Michelin Tires

[rlrich]

I want to replace the two Michelin LTX M/S 235/85R16 tires on the rear axle of my F250 4X4 with Michelin LTX M/S 265/75R16 for the higher weight rating. What issues other than speedometer could the diameter difference cause. The rear tires would be 31.8" diameter with the fronts being 32".

[RustyJC]

Probably not much unless you have a tire pressure monitoring system that uses ABS sensor input to impute tire pressure. Depending on its alarm threshold, it might think the rear tires are low on air with the smaller diameter (i.e., more revs/mile).

I doubt the ABS system itself would be upset by the differing wheel speeds since we're talking about a <1% difference, but you never know!

I guess I'm surprised that you had LT235/85R-16 tires on an F250 to begin with as that's a size normally found on duallies - I'm running 6 each Michelin XPS Rib LT235/85R-16E tires on mine. Out of curiosity, what size is called for on the driver's door GVWR/GAWR sticker?

Rusty

[Hitchhiker]

Would the slight difference of .2" in diameter cause any issues when the four wheel drive is engaged? I'm wondering if the transfer case and drive train will try to make the smaller diameter tires rotate a little faster thus causing a strain on the drive train and/or tire tread scuffing?

I suppose you could find out by sticking them on and driving a while to see what happens.

[Rick Long]

Stop and think about it... Your front and rear tires are never at the same ride height because of different weight loads and different air pressures.

[TXiceman]

Rick, rather than ride height, you need to match the rolling circumference of the tires. Like tires with differnt loads will still have nearly the same rolling circumference.

The rolling circimference is set by the extreme OD of the tire to the outer edge of the thread. This circumberence is pretty much set by the belts in the tire which do not grw appreciably. In a dragster, the tire is designed to grow in fiameter and circumference.

Ken

[RustyJC]

Following up on Ken's post, that's why tire manufacturers provide revolutions per mile (revs/mile) data for their tires. Revs/mile can be more useful than diameters and loaded radii for purposes such as this.

Rusty

[rlrich]

My door sticker shows LT235/85R16E. Revs per mile on the 235's is 654. Revs per mile on the 265's is 658.

[RustyJC]

Wow, that's surprising regarding the OEM tire size. I think the Dodge 2500's used a LT245/75R-16 as a standard tire, IIRC.

The difference in revs/mile is 4/654, or 0.6% - probably not enough to worry about.

Rusty

[CD]

rlrich
In reference to tire dia. I would suggest to only use 4X4 on slick surfaces. The front being slightly smaller is not good. The front tires need to turn more revs. when turning a corner. Years ago I had 2 sizes of tires like you but probably more difference. My hired man pulled out of a plowed field in 4X4 onto a dry blacktop road. After 1 90 degree corner and about 1/2 mile I had a broken front drive shaft.

[JimmyDee]

You will not like the way your truck handles in 4x4 with different size tires on the front and back. Your transfer case, U-joints and every thing else in the drive train will not like it either. I once drove a 4x4 in the winter on snow with smaller tires in the front and it was a wild ride.
Don't mix the sizes on a 4x4.
Jim

[Ray,IN]

Quote:

Originally Posted by JimmyDee You will not like the way your truck handles in 4x4 with different size tires on the front and back. Your transfer case, U-joints and every thing else in the drive train will not like it either. I once drove a 4x4 in the winter on snow with smaller tires in the front and it was a wild ride. Don't mix the sizes on a 4x4. Jim

Ditto;

[RVDude]

Quote:

Originally Posted by rlrich Revs per mile on the 235's is 654. Revs per mile on the 265's is 658.

So, when locked in 4x4 for every mile you roll you will scrub 658-654=4 tire circumferences worth of rubber. Does that sound like something you really want to do either on a slick or (especially) dry surface?

With 4x4, especially if AWD you want exact size (and wear) tires on all.

[RustyJC]

1. A 4x4 with no center differential is not designed to run on any high traction surface with 4WD engaged. Tire scuffing will occur on every turn.

2. A 4x4 with a center differential (aka AWD) will not scuff the tires because of a size difference. Any number of AWD vehicles come with different size tires on the front and rear - a Porsche Turbo, for example.

Rusty

[RVDude]

My bad on generalizing AWD - there are a variety of shapes and flavors of these systems.

My reference was specific Lexus (a "full time" AWD system) where the vehicle manufacturer tire spec will not allow a difference of more than 1/16 inch in tread depth between tires. Yep, a big all 4 tire expense if you trash a single tire halfway through their life.

I don't think we can generalize about AWD design and everyone would be best served to verify with their 4x4 vehicle manufacturer as to the allowance on tire diameter differences. Every system is different and just because you have a center diff does not imply anything. Always check with your vehicle manufacturer.