Theoretical question. If you could increase wheel/tire width (simultaneously increasing both grip and unsprung weight) at what point is it either worth it, or no longer worth it? Is half an inch wider wheel/20mm wider tire going to improve grip enough to justify the increase in unsprung weight? I guess what I'm really asking, is which should take a higher priority adding grip, or keeping unsprung weight low?
Happiness is not a matter of intensity but of balance, order, rhythm and harmony.
No way to know without testing and data.
The belief has always been that if your tires aren't getting up to temp, then they're too wide. However I've seen plenty of statements from drivers who have picked up time by going to the widest tire that can be stuffed under the car.
I expect there are practical limits - a 100hp 1500lb car likely won't be any faster on 335-width tires than it would be on something in the 225-245 range, nor do I imagine that running a Vette on 225-series tires would make it any faster due to the narrower track. 
This is all presuming your car has an alignment that allows enough static/dynamic camber to actually keep the contact patch on the pavement.
Edit: Unsprung weight doesn't mean much if the surface is smooth, but on rough/broken concrete it can become a factor. Just another element to complicate your question. 
Grip is more important. I can't think of a situation where someone has reached that point. Almost all tire sizes on competitive cars are limited by compound/tire availability, chassis fitment, or class rules/weight penalties.
In the mythical land where you can just add grip/tire with no limitations... test until you find that sweet spot.
That's a pretty complex question...but if you're trying to find out if gains from a wider tire are worth the unsprung weight, yes they absolutely are. You'll run into sharply diminishing returns on tire width before you run into unsprung weight problems.
GameboyRMH wrote: That's a pretty complex question...but if you're trying to find out if gains from a wider tire are worth the unsprung weight, yes they absolutely are. You'll run into sharply diminishing returns on tire width before you run into unsprung weight problems.
This^^^ As an example, I have seen plenty of 100 whp 2000lb to 2400lb class cars turn faster laps on 205s than they did on 225s, particularly on full-size road courses. There are other factors to grip than just tire and wheel width.
Let's start with; car, rules/class that he car is being run/built too and tire's.
My bench mark is the 1995 Miata that I have been running this year. I was not going to run the car but as fate would have it I am very happy that I have.
The path to SCCA E Street is very straight forward. Keep the stock springs and then, spend as much as you like on everything else as to suspension.
The first thing completed, on a 129,000 mile car, was a refresh of the ball joints, KYB struts with Fat Cat bump stops, front hubs, rotors/pads, fluids, and a set of 195/60 14 Falken 615K's.
These tires were take off's from another Autocross car of mine, so not brand new but only five months old. From the start this combination was competitive. In three events with 5 to 7 cars in the class I ended up number three twice and once in second. Then I upgraded to an aggressive alignment and Bridgstone RE-71 205/50 15's on 15x6 rims.
This combination has started the next round of upgrades as the grip generated by the tires has overwhelmed the shocks. The Koni's are in the car, two events are on the books, now I have a second and a first at the last event.
The Racing Beat sway bar is going in the car this week, now it's done. The wider tire on a narrow rim is working me on what tire pressure that the car wants. Not on what I am used too.
WildScotsRacing wrote:GameboyRMH wrote: That's a pretty complex question...but if you're trying to find out if gains from a wider tire are worth the unsprung weight, yes they absolutely are. You'll run into sharply diminishing returns on tire width before you run into unsprung weight problems.This^^^ As an example, I have seen plenty of 100 whp 2000lb to 2400lb class cars turn faster laps on 205s than they did on 225s, particularly on full-size road courses. There are other factors to grip than just tire and wheel width.
Yeah. That makes sense. I specifically asked about autocross since it seems to me that grip would be more of an advantage on tighter more technical courses than on an open road course, where the unsprung weight might have a larger effect.
On an autocross course I think you need as much grip as you can fit.
For a road race car there has to be a balance between tire width, aero drag, frictional drag, rotational weight, and un-sprung weight. You spend as much if not more time going straight on a road course as you do turning, and lets not forget braking too.
In reply to STM317: In autocross, lightweight wheels have been proven to not speed up the car. Adding grip is what makes you faster, along with driving well, so go with the widest wheels that will fit and match the tire width to the wheel width.
Back in the Pleistocene era (or was it the Plasticine era?), this very publication did a test comparing the effects of standard diameter, plus-1, and plus-2 wheel and tire combos. If I recall, the increased weight of the increasing wheel diameter did have an effect on straight-line acceleration. However, I believe the test car was a low-hp Honda Civic which needed a couple of tenths extra to twist the bigger, heavier wheels. Note: All tire ODs were pretty much the same. So perhaps a heavier combo would create an even greater disadvantage for the low-hp entrant on a "power" course.
Look at FSP for low power cars. The fast ones run 275 wide Hoosiers(at least up front on the FWD cars anyway). As opposed to 205-225-245 also available.
In reply to Jerry From LA: A couple of Tire Rack employees autocrossed for a few seasons and tested it out, too. Also, the mag did a more on-topic test recently about using different weight wheels in the same diameter.
I agree that unsprung weigh is not an important consideration on a smooth surface.
Personal experience- Neon IT roadrace car, lost almost 10 pounds per corner with a wheel and tire change. It was astounding how much better the shocks got all of a sudden
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MY 275/30 hoosiers weigh within a pound of my 205/50 street tires (miata sized). Case in point, the car is metric trucks faster on the hoosiers.
It's worth it until you get too wide and heavy and your time is affected. Test, test, and test to find out where it crosses the line and becomes too much is pretty much how you find out. Personally, if I still autocrossed, I would go with widest tires in the stickiest compound allowable that would fit the widest (and lightest weight that fits in budget) wheels available in class. For the street though I've gone with the smallest fit and lightest tires for a 17x7.5 wheel for my FR-S, 205/45-17. Over 4 lbs lighter a corner than stock (wheels) really noticeably improves the ride quality. The tires are smaller diameter than stock (215/45-17) and the car has never been as quick and agile in responsiveness and acceleration while having more grip. Tire compound and matching tire tread width to wheel width is everything in maximizing responsiveness, which I value more than maximizing grip for daily driving.
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