Scott at Team Harco

The obvious thing to me, is that both front and rear are on the same size wheel (rim diameter) and both have the same aspect ratio. There is a section width difference of 50 (235 vs 285). This means the front is of a smaller diameter than the rear. I could calculate this, if it were important to me.

So let's assume Porsche intended the fronts and rears to be close in diameter. Either a larger aspect ratio would be needed in front (40?) or less in the rear (30?) - assuming the section width remains the same. It may be worth checking the owners manual, the tire placard on the car, or even visiting a site such as Tire Rack to determine what size the car came with. Then some size calculations would be in order.

Coopduc

Fairly certain the OEM sizes are
front: 225/40-18
rear: 295/30-18
this could explain your issue

tom__w

I believe that these are the standard sizes (above).

So the overall circumference is what I am concerned with. As long as the ratios of the front to the back remain the same, regardless of wheel size (18 vs 19), would be my expectation?

Scott at Team Harco

Yes - circumference or diameter (and radius for that matter - they're all mathematically related). Interesting that your rears have gone up one inch on rim diameter and 05 in aspect ratio from stock. I would be questioning what the previous owner was thinking when he was buying tires.

tom__w

So here you go:

Stock:
Front 78.8" Circumference / 804 Revs per mile
Back 78.4" Circumference / 808 Revs per mile

What I have:
Front 80.0" Circumference / 792 Revs per mile
Back 84.3" Circumference / 751 Revs per mile

The stock seem to be dead on. My setup is just over 5% difference on the revs between front and rear. I suppose that could be triggering the PSM light to flash in certain instances.

Does anyone know what the allowable difference in front to back is supposed to be?

Looks like I also need to add 5-7% to my displayed speedo
(honestly officer, I was only doing 65...sigh

Ahsai

Yes, it's very likely that 5% is your issue.

Speedo under-reading a few % is normal and by design.

lowpue

^^this is definitely issue for c4s

tom__w

But nothing to worry about I presume. Even if it does tap the brakes every now and again.

I do love the look of those wheels. Quite large they are.

De Jeeper

I believe the mismatch may effect the awd systems and it will certainly effect the handling. I got my car at a $15k discount because the po slid off the track and cracked the motor partly because of tires. Now its a c2 and there were a few other issues i found but there r may threads about the wrong size tires causing the front ends to skip out in hard cornering. I switched out the tires and rims, he had the same size as u and put 18" hollow twists on an factory size tires. The car handles much better.

tom__w

Makes you wonder WHO DOES THIS!!!

Wouldn't someone check to see if mis-matched tires on a $102K car (when new) would cause a problem. WTF..

Scott at Team Harco

Yes - the AWD system wants a closer match between front and rear.

No. If you were to disconnect the drive to the front differential, maybe you'd have less to worry about.

Even then, the wheel speed sensors are going to send information to the vehicle dynamics program that will initiate action - unwanted and unwarranted in some cases. That's why you are getting the PSM light. To minimize your risk, I would suggest going up in aspect ratio on the front. Maybe a 235/45 or 225/45-19. I'd have to check the math, but that is directionally correct.

Do we know rim width? You may be able to go with a 245 section up front if the wheel is around 9" wide. Would put even more meat on the ground. Would also add to steering effort, slightly.

Ahsai

Obviously a safety issue plus over-working the PSM with you fighting the car all the time.

tom__w

I used this site. Very cool because it gives you compatible sizes after you enter yours:
https://tiresize.com/calculator/

Here are my numbers:


Seeing that I don't want to downsize the rear tires, I think my preference would be to get 245 in the front, if they would fit. Seeing that my current tires are 25.5" diameter / 9.3" wide, a 26.7" diameter / 9.6" wide tire should fit:


What do you guys think?

As well, I can get Ultra High Performance All-Season tires in that size at a reasonable cost, eliminating the need to swap tires / rims in the winter.

Thoughts?
(this is exhausting)

Porschetech3

The C4s and Turbo have a viscous clutch that applies torque to the front wheels whenever there is a difference in back-front wheel speed. The more the speed difference, the more the clutch applies, you will destroy the viscous clutch with your present set-up , let alone ill handling and dangerous..

Scott at Team Harco

If you plan to use this car in the winter - I would highly recommend going with the smallest diameter wheel that will clear the brakes and the narrowest rim width; within reason. Save your 19" wheels specifically for summer.

Look at a 17" wheel and tire package for winter. Physics work against you with big wide tires in slippery conditions.