I'm confused about something. Say I have a 17x8 rim. People say you get a bigger contact patch if you get a wider tire, say a 235/40/17 vs a 225/45/17. Why is this? I would think that regardless of what tire you put on the rim, you'd have a contact patch that is 8" wide.

The first number is the width of the tire in millimeters. A 235 is 9.25" wide and a 225 is 8.86" wide. They both fit on an 8" wide rim because the sidewalls will reach back to meet the rim.

Howl nailed it. It confused me, too, at first, but a bunch of different tire widths will fit on a rim. For example, you can go from a 205 to a 225 on a 7" wide rim. Huh? Doesn't sound right at first glance. But rubber is pliable when you think about it.

so basically if you go with a wider tire, the sidewalls will just be angled inwards a little bit to compensate?

so basically if you go with a wider tire, the sidewalls will just be angled inwards a little bit to compensate?

Now you're starting to get it.
The ratio between sidewall and wheel width is important for, among other things, turn-in. I've actually been in a car that had tires that were quite oversized for the wheel with respect to the wheel-versus-tire(tread) width and you could actually turn the wheel back and forth a little bit, but continue in a straight path down the road. There was enough sidewall flex that the wheel could actually change angle slightly without tracking differently. granted, this was an extreme case, but it makes a good example.

For drag race, maybe you don't care about turn-in, you just need all the rubber on the ground you get....but if you want to turn and change direction a la auto-x or track days, you need to have the wheel width and the tire(tread) width properly matched.

ok, thanks BadTrip, that makes sense. So, if i have a 17x8 wheel, what is a good fit? 225/45/17? Or maybe 235/40/17?

ok, thanks BadTrip, that makes sense. So, if i have a 17x8 wheel, what is a good fit? 225/45/17? Or maybe 235/40/17?

I think there's a thread around here somewhere stating recommended size tires for specific wheel widths.
With an 8" wheel I'd think either of those size you listed would work.......personally I like to stay pretty tight on the wheel, so I'd tend to go with the 225. Remember, you're in an AWD drive car. unless you're putting down HUGE power you should have pretty decent traction.
For auto-x, I'd tend to think that you'd want very sharp response....also looking to the 225. maybe ask some of the auto-x guys around how they feel about it.

Check this out from luke @ the tire rack:
http://forums.nasioc.com/forums/showthread.php?p=13914993#post13914993

and in the winter (which I see you are from NY) you're definitely looking for a tire that is more narrow.

Wider tire = summer

Narrow tire = winter

FTW!!

Another advantage of going with wider tire on a given rim, is to protect the wheel from curbing damage. If the rubber sticks out a little past the lip, the tire may rub against the curb first before you cause permanent damage (in case you really mess up a parking attempt).

well my main goal is autox, and they are summer only tires, and i have rally armor mudflaps as wheel guards, so it seems like everything points to getting 225/45's...and they're cheaper.

1. All other things being equal, a wider tire will not increase contact patch, it'll only change the shape of it.
2. Wider is not always better, it just depends on your application.
3. While a 235 will give minutely better auto-x cornering abilities, it'll also decrease your torque to the wheels because of increased overall diameter. Beyond that, it'll probably increase your unsprung weight/rotational mass.

Sticky 225 <---my recommendation

The 235/40-17 is shorter than a 225/45-17, so it will accelerate quicker. I run 235/45-17s on my 17x8 wheels, and they fit really well.

Remember, the contact patch is determined by corner weight divided by PSI. So, if you lower the PSI of the tire the patch will increase. Increasing the width of the tire, but using the same PSI, will not increase the contact patch but will only change the shape of it (make it wider and shorter). So:

205/55-16 (---)
(---)

vs.

235/40-17 (------)

The nice thing about having such a wide tire with a short sidewall is that you can drop the pressure a bit to increase the contact patch and still not roll over as much under cornering load. So, you still keep good steering response and increased patch size.

