All minimum recommended wheel fitments (what max tire for a given wheel width also means) . . . and the only way I'll ever run a min-recommended wheel width for whatever tire size is if that's the way the car comes when I buy it. Sure, it's a legitimate fitment, but for overall use it just leaves too much on the table unless you simply can't stretch the sidewalls out any further because of something like strut clearance.
The nutty tire and wheel combinations that you see in SCCA autocrossing aren't done because they're the absolute optimum way to go, it's really an unintended consequence of placing limits on wheel widths but not restricting tire widths/sizing. Not worth emulating on the street.
Norm
Just wanted to add that Norm is 100% spot on here. Read his post, learn it, live it.
The OEMs tend to "run wider tires on skinny wheels" for a number of reasons, but a big part of that is "typical idiots park too close and rub up against curbs". Its easier to fix a scuffed up tire (a serviceable part with a 5 year lifespan, at best) than a curb-rashed wheel (wheels could/should last the life of the vehicle). Many tires now come with "rim protectors", which are extended rub strips near the bead, that are sacrificial and protect the wheel lips. These "wide tires on skinny wheels" are not done for performance reasons.
On this AWD EVO X we ran in STU class, tire width was limited to 245mm but wheel width was not... so we used an 18x10" wheel
Racers know that when they have the choice (rules, budget, clearance), its almost always better to run a given sized tire at the "wider" end of the "recommended wheel width" spectrum than at the narrow end. Like if a class limits tire width but has unlimited wheel width, we stretch out the max tire width onto an even wider wheel a bit. Makes for great feel and feedback, uses ALL of the tire tread, and has few if any drawbacks. Of course the VIP crowd has taken this way too far and made the term "stretched" a dirty word.
Of course if you can fit an uber-wide wheel and the rules don't limit tire width, it is often better still to run a tick more tire width than the "slight stretch" that is text book optimal. Like when we first went to wider wheels on our S197, it had an 18x11" front and 18x12" rear with a 315/30/18 tire (see above). The tire width was limited by the fender width, but the same tire on the wider wheel was sharper turn-in and wore better (the rears). The wheel widths were limited then by the factory fenders and inner structures/strut.
When we added
front flares (or at the event above, just cut/clearanced the fenders - until the flares were completed a few weeks later) we quickly moved to wider 18x12's up front (to match the rear) but kept the same 315mm tire at both ends (the rear wasn't supposed to ever be touched, according to Amy, so 315 was the limit). The car GOT FASTER and the turn-in improved on the front tires - with nothing but a wheel width change at one end.
When the opportunity to add rear flares came along (she was out of town!) we took it. This then allowed the same 18x12" wheels to work with much wider 335 (front) and 345 (rear) tires. The car dropped another 2-3 seconds a lap from the added tire width. BUT, the 345 was visibly squeezed onto that 12" wheel. Ideally the front wheel would have gone to 18x13" and the rear to 18x14"... but the wheel choices got hyper-expensive above 18x12", as you have to go monoblock or 3-piece to get wheels that wide. So we had a known compromise on tire-to-wheel width... due to cost and availability factors of the wheels (we always kept 2 sets of wheels around, to run scrubs in practices and/or in case a wheel or tire was damaged on the "race set"). Wheel costs literally tripled and in some brands quadrupled when going beyond 18x12".
Our next shop car is being built around the same tire package as we ran on the TT3 Mustang. But now we will have them on what I feel are the "correct" wheel sizes: The 335/30/18 will go on an 18x13 and the 345/35/18 will go on an 18x14 wheel. The newly offered M14.2 Forgestar 2-piece wheel (above) bridges the gap - in price and width - between 1-piece rotary forged wheels ($350-400/each) and super costly 3-piece or monoblock wheels ($900-1500+/each).
Just one example, but keep in mind the rule: if the tire manufacturer has a range of wheel widths, err on the side of the WIDEST one they offer, if it fits your class/budget/chassis restrictions. Supporting a tire with the proper wheel width is important for performance, feel, ultimate grip and tire wear.
Cheers,
