Ok pls help me understand this, I know I might have screwed up. Last week my two rear tires were replaced due to camber, the tires and sidewalk were seperated. I didn't want to wait so I made maybe a stupid mistake. My front and rear were 245/30/19 and 275/30/19. Since they didn't have the size I needed, I went with the closest 275/35/19 in the rear. Ever since I noticed at speed passed 80mph, a yellow triangle with the ! appears on the dash board, and when I pressed on the brake...it's hard..like I completely lost the brakes. I had to downshift and bring the car below 80 and everything is fine again. Help me understand this. What is wrong with my car?

 

the higher profile tire (35 vs. 30) is probably taking out the abs and traction systems...for them to function properly the circumference of the front and rear tires have to be within spec

 

I am thinking could it be the PSM hasn't realized I have thicker tires in the rear than the front and simply just need a reset?

 

Call the dealer or a local shop. There's gotta be a way to change the setting manually and change it to what you want.

 

your PSM can't learn that you have a higher profile tire than before..the circumference is either in spec or not...the higher sidewall gives the tire a larger circumference...you will have to consult the manufacturer of that tire to get the exact circumference and compare that to the circumference of your front tire to see if you are still in spec...you might be able to put 245/35/19 in front and get back in spec..

 

Thanks guys. I will find a backroad and try with PSM turned off see if that fix the problem. If it does, then I will try resetting the ECU.
Anyone has similar experience? I should have waited for the proper size tires =T

 

If it was a C2 it wouldn't matter. On a C4, don't drive it. Your diameters off front and back and as speed increases, you're throwing it off more. You're gonna burn up the differential and possible additional damage.

 

Oooh I guess the only fix is to get 245/35 for the front then. You are right, the awd is gonna get messed up when front and rear wheels diameter are different. Thanks for the info. Case closed! This is why I love 6speed!

 

You were running 245/30 front and 275/30 back? That's still cutting it close with almost a 3% diameter difference at 60mph. With your current 275/35 in the back with the same fronts, your almost 7% off. Even if you go 245/35 in the front with your current 275/35 in the back, that'll put you close to 3%. If you're keeping the 275/35's, I'd go 245/40 in the front which is less than 1% difference.
Especially with the C4, I'd try keep with within 2% difference at 60mph as I'm sure your gonna be driving faster than 60mph.
Go to 1010tires and it'll let you calculate diameter difference.

edit: if you go 245/40/19 in the front to match rear diameter, the tires will be taller than factory spec and if you're lowered, I'd be worried about tires rubbing.
I'm not sure if the speedometer will be off as I'm not sure where the speed sensor is on Porsches

 

You are Running the Wrong Size Regardless in the Front... You should be Running 235/35/19 not 245/30/19... Thats a 2.70% Diffrence compare to the Correct Rear Size 275/30/19...

Now since you have gone from 275/30/19 to 275/35/19 you have made the Tire Taller by 4.07%... Now compare to your Front which is 245/30/19 (Wrong Size) to your new Rear 275/35/19 (Wrong Size) now the diffrence is 6.74%... Way over the 3% Limit

BTW you are going to Kill your AWD System runing those Sizes... Turning PSM wont help, you are just burning the Viscus Clutch in your AWD System... thats a lot of $$$$ to fix

I suggest you get rid of your Tires and get the Correct Size 235/35/19 and 275/30/19...

 

Awesome! Thanks for the info. Am getting the right size tires, car sits until new tires arrived. Learned a lot today =)

 

Smart man... Good Luck

What tires are you getting and how much?

 

you can check TIRERACK.COM, too. they give you the numbers for revolutions/mile. Dont forget as tires wear they go from 10/32 to 2/32 depth, a change of 16/32 or 1/2 in.

Just looking for my rears, 295/30/18 stock, various tires go from 827 to 837 revolutions/mile, a 1% variation. And wearing tires will drop another 2% off.

This suggests that a 3% variation should be built into the calculations.

best advice is to talk to Porsche.

by the way I have 2 sets of rears. one is 285/30/18 and the other is 295/30 (stock size). No problems with either as Rev/mi on these tires is listed at 830 and 832, respectively.

JB



As mentioned above, tires are legally "worn out" In most States when they reach 2/32" of remaining tread depth. For example, a typical tire that starts with 10/32" of original tread depth has only 8/32" of useable tread depth. from TIRERACK.COM

 

Its probably not psm, doesn't that automatically turn off around 55mph? Its best to be safe and just get the proper sized tires.

 

I swear I was just paranoid when I first typed up my post...I had 245/30/20 on my other car...and I mixed up the numbers =P....I must be crazy!
But correction here: I have 225/35/19 in the front and now I am getting 275/30/19 in the rear. Just waiting for the tires to come in. The front is at 0.48% and the rear is 1.61% , so I think I am good with that set up.