17x7.5, would you rather a 225/45, or 245/40?

[Joe-G]

At a crossroads in choosing a tire size, and I'm unsure what size tire would be best.

I don't have much experience driving on wide tires crammed onto narrow wheels, but can anyone share their experience? Strictly for auto cross use of course.

The car is FWD, and oem size is 225/45/17. I guess 235/40 is an option, but I'd rather try a wider 245/40 if I m going bigger.

Is the extra width actually worth a few more seconds at our local venues? I know driving feel may go down the drain, but is it worth the tradeoff?

[nnywg]

IMO more tread made a massive difference in my truck - 255 OEM to 275 now

You'll barely notice that it's a wider tire on the rim.  You're talking about 10mm inside and outside.  I say go for it.

[PedalFaster]

People tend to hugely overestimate the advantage of stuffing wide tires onto narrow rims. You absolutely won't gain "seconds" -- at most you'll find a tenth or two, and you may not see any benefit at all.

My last three cars:

FS BMW M3: Tried both 245 and 275 front tires on the 8" front wheels. They felt very subtly different, but I couldn't tell the difference on the clock. A buddy of mine tried the same test on his E46 M3, and couldn't tell the difference either.

BS Chevrolet Corvette: Tried both 255 and 275 front tires on the 8.5" front wheels. Couldn't tell the difference in either feel or on the clock. A bunch of other guys tried the same test, and amusingly, some preferred the 255s, some preferred the 275s, and some didn't have a preference.

GS Mini Cooper S: Tried both 205 and 225 front tires on the 7" front wheels. Couldn't tell the difference in either feel or on the clock.

What's the car? The fast setup on a Focus ST, which has 8"-wide wheels, is a 255 tire, so if you have 7.5" wheels, a 245 should be reasonable.

[Ryan S]

I wouldn't go wider than 235 on a 7.5 inch wide rim.  Refer to the 'measured rim width' published in the tire specs, you want to be as close to this as possible, and no more than half an inch over.  As you start stuffing too wide tires on, you lose sidewall stability for the increase in contact patch area.  And for smaller cars (as opposed to Gwynn's truck), an increase in contact patch area does not always equal an equal increase in traction, as you lose surface pressure and tire heating as a result.

Sometimes you don't have a lot of options size-wise (I would prefer a 215 on the back of my MR2 with the 7" wide wheels, but there isn't an appropriate aspect ratio, so I need to go to a 225), but if there is a close to stock revs/mile tire in 235 width, that is what I would go for.

[Joe-G]

IMO more tread made a massive difference in my truck - 255 OEM to 275 now

You'll barely notice that it's a wider tire on the rim.  You're talking about 10mm inside and outside.  I say go for it.

I guess it's not so bad. Look like it has been done before, and the tires aren't too pinched.

What's the car? The fast setup on a Focus ST, which has 8"-wide wheels, is a 255 tire, so if you have 7.5" wheels, a 245 should be reasonable.

Thanks for sharing your experience. This was the exact type of answer I was hoping to get. I guess it really does't hurt to try one vs the other. Even if I did pit them up against each other, I don't think I'm consistent enough to meaningfully record any difference anyways...

The car is a first gen Audi A3. From what I've read, it's not the best car for AX since traction is an issue. Maybe a bigger footprint could benefit the car? 6speed, not the DSG :/ ...

I wouldn't go wider than 235 on a 7.5 inch wide rim.  Refer to the 'measured rim width' published in the tire specs, you want to be as close to this as possible, and no more than half an inch over.  As you start stuffing too wide tires on, you lose sidewall stability for the increase in contact patch area.  And for smaller cars (as opposed to Gwynn's truck), an increase in contact patch area does not always equal an equal increase in traction, as you lose surface pressure and tire heating as a result.

Fair point, I'll keep this in mind.  I guess I'll have to see what's in  stock/can make it on time for the 1st event.

[MurrayPeterson]

I have raced on various sized tires on my 17x7.5 wheels (225, 215, 205), and I can easily assert that the 215's are no slower than the 225's.  I believe that Andy Hollis has done some serious testing, and has shown that tires wider than the rim size just aren't any faster.  The only reason I run 215's is because of the gearing issue with my Miata -- the larger diameter gets me a few KPH before needing to change to 3rd gear (which sucks).

[Reijo]

Andy's testing was done a long time ago and on street tires.  I wonder if this is so with these new street tires, such as the RE-71's, and their stiffer sidewalls and handling characteristics that are closer to Hoosiers.  With Hoosiers, of course, the wider you go the faster it is - not ideal, of course, but more stick nevertheless.

Time for some testing?

[PedalFaster]

My Corvette and Mini testing described above was all on RE-71Rs.