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All else equal, within a reasonable range, wider tires will generally give more lateral grip for reasons of contact patch shape, how it deforms under lateral load, and how that affects grip vs. load curve. Additional grip *by itself* would make you faster. The only question is where is the point of diminishing returns vs. heavier wheels/tires, aero drag, and rolling resistance. Here's what I think:
Additional aero drag from wider tires is going to be pretty damn small. Difference in vehicle frontal area with 245s vs. 215s at stock ride height is about +0.005 or one half of 1%, nothing. Coefficient of drag will be slightly different as well, but considering that the 245s will put the outside surfaces of the wheel/tire closer to the profile of the body it's possible that wider wheels/tires on stockish wheel offsets might actually reduce Cd, but either way it's going to be a small difference within a reasonable range of widths.
Rolling resistance is theoretically *less* with wider tires, as a wider/shorter contact patch undergoes less hysteresis. Tires with a narrower/longer contact patch have to have more deformation in the middle of the tread and hence lose more energy while rolling. On the other hand, wider tires with a greater mass of rubber being deformed could tilt the advantage in the other direction... In practice for a reasonable range of tire widths this effect is going to also be very small.
Additional rolling resistance due to *adhesion* (sticky rubber not wanting to let go of tarmac) is not really going to be a factor on the street on street-compound tires as in those conditions the tires don't get hot and sticky. At the track it could be a factor in lap times for low power/weight cars like the FT86 and Miata, but due to the nature of how the tire deforms I'm not sure it wouldn't be *worse* for narrower tires vs. wider. The narrower tire with longer contact patch is having to be "unpeeled" from the ground over a longer circumferential distance vs. wider tire with shorter contact patch.
The biggie in terms of how going too wide on wheels/tires can hurt you in terms of performance has got to be WEIGHT. More rotational mass will hurt acceleration of a modest power/weight car, and at some point this will hurt lap times more than increased lateral grip from wider tires helps.
In reality *other factors* have a MUCH greater impact on performance, to the point that it's difficult to isolate the effects of *only* going wider on wheels/tires without changing tire diameter and/or the stretch/pooch of the tire on the wheel (inherently an issue as tire widths are in 10mm millimeter increments and wheel widths are in 1/2" increments!).
I'm on the verge of deciding between running 225/45-17 on 17x8 or 245/40-17 on 17x9 for track events this year, and honestly I doubt that there's more than a tenth or two difference, all else equal. But I might go 245s based on vanity...
My 0.02, FWIW...
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