With respect to undersizing the tire (using a lower width tire on a wider rim, a.k.a. "stretching") your contact patch shape may stay small if the sidewalls are rigid. So, while my Azenis 205/55-16 was square on the 16x6.5 rim, it will stretch on an 8" rim but still be limited to the tread width and not roll on the sidewalls (that would be bad anyway, but this is an extreme example). I would like to try 245/40-17 to get the widest contact patch, but after driving on the 235s I am not sure it's worth it.

The 235/40-17 is shorter than a 225/45-17, so it will accelerate quicker.

Not always, some 225/45's are less in overall diameter than their 235/40 counterparts, and secondly, the 235 will usually be a heavier tire.

The 235 will increase absolute cornering because of the increased width, but because of the increased weight it'll probably decrease acceleration.

All in all, it's probably a wash between the 2 sizes, but I'd always choose the lighter tire.

the width of a tire (225,235 etc.) is not the contact patch. stocks s02s on my S2000 have a contact patch equivalent to a 245 tire, but are 225/50/16. There is a specific measure for the contact patch.

Not always, some 225/45's are less in overall diameter than their 235/40 counterparts, and secondly, the 235 will usually be a heavier tire.

The 235 will increase absolute cornering because of the increased width, but because of the increased weight it'll probably decrease acceleration.

All in all, it's probably a wash between the 2 sizes, but I'd always choose the lighter tire.

Hankook R-S2s are definately shorter, and there is a noticable increase in acceleration from my Azenis 215 in 205/55-16. The total of the new tire and wheel (5Zigen FN01R-C 17x8 vs. the stock 16) was within half a pound from what I remember. I will probably weigh my old stock wheel and Azenis combo again to refresh my memory.

Also, while tire width is not an absolute determining factor for contact patch, it is typical for a given tire make/model that a wider width will yield a wider shape for its contact patch. When comparing different manufacturers, or even different product lines for a given manufacturer, you have to use the tread width spec to start figuring out your shape.

People say you get a bigger contact patch if you get a wider tirefrom another post of mine:

contact patch area is determined by pressure, wheel rim diameter, tread width and section height for a given load. the static contact patch area is (virtually) the same for the following sizes:

205/55x16
225/45x17
225/40x18

what changes as you lower the profiile of the tire, reducing it's section height and increasing it's tread width, is that you increase the width of the contact patch and shorten it's length. A shorter, wider contact patch will provide better dry cornering capability, while a longer, narrower contact patch can be better for resistance to aquaplaning and dealing with snow.

the contact patch shape becomes wider - the size (area) remains the same

a shorter, wider contact patch at the same slip angle begins to slip at the same distance from the tire's leading edge as a tire with a longer, narrower contact patch. However, the wider contact patch has more of its length in contact with the road than the narrower contact patch and therefore a larger portion of the overall contact patch area is creating grip. For the same load and slip angle, a wider contact patch generates more grip in dry road conditions.

of course, when you are actually using the tire as intended, the dynamic contact patch of the tire constantly changes shape depending upon the load forces acting on the tire. once you choose your tire the section height, tread width, and tire diameter become fixed. increasing the width of the wheel by .5" will increase the tire's section width .2" resulting in a wider contact patch. the remaining variable - tire pressure - becomes crucial in determing contact patch shape and therefore the performance of the tire.

for the track, research done by Jim Hall of Chapparal Can-Am racing fame along with Firestone led to a doubling in width of race tires in the mid-60's. Every time Firestone sent Hall a wider tire, he and his team tested them on consecutively wider wheels, going from 15x7 to 15x9. Each time the increase in tire width and wheel width led to greater cornering forces during skidpad testing. Then Firestone would create a new tire designed for the 9" wheel and Hall would increase wheel width again with the same results. The difference in grip was enough to force Hall to redesign his transmission to handle the greater loads. the end result was an increase in Can-Am race tire width from 6" to 12" in less than 18 months and the acceptance of the fact that a wider contact patch translates into better dry cornering and braking.