situation I may actually use 4wd I would likely be fine. But I want to ensure my bad day when I need 4wd doesnt turn into a worse day when I rip the 205 out of the truck for mismatched gears. lol

Thanks.

Robert

 

[Mark31]

You might get away with it in slush and mush or with a light weight sled. I envision you busting something with that Cummins sitting on a front axle nailing it to the ground.

 

[Hoefler]

Your difference would be more like 7%. It is all about drive shaft rotations. In 2000 revolutions of the driveshaft, the 3.08 would have 650 axle revolutions, the 3.31 would have 604 axle revolutions. If you run the 31.6 tall tires with the 3.08, you will travel 2112' with 2000 driveshaft revolutions, while the 32.7 tall tires with the 3.31 will travel 2013' with the same revolutions of the driveshaft. That is a big difference, something has to give. The farther you travel, the greater the difference. Most likely will grenade the transfer case, twist a driveshaft, break an axle or ujoint, etc..

 

[Jim W]

My question is why do you want to do this? I have been around powertrains on and off my entire engineering career at Cat. We never would put in different sets of differential gears in the powertrain. They always matched in gear sizes front to back.

I also do not see a cost advantage either if you are purchasing new gear sets for your truck. Why go down that rabbit hole and spend the money for something that may not work out and you than end up rebuild something that grenades and changing out the mismatched gears

 

[RSchwarzli]

Hoefler - Hmmm. That math is different than mine. So it makes sense then that it is about driveshaft rotations. But in theory I could still compensate with tire sizes no?

Jim - The Dana 70 on the 90 is not liking my 1200 ft lbs of torque mixed with my 15,000 lb trailers. Now add a stick trans. I am playing in Dana 80 territory. So I want to swap the back to a 80, but dont want to give up the fuel economy and speed capability of my 3.07s. So I am trying to find a way of sticking the 80 in and not losing the good aspects of gears.

 

[Nigel]

I thought the crew cab was built to tow and the 90 was to be your daily.

What do I know?
Nigel

 

[Hoefler]

Those numbers are for static diameter, loaded or rolling diameter is signifacantly different. Tire pressure, temperature, tread design, etc will have to be considered as well.

 

[AH64ID]

Can you increase the strength of the Dana 70 and keep your ratios.

 

[RSchwarzli]

I thought the crew cab was built to tow and the 90 was to be your daily.

What do I know?
Nigel

Bahahahaha. Of course you would be the one to call me out on this! Well, ahem, see, while that is true, the crew cab sorta, well, became a bit of a trailer queen and I need to tow it to the shows...

Those numbers are for static diameter, loaded or rolling diameter is signifacantly different. Tire pressure, temperature, tread design, etc will have to be considered as well.

Valid point. So having done some more math, if I ran a 245/75/17 at a 30.5 diameter, then I would be close. Really close.

Can you increase the strength of the Dana 70 and keep your ratios.

I dont think I can. I sheared the carrier in half. lol Not sure that I can increase strength much more.

 

[Nigel]

I dont think I can. I sheared the carrier in half. lol Not sure that I can increase strength much more.

Ouch

 

[RWherley]

A friend of mine had miss matched gears between front and rear in his truck after swapping rear axles on his truck, he tried using 4wd in a emergency in a muddy hay field, while he didnt hurt the truck he said the 2 ends fighting each other made the 4x4 almost worthless.

 

[RSchwarzli]

Ouch

It was ouch!! lol Bit crunch too!

A friend of mine had miss matched gears between front and rear in his truck after swapping rear axles on his truck, he tried using 4wd in a emergency in a muddy hay field, while he didnt hurt the truck he said the 2 ends fighting each other made the 4x4 almost worthless.

Lovely. How far was he out? Ratio wise.

 

[RWherley]

I think he was 373 in one end and 410 in the other

 

[RWherley]

Just another thought, could you put a standard D60 ring gear carrier up front and change the front down to 3:31 or what ever to match the rear?

 

[SRay]

Easy fix. Tape measure the height of tires from the ground to center.

Deflate the front tires down. Calculate the ratios as if the tires are smaller. Even though the tire size hasn’t changed.

Or inflate the back tires until it looks like a balloon.

fixed.

If we watch the sled pullers pull, all the weight on the front really pushes the front tires down. We will then see the back tires kicking dirt as it’s way up with no load on.

 

[RSchwarzli]

Just another thought, could you put a standard D60 ring gear carrier up front and change the front down to 3:31 or what ever to match the rear?

I can. But the issue is available ratios. Front ratios jump from 3.08 to 3.55. Thats the issue.

Easy fix. Tape measure the height of tires from the ground to center.

Deflate the front tires down. Calculate the ratios as if the tires are smaller. Even though the tire size hasn’t changed.

Or inflate the back tires until it looks like a balloon.

fixed.

If we watch the sled pullers pull, all the weight on the front really pushes the front tires down. We will then see the back tires kicking dirt as it’s way up with no load on.

Haha. Yes. It will involve some dicking around with that. But I need to be closer to start with. Sled pullers at least have same gears from to back so less of a delta.

 

[SRay]

Another easy fix.... who needs 4 wheel drive? We live in Canada where it snows and freezes. Just remove the drive shaft.

 

[NIsaacs]

Looking for guidance. I am aware front to rear gear ratios do not have to be 100% the same. Many new fords even come with a slight difference in ratio (example 4.10 to 4.11).

The 90 has 3.08 axles. I want to stick a Dana 80 into the truck, but dont want to lose my lovely highway gears.

With the stock ratio of 3.55 for the D-80 I don't think you will miss 3.07's. Maybe go up a tire size or two. Generally you gain weight capacity too.

 

[RSchwarzli]

With the stock ratio of 3.55 for the D-80 I don't think you will miss 3.07's. Maybe go up a tire size or two. Generally you gain weight capacity too.

Oh for sure. Thats why the original post I did said 3.31. Not a fan of 3.55. I know I will miss it. But how do I make it work......

 

[Ozymandias]

Put in an ARB Carrier.
Way more rigid then the sticker.