I see a lot of people running 255s and not as many running 245s. Is there a reason why? I would imagine that 255s would feel slightly floaty compared to 245s.

 

The advantage of the 255 is more rubber on the road. The difference in height is only about 2/10". If you go with the same size all the way around I would think the 245/40R17 is going to fit easier. If I can help with the tires let me know.

 

I have a lot of experience running both sizes on the same wheels. The delta between the two is negligible for casual HPDE use. The data I saw, testing same day back to back had the 255 ~.1 seconds faster but there are too many other factors to definitely say so. I would be willing to bet the 245 would be faster on some more open, higher speed tracks. Depending on the tire construction a tire may prefer to be a little more stretched or not be as bothered by running a little wider on the same rim width. Also remember that was just the data from a specific tire, car setup, track, driver, etc. Others may find different results.

Take note that this is in a time attack setting on big road courses so times were for a single fast lap. I prefer the feel of the 245 and more often than not they are less expensive than the larger size but the 255 may perhaps be a tick faster. The biggest benefit of the 255 is that it will go a little longer before overheating so if you're looking for a little more consistency throughout a session then a 255 may be the way to go. But the difference isn't night and day.

-Matt M.

 

For a 9" wide rim, the 245 would be more optimum a profile. There will be less sidewall deflection so the handling is going to feel tighter and more precise. I run a 255 up front, but I pair it with 10" wide rim for the same reasons stated for the 245/9".

You could consider running a staggered 245/255 on a non stag 9" as well.

 

Thanks for the nice replies...

Looking to get as much info as possible until I own an S one day

 

I do 245/255 staggered. Fits and drives great. If you track and auto x a lot, go 255

 

The 17x9 255 setup is popular with autocrossers as that is the maximum rim and tire width allowed in SCCA STR. I'm guessing that may be why you see a lot of people running that combination.

 

FWIW, tire rack seems to measure all of the 255's on 9" wheels and 245's on 8.5" wheels.

 

For me, I've found that tracks with long, open straights, the narrower/smaller diameter tires have a top speed advantage. Shorter, tighter tracks the advantage goes to the 255's.

Unless you're competing, just get whatever size is available in the tire you want, or whatever size looks better to you. Differences are gonna be subtle. For feel on the street, I like the skinnier tires. But the meaty 255's look nicer to me.

 

Unless you're measuring your speed with GPS, the difference you're talking about might just be the change in speedometer calibration.