javahut

Anyone have any input on the advantages or disadvantages of running 245/40 18s on the stock rims as opposed to the OE 225/45 18s?

lourx8

Advantage - more grip
Disadvantage - more expensive

:D

zaglo6204

any comments on side wall 'feel' (squirmy, squishy...), or turn in, etc? i kinda want to know what to expect on friday when i get my new babies (t1-r's) mounted.

javahut

Yeah, I definitely don't want any less firmness in handling or cornering than the stock tire size provides. I'm kinda worried about the sidewall issue, too, with the wider tire. I'd rather have handling with stock tire size over the looks of a slightly wider tire... though I realize there'd be a little more rubber on the road.

OdinGuru

I've run three tires on the 8 so far:
1) 225/45-18 - Bridgestone RE040 - the original stock tires in stock size
2) 245/40-18 - Toyo Proxes 4 - I used these ultra-high performance all-seasons as my "winter" tires
3) 235/40-18 - Michelin Pilot Sport PS2 - my current Spring/Summer/Fall/Track tires

In terms of overall grip, the stock RE040's and the Proxes 4's are similar. The PS2's have noticably more grip than either. Tire compound make much more difference than small size changes for grip. In comparison to the stock RE040's and PS2's, the Proxes 4's have noticibly less turn in responsiveness. Obviously this isn't an apples to apples comparison because being an all-season tire the Proxes 4 doesn't have as stiff a side wall as the other two tires. However, from what I've read, the Toyo T1-R isn't as quite as stiff as some of the other Max Summer tires (e.g. the PS2's).

I see a lot of people assuming that the wider a tire you get, the greater the overall grip. However, keep in mind that the area of your contact patch isn't increased by a wider tire, it changes shape to be wider but also gets shorter (same area). On a track this shape change is useful because it allows the friction heat to be dissipated from a greater total tire surface area (helping keep the tires from overheating), but this isn't really applicable in a street environment. I never had the RE040's on a track, but I had the Proxes 4's at my first track event, and the PS2's since. Once again, as to be expected, the compound makes more difference than size, the PS2's take the extreme heat buildup WAY better than the Proxes 4's.

While there is no major disadvantage to going to a wider tire on a wider wheel, you do start loosing sidewall control as you go to a wider tire on the same sized wheel. If you want to make sure you perserve the turn-in response that makes the 8 so enjoyable to drive, I'd suggest thinking about not going with the 245/40-18. The Toyo T1-R do come in both the 225/45-18 and 235/40-18 sizes which I would concider as viable alternatives. I picked the PS2's in the 235/40-18 size because it was only half as much wider than stock as the 245, and also has a shorter overall sidewall height (which also helps with turn-in). The result was slighlty better turn-in than the RE040's. Overall diameter is 2.2% less than stock (as opposed to 1.0% less for the 245-40). I haven't found this to be a problem (Tirerack recoments staying withing 3% of stock).

Also, depending on where you are buying the 235/40-18 size may be conciderably less expensive than the other size options for Toyo T1-R's. The best online price I've seen recently were at sportcompactonly.com and are $177 for 225/45-18, $148 for 245/40-18, and $179 for 245/40-18. The price difference is almost enough to be a "buy 3 get one free" deal. For me this alone would be enough to make my desision.

P.S. I'd be interested to know if anyone on the forum has had actual experience with the same tire in different sizes on the same or different sized wheels. I'd be currious to hear their take on the "wider" is better ethos I see so often but can't quite reconsile with my understanding of tire physics.