Tire Sizes for a '07 C4S

[dphatch]

I got a question for you guys. OEM tire sizes for my car 235-35-19 and 305-30-19 . Diameter of these is 25.5 " and 26.3 " respectively. Now given the obvious concern over rolling diameter of the tires on C4S and the impact of too much variation in front-to-back tire diameter on the drive train why not use a 235-40-19 on the front which at 26.4" and nearly identical to the rears 26.3 ? Tires in question are Michelin Pilot Super Sports.

Edited March 18, 2015 by dphatch

[qikqbn]

I got a question for you guys. OEM tire sizes for my car 235-35-19 and 305-30-19 . Diameter of these is 25.5 " and 26.3 " respectively. Now given the obvious concern over rolling diameter of the tires on C4S and the impact of too much variation in front-to-back tire diameter on the drive train why not use a 235-40-19 on the front which at 26.4" and nearly identical to the rears 26.3 ? Tires in question are Michelin Pilot Super Sports.

 

 

I have an 06 C4S and recently switched to 245/35/19's on the front and I have 305/30/19s on the rear.    the 245  is 25.8" diameter compared to 235 25.5".   The GT3's wear the 245 size too.  adds just a touch more width and looks Great!!  Just my opinion, but going 235-40 route, the tire may look a bit too tall compared to the rears.  Even though my current setup still has the front diameter slightly smaller than the rears, they look almost the same diameter when looking at the car.  the 40 aspect ratio may make it look a bit off balance with the fronts being taller.  just something to think about ;^)

[api]

Hi dphatch and qikqbn , sorry I didn't see your post sooner, here’s what info I have anyway, might be useful for someone else. 

 

I had the same question when shopping for winter tires for my 2007 C4S. Basically, there is a given ratio of front/rear rotations (or circumferences or diameters or revolutions per mile.) If that ratio is changed it is like your front or back wheels were slipping on ice. So, to find the difference or ratio you can use an online tire size calculator such as http://www.tyresizecalculator.com/ 

(or you can just use the diameter difference that you already calculated. 26.4-25.5=0.9”, 0.9/25.5=0.0353 =3.53%) I’m pasting below the results from the calculator which are similar with a little more precision. In either case it exceeds the 3% “slippage” mentioned to me by one Porsche technician and the 1.5% mentioned by the calculator. In the case of 245-35-19 the difference is 1.2% so acceptable. 

 

 

 

 

Edited May 13, 2015 by api