315 vs 295 Comparison
 

I just picked up a used set of wheels, which had 315/25R19s mounted. I currently run 295/30R19s, but have always wondered if the 315 would look noticeably wider.

Both tires were mounted on an 19x11" wheel. The 315/25R19 is a Michelin PS2 and the 295/30R19 is a Michelin PSS.

Interestingly the section width of the 295 was slightly wider (shown buy the washers needed to get the level, level). You can also see that the usable tread width was almost identical.

Given the rare nature of a 315/25R19 PS2, I'm not sure this is relavent, but I thought it was interesting.

The 295 is on the left and the 315 is on the right.


https://cimg8.ibsrv.net/gimg/www.6sp...17a1cdc029.jpg


https://cimg0.ibsrv.net/gimg/www.6sp...c04790f82e.jpg

I work with wheel/tire applications all day on exotics and the PSS/PSC2's are huge compared to any other normal tire.

We always tell people that certain setups ONLY work with a lowered Pirelli tire for example because if you do the same size tire in a PSS it will rub like crazy.

Pirelli's also have more of a rounded side profile vs. the square side profile of a PSS which again, tends to reach out and grab suspension components and fender liners unlike most tires.

Back in the day on 21x12.5" rear GTR fitments they would run a 375/25R21 PZero and when it was discontinued the 355 PSS was even larger than the 375.

So you really can't just go off the treadwidth anymore... actual tire fitment is very brand dependent these days as well.

My issue was that a 295 won't fit on a 12 inch rim, otherwise that is a great option. That's why I scoured the earth for good 315/25's.

The 295/30x19 tire makes 800 revs/mile.

The 315/25x19 tire makes 825 revs/mile. This increase in revs/mile probably exceeds the safe number of revs/mile over what the front tires deliver and as a result the viscous coupling could suffer a failure due to a too high operating temperature.

Unless you're running a 245/35R19 front tire or have converted to rwd.

The 245 front with the 295 rear is pretty close at 25.8 and 26. The 305 rear is 26.3, close enough to not cause issues but I think it reduces the usefulness of the AWD system.

Compare something like a Direzza Star Spec Z1 to a Yokohama in the same size. The Z1 has much beefier shoulders in the same size tire.

My understanding is that you need the difference in rolling diameter to transfer torque to the front. It's a mechanical system.

Yes, if your fronts are larger than the rears the fronts will spin more slowly thus transferring more torque to the front wheels. If the fronts are smaller it has the opposite effect. So you want them the same size or have the fronts a bit taller.

By that same line of reasoning, could you go with smaller front, to effect more RWD-like dynamics?

Yes it likely would let you spin a bit more in corners. Without a limited slip diff i don't know f it would be any good though.

If your car has been converted to RWD only then no problem.

If your car has not been converted to RWD then a problem.

The wheels/tires you are considering have a front/rear revs/mile of 807/825 for a difference of 18.

The stock wheels/tires of 225/40x18 and 285/30x18 have a revs/mile of 828/840 for a difference of 12.

Running 295/30x18 rear tires has the front/rear revs/mile of 828/832 for a difference of just 4.

The tire setup you are considering has the rear tires making 33% more revs per mile than the 225/40x18 and 285/30x18 wheels /tires. 33% more revs per mile is probably going to over heat the viscous coupling.

If your car isn't currently RWD only it soon will be when the front diff konks out and you have to remove it.

The 245/35/19 spins 807 per mile and the 295/30/19 spins 799 in Michelin PSS. That is a good setup.

The car has almost 90,000 miles on it and over half of that mileage is with the 245/35 and 295/30 setup without any issues.

Later, Steve

Ok then. Keep doing what you've been doing and best of luck.