Different tire diameters on front and rear

[Sevik]

Just something to think about, not yet planning to do that to my Jeep. 

 

Let's say, I decide to run 31 on the front and 33 on the back. According to the gear ratio chart (XJ in this case), different gears need to be used. 

 

31 - 3.91 (2756 rpm @ 55 mph and 4th gear)

33 - 4.11 (2721)

 

or 33/35:

33 - 4.56 (2847)

33 - 4.27 (2827)

 

The difference is mere 1.3%/0.7% in engine rpm, will this have severe effect on a transfer case? I mean, these numbers are pretty much within tire wear tolerance.

 

Some may ask "why bother?", well MJ can have a flatbed and thus accommodate bigger tires there while getting 2 inch extra height of tires on the front will cost a lot in a lift kit, ride quality etc. 

[Marine1Texas]

in 4wd you will cause the transfer case a lot of extra load on your chain. This will also effect traction issue since your rear tires will want to go faster then your front tires all the time in 4wd. if it was 2wd no issue but 4wd I would not recommend it at all.

[Sevik]

  On 5/15/2016 at 3:26 AM, Noriyori_Kudo said:

in 4wd you will cause the transfer case a lot of extra load on your chain. This will also effect traction issue since your rear tires will want to go faster then your front tires all the time in 4wd. if it was 2wd no issue but 4wd I would not recommend it at all.

Just as an example, a thread is 18.5/32" (MickeyThompson), that is 1.15", on a 35" tire it is 3.3% so if you put half worn tires on the front and brand new on the back, you will get about 1.6% diameter difference. Here I shown 0.7%, less than a half of that. 

[buckwheat]

So, just making sure I understand your arguement:

 

You would run different tires sizes between front and back. Then to accommodate the strain on the transfer case you would regear each axle to something different. In the cases you mentioned, regear it to a ratio that doesn't even exist so far as I know. Just to save yourself the money of a pair of Acos spacers?

 

Then lets say you get a flat. Are you going to carry two spare tires since you have two sizes on your truck? And none of this is factoring in the cost of building a flat bed and the associated work. I think you are looking in very much the wrong direction. Compared to what you are talking about, a lift kit is cheap.

 

Then again I am wondering if this is just a troll post. . . 

[Sevik]

  On 5/15/2016 at 4:57 AM, buckwheat said:

So, just making sure I understand your arguement:

 

You would run different tires sizes between front and back. Then to accommodate the strain on the transfer case you would regear each axle to something different. In the cases you mentioned, regear it to a ratio that doesn't even exist so far as I know. Just to save yourself the money of a pair of Acos spacers?

 

Then lets say you get a flat. Are you going to carry two spare tires since you have two sizes on your truck? And none of this is factoring in the cost of building a flat bed and the associated work. I think you are looking in very much the wrong direction. Compared to what you are talking about, a lift kit is cheap.

 

Then again I am wondering if this is just a troll post. . .

Gears exist, 4.27 for D44, rest are there for d30/35, not very common though.

 

Regarding flat: not all cars equipped with a full size spare and live with it. One 33" will do.

 

Flat bed or bedless setup is another discussion, assume it has to be done for other reasons for the sake of this topic. (In my case, box is so rotten, I will get rid of it, I got another one but that is low priority now)

 

This topic is for discussion only, just let people think about something not commonly done.

[Blue88Comanche]

In theory the 242 in full time wouldn't have any issues. I'd be like having a full size tire with a doughnut spare on the same axle. The open carrier should allow the difference in rotational speed. Part time is where issues may arise.

 

I'm sure someone mentioned this before. It's like 4wd tractors, small tires paired with large tires.

 

 

Sent from my iPhone using Tapatalk

[gogmorgo]

Keep in mind that the XJ/MJ d30 is high-pinion and requires reverse-cut gears. If you're looking for 3.91's, make sure they exist before committing to anything. It's not a factory ratio and not something many people in the aftermarket would be looking for.

And frankly what is the benefit? You lose all advantages of running a taller tire by not running them on the front as well, but still have the inconvenience, lower mpg, higher COG, etc. Split the difference, go with 32's front and rear (should fit with no lift) and 4.10's. You'll be happier.

[Eagle]

  On 5/15/2016 at 3:37 AM, Sevik said:

 

  On 5/15/2016 at 3:26 AM, Noriyori_Kudo said:

in 4wd you will cause the transfer case a lot of extra load on your chain. This will also effect traction issue since your rear tires will want to go faster then your front tires all the time in 4wd. if it was 2wd no issue but 4wd I would not recommend it at all.

Just as an example, a thread is 18.5/32" (MickeyThompson), that is 1.15", on a 35" tire it is 3.3% so if you put half worn tires on the front and brand new on the back, you will get about 1.6% diameter difference. Here I shown 0.7%, less than a half of that. 

 

 

That's in theory. What you are ignoring is that tire revolutions per mile aren't based on theoretical, unloaded tire dimensions. When I made up my spreadsheet for tire sizes and speeds by gear ratio, I went to various tire manufacturers' web sites and printed brochures (this was back when they still had printed brochures) and I looked up their published specs for revolutions per mile. They can be quite different from the theoretical, due to deflection of the sidewall when loaded.

 

Example: Take a 31x10.50-15. The revolutions-per-mile I got from some manufacturer (probably BFG, but I don't remember) is 680.

 

Theoretically, most 31x10.50s have an actual diameter (listed) closer to 30.5. That makes the circumference 30.5xpi = 95.8066", or 7.9839 feet. A mile is 5280 feet, so to travel one mile that tire would theoretically rotate 661.33 times. That's a difference of 2.8 percent between actual and theoretical.

 

For mismatching of gear, what we can get away with is very small. Like to the second decimal place -- due to different ring gear sizes, we consider 3.54 and 3.55 to be the same, or 4.10 and 4.11. But those differences are tiny -- using 4.10, the difference is .01/4.10 = 0.24 percent. You want to run with a difference that five orders of magnitude greater.

 

I don't think it will work. And that's ignoring the need to carry two spare tires.

[benjy_26]

That's a lot of complexity and cash wasted just to have a goofy looking Comanche.

[Marine1Texas]

I am not saying it can not be done, but remember this, if your gearing is off even 1/64 of a turn while you are in 4wd low you are going to rip up your transfer case chain, then any other weak spots. I am all for trying new stuff, I see no advantage at all to doing this my self, but WTH right The money to do this verse putting a lift on the front so you can run the same size tires. I say go for it if you can work out the engineering, then your have a one of a kind.

 

I use 4wd low ever single day I need to work on my ranch here in Texas at this time of the year with the raining season. I use 235/75r15s with 2 inch lift. torque is a major factor for me, because I am hauling equipment thru the streams and run off area's. I did try 31's at one point and in one area of my ranch got stuck a few times. When I went back to the 235's I had no problems. My MJ is a work truck and not everyone is going to be in the same environment. We are in a soft black clay area, we call it Texas Gumbo here. Others offroad in sand and rock. and lots people need height. It all comes down to what you going to use it for.

 

I say go for it if you have the money to throw at it and the time to dedicate to it.