What combination is going to work better for track/autocross on a small light (2000 lbs) car: 205/50-15 on 6.5" wide wheels, or 205/40-17 on 7" wide wheels. Obviously the 15's will ride nicer, but would there be any difference in cornering and forward bite?
This is assuming the same tire, yes? Our wheel choices are fixed?
The 17" should have less flex in the sidewall - but a 205/50-15" is right in the middle of the recommend wheel width at 6.5" and a 205/40-17 is at the minimum wheel width at 7". So that's probably a wash.
I'd go with the one that has a better tire option or use a wider wheel.
Weight difference between the wheel and tire packages?
I'm biased that the 15" tire will work better on the lighter car. The 17" wheel will be heavier and its mass further out, and the reduced sidewall means you're really going to be taxing the suspension.
Don't forget that "nice ride" works both ways. No matter how smooth your inputs are, it doesn't mean anything if the tires aren't following the road very well.
Also, tiny sidewalls make for a much less forgiving slip angle curve, less forgiving is generally less fun.
Assuming both are using the same tire design/compound/construction, there are technical arguments that could be made to favor either for a given set of circumstances...But ultimately there are too many variables to say for certain without personally testing both to see how each performs on your car with your driving style at your track.
My main argument would be that in autox/track tires, there is actually considerably more available (competitive) compound options in 205/50R15 than 205/40R17...That and I think the 15's look better. 
But faster turn-in! 
The lack of sidewall deflection is what made the RT215 such a wonder when it was introduced. I swear they were filled with concrete.
Didn't GRM do a test on the effects of wheel weight a few years back? *
*by "a few years" I think I mean "about 15"
In reply to Keith Tanner:
Sport Compact Car did it around 15ish years ago. They took a loaner vehicle (might have been an Evo VI) and set it out on a track with varying wheel diameters on it, and also varying amounts of weight in a plastic bin in the back seat/trunk. Without allowing the driver to know what changes were made.
I believe that was one of Dave Coleman's articles as it had a decent amount of scientific rigor.
Keith Tanner wrote: But faster turn-in!
Yes. And both drivers were faster on the heavier larger diameter set if I remember correctly.
I remember it being non-intuitive as well. The SCC test was around the same time.
The 205/40/17 would also be almost 1/2" taller, so you may need to take the effect on the gearing into account.
I'm pretty sure GRM did a comparo of light vs. heavy wheels of the same diameter and found that even with added weight in the trunk to make the car weigh the same with the light wheels as the heavy, the light wheel combo was still faster due to reduced rotational mass.
As far as I'm aware, none of the fastest tires (street or R-comp) are available in 205/40-17...Just sayin'.
Don't forget, just because the bigger wheels are heavier, that doesn't mean the overall wheel / tire package is heavier. I've seen cases where the weight gained in the bigger wheels is lost from lighter tires and it's a wash in the end.
Also, if the smaller wheel option in question is an OEM wheel, a larger aftermarket wheel very well may be bigger at the same weight.
In reply to rslifkin:
Yeah, well.... I'm pretty sure I'd get a flat at least once a week if I had rubberbands on 17s, while my 195/55s on 15s (same sidewall as 205/50) merely have my wheels "pleasingly oblong", with no inflation-negating dents or cracks. (Sometimes curbs or potholes get in the way. What else should I do, slow down? Driving slow is for timid weenies in mall crawlers)
Also, 205/40-17 is a ricer size, not a performance size. So neener neener 
So I guess I'll give a little more detail. I've been running 17" wheels for the past few years and have used a couple different size tires on them. Current street wheels are 17x8" with 215/40 front and 235/40 rear.
My challenge set are 17x7 Kosei K1 TS that I mounted used 245/40 RS3's to. 
I've heard comments more than once about how tall the 17's look. The 245/40's are really too tall in my opinion and the 235/40 seem borderline. I think the 214/40's look about right. In the interest of stepping up my game (without adding a lot of cost) I've been considering if it would be better to downsize the tire diameter. If I keep the 17's that means dropping down to a 205/40 with the accompanying stiff ride, or dropping down to 15" wheels which means hunting for deal on used wheels. The car is also at about 10 lbs per hp, so I'm not sure 205's would provide enough grip on the rear. 15x8 or 15x9 would be great but are not likely to be found cheap. 15x6.5 however should be very common with a greater chance at catching a deal. So really I'm trying to figure out if I should stick with the Kosei's or start an exhaustive search for low buck 15's.
Damn that Opel GT is tight!
949 has pretty affordable 15x8, 9, 10, 11s that might work, depending on your backspacing and bolt pattern.
I'm not so sure that finding a used set of wide 15 inch wheels in 4x100 will be that hard. There are a ton of options on my local Craigslist thanks to Civic and Miata stance bros looking for "dat fitment".
Knurled wrote: Also, 205/40-17 is a ricer size, not a performance size. So neener neener
This is a good point. Sometimes wheel size is determined more by "what size does the tire I need come in?" than "what size do I want to run?"
That's why I've got 17s on the Jeep for the street summers and not 16s. In 16s, there's 1 choice of mediocre street tire. In 17s, there's that same choice plus 2 other much better choices. If I go to 18s, there's tons of choices, but they're even more expensive and I can't stomach the look of 18s on the thing, so 17s it is!
yupididit wrote: Damn that Opel GT is tight!
Yes it is! Check out the Pinto Cruising Wagon.
thatsnowinnebago wrote: 949 has pretty affordable 15x8, 9, 10, 11s that might work, depending on your backspacing and bolt pattern.
This. Or tirerack. Or a few other places. If you are running 4x100, you should be able to get a 15x8 just as cheap as a 15x6.5
In terms of looks, both of those setups look awesome
In terms of performance, I'll second what Keith said. A wider 17" wheel would be a great option, but between those two sizes, the performance is going to be very close.
ProDarwin wrote:thatsnowinnebago wrote: 949 has pretty affordable 15x8, 9, 10, 11s that might work, depending on your backspacing and bolt pattern.This. Or tirerack. Or a few other places. If you are running 4x100, you should be able to get a 15x8 just as cheap as a 15x6.5
True, if you are purchasing new. Trying to get a set on a Challenge budget is a bit more difficult. I scored the set of 17x7 Kosei's for $100 a couple of years ago and that price will be hard to match for anything other than stock Miata wheels.
Knurled wrote: In reply to rslifkin: Yeah, well.... I'm pretty sure I'd get a flat at least once a week if I had rubberbands on 17s, while my 195/55s on 15s (same sidewall as 205/50) merely have my wheels "pleasingly oblong", with no inflation-negating dents or cracks. (Sometimes curbs or potholes get in the way. What else should I do, slow down? Driving slow is for timid weenies in mall crawlers) Also, 205/40-17 is a ricer size, not a performance size. So neener neener
Weird.
I've run "rubber band" tires on multiple vehicles on multiple different wheel setups.
I ran 255/35/18 Star Specs on 18x9.5 RPF1s on the BRZ until recently. Put nearly 25k street miles without a flat tire or bent wheel.
And I live in a state ranked in like the Top 6 in the country for % of roads in need or repair.
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