[GrandSport]

Originally Posted by dave99rt

Yep, for autocross most go wider or widest tire when restricted to narrow OEM wheels. Height for higher peak speed is also important. Bigger tires help with heat management also.
245/45/17 on 7.5" is commonly used in DS. It's the perfect blend of higher top speed and not overly pinched with the taller sidewall. 235/45/17 would be my second choice.

Heat and wear management are the biggest reasons as its endurance racing (WRL). 8% more tire, 8% more thermal mass. 8% more surface area to cool down the tire. Plus, less sidewall is nice.
Also, narrower tire on a wider wheel is harder to drive, even if it's faster. That's an issue for a 16 hour race.

Regarding the actual test results, it looks like 225 on 9" wheel is the fastest, but 245 on 9 is just a hair behind it and the 245 on 9 had the fastest lap (just not averaged out). 275 on 10 is basically the same as 245 on 9 (1 inch is 25, so it's almost exactly 270 on 10). Given this is enduro, it's hard to see why those test results do not support 245 +9 which would say 275 on 10.
It does support running a 245 over a 255 though.

IDK, can we fit a 10.5 or 11 inch wide wheel? 295/30/18 is really the "best" size because of gearing/height. The 275/35/18 is more than half an inch taller than the 255 and nearly an inch taller than the 245.
The 255 might be the worst size.