techweenie

My car came to me with 235/35 19 front and 295/30 19 rear. If I look up rotations per mile on Tire Rack, these are, respectively 817 and 799. Is that within Porsche's requirements?

I'm sure there are already several threads on this topic, but I was unable to find one.

Carlo_Carrera

The rears are significantly larger diameter than the fronts. Idealy you would want the fronts slightly larger.

If it was my car I would not run those sizes.

techweenie

The difference appears to be 2.3% front to rear. Is that really too much?

Road King

Agreed.

Our cars were spec'd to have slightly taller front tires based on the AWD system's design. I wouldn't run taller back tires. Others have, with apparently no issues.

manimal

That's fine if the fronts are the larger ones, but consensus seems to be that if the rears are larger at all, it could damage the viscous coupling.

Third-Reef

The rears being slightly larger will not kill the coupling. It does not care if the front or rear is spinning faster. it is the different rotation speeds that will kill it. at low speeds a large stagger may be OK. As the speed increases the rotational difference increases proportionally. So something that works just fine at 60 mph may fry your coupling at 150.

Now the stagger defines your torque split and effects handling and PSM response. With rears spinning faster, there is a small amount of torque driven to the front wheels. When the rears slip then there is more torque driven to the fronts. If the fronts are spinning faster they are actually dragging a bit and when the rears slip they will need to slip even more to get the torque driven to the fronts. I had slightly larger rears, swapped them out for stock sizes and no more weird PSM interventions and the car is more stable at the limit. The car was designed by some smart dudes, maintain the proper stagger they designed in for a reason. Or go 2WD and do whatever you want with tire sizes.

Carlo_Carrera

I believe Porsche specificities 2% maximum and staggered with the tailer tire on the front.

Carlo_Carrera

I had the exact same experience.

techweenie

Thanks for the informed input. I'm actually selling off the 19" setup and going back to stock 18". Looks like a pair of 225/40 19s on the front is what I should plan to recommend to my buyer.

"02996ttx50

anything within 3% ( revs per mile differential ) btw front and rear is considered ok.

Road King

Theoretically yes, but at 26.1" tall, the 225/40 is a TALL tire to fit up front. I'm sure it's been done, but a lot depends on ride height and potentially radiator relocation mods.

wross996tt

So have you actually measured the tires or are you going by theoretical sizes?.. Realize the tires actual sizes may differ significantly from theoretical.

Road King

No, of course not, but Tire Rack publishes detailed specifications provided by the manufacturers. Here's the Michelin 4S:



Occasionally a spec on Tire Rack doesn't look right, I attribute this to a typo or transposition error. A quick visit the Michelin, Yokohama, etc. generally yields the right numbers.

While these 19s are tall, they have a similar revs per mile differential to the factory 18s.

wross996tt

Yeah and these are at best averages. Not one of those pieces of data has any variation in it (standard deviation). So on average (head in the oven, feet in the freezer) they are OK. This does not mean the tires you actually have will have those identical numbers...you do understand the concept of variation?

Road King

It's close enough for this exercise, isn't it? The conversation was about taller front vs. taller rear tire.