pszikla

ANy of you guys ever try and change the profile size?
Right now I have the standard 205-50ZR17 in front and 255-40 in back.
Was wondering why Porsche did that since in new models it seems to be the same ###-40-####, front and back.
What would be the problem putting both profiles the same front and back, i.e. 205-40ZR17 and 255-40ZR17.
It seems to me that it would bring the front down a bit and that's all?
Pete

eDoug

The profile refers to the ratio of sidewall height to tire width. Unless you change your wheels to another stock size, say 18" w/ 7.5" wide front / 10" rear rims, then I think you'd be foolish to try to mess with your tires. Not the least reason of which is you can throw off the traction control (if you have that), by messing with tire circumference indiscriminately. Tires front and rear should have roughly similar sidewall heights, which means similar rotational speeds when on the road. The staggered front/rear fitment of these cars means different aspect ratios to keep the tire diameter/circumference/rotational speed roughly equivalent front/rear. Check for an online tire size calculator, I'm sure that might help explain better than I can.

bgrpph

i just replaced my 205/50-17 fronts with 225/45-17- tire height of the 225/45 are almost identical to the 205/50. kept rears at stock size 255/40-17. check out the calculator http://www.1010tires.com/TireSizeCalculator.asp

htny

OK so in a nutshell, from what I've seen:

205/50 is 102.5mm sidewall height
255/40 is 102mm sidewall height

near perfect match and right on the target rolling diameter assuming 17 inch wheel.

If you wanted to go wider you could and make the necessary adjustments to profile to retain the 102mm height, but don't change the front profile at the same 205 section width, as it currently does match. So when BGRPPH went to 225/45, that resulted in a sidewall height of 101.25, that's perfectly fine.

But to further illustrate the point, you'd have to go to 255 section width up front to run 40 series and retain the target sidewall height / rolling diameter, and you wouldn't want t do that and retain 255 rears on these cars.

I would say that if the only thing bothering you is the presumption that 50 series fronts are thicker or don't match the 40 series rears, don't worry, it isn't absolute, it's a ratio and in your posted configuration is perfectly matched.

Formula is "section width" X "series%" so 205/50 is 205 X 50% = 102.5mm

pszikla

Thanks Hans
That makes sense!
Pete