Hi all - merry christmas

I gave up on finding stock 18" wheels and TPMS parts so I ordered a set of snows from tirerack. Couldnt be happier with service and the ease. Sensors installed etc and everything mounted no probelm. I went with recommended set for the car, but after putting the wheels on and driving for a short bit, I thought doing the math on the diameter / circumference may be a good idea. Of course, after fact.

From calculator the difference between stock front (235/35/19) and rear (305/30/19) is 2.8% difference.

The tires I got, Michelin Alpin, front (235/40/18) and rear (265/40/18) are showing a 3.5%+ difference.

Will I burn out / stressthe Viscous Coupling / Center Diff or is this still within limits????

 

Any opinions on whether I should worry about this / get different tires sizes or just drive it and assume the tolerances are within limits??

Thanks for any info.

 

I have the same sizes on mine. The tirerack rep said it's the only 18" package they recommend for our cars.

From all the research I have done, the consensus is that anything within a couple % difference of stock is fine.

 

I bought an OEM 18" Carrera IV winter tire setup from here

http://www.wheelenhancement.com/

The rear tires are 295 wide. They grip very well in the cold but feel like they are floating a little when driving in snow.

The wheel and tire combination looks very nice on the car.

 

thanks guys - not sure i would do it again just to avoid the risk, but I am going to stick it out. They drive great in the snow. Would just hate to replace the viscous coupling after the warranty ends.

Thanks!

 

I had a similar issue when hunting for a 2nd set of wheels. I don't recall the exact % but it is stated in the owner's manual, something like 1.1% is acceptable.

If you want a tire size calculator:
http://www.miata.net/garage/tirecalc.html

 

I'm glad you asked. I bought a winter set for my 2007 C4S from the tirerack last year. Like you, after I took delivery of the wheels, I started looking into the math on the revs/per mile and the sizes specified in the manual. I exchanged several emails with the TireRack on this exact subject. Here's the correspondence: the TR response is on top, my question to them on the bottom.

From: [xxxxxx] The Tire Rack
To: CMOS

The magic number is the overall diameter difference front to rear, not revs per mile. Keep in mind it changes, and at different rates on this car, as they wear down. The small difference in revolutions per mile between the two will not do anything to the car. Porsche calls for a maximum of 4% difference in overall diameter between front and rear tires, which you are well within. Keep in mind that the combination is N rated, so Porsche has put it's stamp of approval on them. Your warranty is in good shape by running N spec tires in approved sizes, which these are.

We've never run into this combintaion causing any differential or drivetrain problems. The fitment is fine.

Thanks again.
__________________________________________________ ________

Tire Rack
7101 Vorden Parkway
South Bend, IN 46628


From: CMOS
To: Customer Service


When I placed Order No. [xxxxxxx], I indicated that these tires were for an AWD Porsche 911 (2007 C4S). TireRack sold me the following setup, which is now on the car:
Rear: Pirelli Sottozero IIs in 265/40/18, which has revs/mi of 765
Front: Pirelli Sottozero IIs in 235/40/18, which as revs/mi of 794
Difference: 29 revs/mi or 3.79% more revs/mi on the front.

The Porsche recommended setup for winter tires is:
Rear: 295/30/19, which has revs/mi of 777
Front: 235/35/19, which has revs/mi of 792.
Difference: 15 revs/mi or 1.93% more revs/mi on the front.

The TireRack setup has almost twice as many revs/mile Can you please confirm to me what the difference from 29 revs/mi from 15 revs/mi is OK and will not harm the car (in particular the center differential) or void the warranty?

 

Hi CMOS

Thanks for sharing - can I assume you have kept your tire set with the approx 3.6% diameter difference and have put a bunch of miles with no issues?

Thanks!
Kaan

 

been running the same setup. No problems thus far.