[mm5300]

I know this may be a strange question, but I'm thinking about replacing the tires on my '19 M5C. The backs are pretty bad, but the fronts have a bit of time left on them. I'm thinking about replacing the rears first with something else, maybe Michelin. It's not a daily driver, more like a fun, nice day car. Do you think I'd have an issue if I replaced the rear with Michelin's and waited to swap over the fronts to Michelin's? I'd be using stock sizes and was curious if anyone thought that would be a problem. Thanks!

[irablumberg]

I have no direct personal experience, but the conventional wisdom is that it is a bad idea to mix older worn tires with new tires of the same brand. Mixing brands on top of that sounds like a risky proposition.

[shawnhayes]

Originally Posted by irablumberg

I have no direct personal experience, but the conventional wisdom is that it is a bad idea to mix older worn tires with new tires of the same brand. Mixing brands on top of that sounds like a risky proposition.

This is conventional wisdom.

You could luck up and get spot on or you could overheat or destroy the center differential function of the transmission.

Shawn

[pbonsalb]

I would not worry the least bit about changing the rears only. Many heavy or high performance cars will wear out the rears up to twice as fast as the fronts. You won’t do any harm to your transmission or differential.

[mm5300]

Thanks for all the information guys!!

[irablumberg]

Originally Posted by pbonsalb

I would not worry the least bit about changing the rears only. Many heavy or high performance cars will wear out the rears up to twice as fast as the fronts. You won’t do any harm to your transmission or differential.

Just to be clear, my post was not referring to potential diff issues as I agree different tire brands have different profiles for the same size tires, so there is a fair amount of slack in most AWD systems to accommodate such variations.

My point was that old vs. new tires and even more so different brands of tires will have different traction and grip characteristics and mixing them can create unexpected and unwanted handling issues at or near traction limits.

[shawnhayes]

Originally Posted by pbonsalb

I would not worry the least bit about changing the rears only. Many heavy or high performance cars will wear out the rears up to twice as fast as the fronts. You won’t do any harm to your transmission or differential.

This is a four wheel drive car. The front and rear tires are linked by transmission and diff. You SURE about that?

Manual also specifically states not to mix tires from different manufacturers and different tread. MOST manufacturers recommend replacing all four tires simultaneously on any all wheel drive car.

So, just sayin'

Shawn

[pbonsalb]

Originally Posted by shawnhayes

This is a four wheel drive car. The front and rear tires are linked by transmission and diff. You SURE about that?

Manual also specifically states not to mix tires from different manufacturers and different tread. MOST manufacturers recommend replacing all four tires simultaneously on any all wheel drive car.

So, just sayin'

Shawn

Again, I would not worry the least, but I know many people here will only do what BMW says and only take their car to a BMW dealer for service. Those people will be most comfortable doing whatever BMW says. If you want to vary, and don't know much, educate yourself so you can make an informed decision. I have been playing with cars for a while so I am comfortable with my decisions.

Yes, there could be minor differences in grip and minor differences in diameter or width with different brands. They are minor, very minor. I would not put 265 snows on the rear and 275 PS4 on the front, but I would not worry about substituting Conti EC Sport for PS4 at one end and I would not worry about running new PS4 at one end and worn out PS4 at the other end. I would not even worry about running new PS4 at one end and worn out EC Sport at the other end. I would not, except very temporarily, run any more extreme combination such as my first example of the the narrow snows in the rear and the wider high grip summers in the rear.

[shawnhayes]

Originally Posted by pbonsalb

Again, I would not worry the least, but I know many people here will only do what BMW says and only take their car to a BMW dealer for service. Those people will be most comfortable doing whatever BMW says. If you want to vary, and don't know much, educate yourself so you can make an informed decision. I have been playing with cars for a while so I am comfortable with my decisions.

Yes, there could be minor differences in grip and minor differences in diameter or width with different brands. They are minor, very minor. I would not put 265 snows on the rear and 275 PS4 on the front, but I would not worry about substituting Conti EC Sport for PS4 at one end and I would not worry about running new PS4 at one end and worn out PS4 at the other end. I would not even worry about running new PS4 at one end and worn out EC Sport at the other end. I would not, except very temporarily, run any more extreme combination such as my first example of the the narrow snows in the rear and the wider high grip summers in the rear.

My last car was a R35 GT-R, we had MULTIPLE people blow transmissions mixing tires front to back. (more correctly, the FWD controller was within the transmission, it was called the ATS unit), many of them had lots of experience with Subaru and Audi AWD systems and believed, like you do, that there was enough slack. It was an $18,000 repair from Nissan to fix that boo boo, and about $3000-$4000 from most of the great repair shops.

From the looks of it to me, the ZF unit looks robust enough to handle a fairly significant difference. But, I just don't KNOW for sure. Frankly, I'm not chancing it. I've been tracking cars for so long it's just too easy to put the same tire front to back on the same wheel on a $100,000 car, that's what I do because at 160 MPH on the back straight I cannot afford to be guessing.

Been burned one too many times going off the beaten path of conventional wisdom.

Seen people get away with a lot of sketch stuff, though, stuff that's KNOWN to be dumb. This, not known to me.

So, to OP, conventional wisdom vs. what you can probably do. I dunno personally.

Shawn

[pbonsalb]

I don't know anything about GTRs, but I think you and I recently had a discussion about going up or down in tread width on one end, which has a similar effect. You agreed in that thread that there was no issue. The situation at issue here is much the same -- wider tires than stock at one end will have more grip and be a slightly different diameter. Likewise, similar performance tires from 2 different brands will have slightly different handling, and worn performance tires compared to new ones will have slightly different diameters as the 8mm of tread depth wears down to 2mm. Are you comfortable driving your car with half worn tires at one end (pretty common to see the rears much more worn than the fronts)? Where do you draw the line? But each person should do what he or she feels comfortable with.

[shawnhayes]

Originally Posted by pbonsalb

I don't know anything about GTRs, but I think you and I recently had a discussion about going up or down in tread width on one end, which has a similar effect. You agreed in that thread that there was no issue. The situation at issue here is much the same -- wider tires than stock at one end will have more grip and be a slightly different diameter. Likewise, similar performance tires from 2 different brands will have slightly different handling, and worn performance tires compared to new ones will have slightly different diameters as the 8mm of tread depth wears down to 2mm. Are you comfortable driving your car with half worn tires at one end (pretty common to see the rears much more worn than the fronts)? Where do you draw the line? But each person should do what he or she feels comfortable with.

I certainly agree then as now that "close" will be good enough. But there's some percentage off that some AWD cars won't be able to tolerate.

The implied data from OP suggests that he will be changing brands of tires. So, at first he would be worn Pirelli's in front and new Michelins in rear. Will that exceed tolerances? Hell I don't know honestly. I do think you can get away with mixed worn sane brand same size front to back, but I've only had 3 brands of AWD track cars, and one of them was sensitive to mixed diameters that were supposedly pretty close.

So, to OP, might be fine, might not. No one has tried what you are thinking yet.

Shawn