Wheel Width

[Redleg94]

I'm planning on sticking with 19" wheels when I get new wheels for the summer for my GT1 (using the stock wheels w/ winter tires for the winter). I know that I could go to a 9" front with 255/35 and a 10" rear with 295/30 and still fit with no rubbing or scrubbing. I know that I could also continue to use the stock tire sizes, although they'll be a little stretched. If I decide to stick with the stock tire sizes, is there any reason to go with wider wheels than stock? Is there any reason not to? On another note, is there any downside to going with wider wheels and rubber? Will it impact ride quality at all?

 

[MerlintheMad]

You might have clearance issues with a 9" wide wheel and 255 tire in front: the offset will determine. By running a minimum width tire for a given rim, you make the side wall stiffer. The cornering performance is countered by the stretched look, which bothers some people. Ride quality (as in comfort) is not impacted by width so much as sidewall height (cushion).

 

[RM7]

By running a minimum width tire for a given rim, you make the side wall stiffer.

This, running the other way around, wider tire on a skinnier rim, you start folding/rolling over the tire during hard cornering, autocross, track driving, etc. That makes you slide. Not likely anyone would push it that hard on the street. But for max performance...

And really if that's the goal, you want the smallest and lightest wheels that will fit over the brake calipers, as wide as possible, with the tire that is at the minimum width for that rim.

 

[Redleg94]

You might have clearance issues with a 9" wide wheel and 255 tire in front: the offset will determine. By running a minimum width tire for a given rim, you make the side wall stiffer. The cornering performance is countered by the stretched look, which bothers some people. Ride quality (as in comfort) is not impacted by width so much as sidewall height (cushion).

So you're saying that running the stock tire sizes on wider rims will improve the cornering, right? Is that because the contact patch is slightly smaller? It doesn't make the car feel any more skittish on the highway does it?

 

[MerlintheMad]

So you're saying that running the stock tire sizes on wider rims will improve the cornering, right? Is that because the contact patch is slightly smaller? It doesn't make the car feel any more skittish on the highway does it?

Stretching the side wall makes the tire stiffer when cornering. That's the only change. The patch is the same size, but on a stretched tire the outer edge doesn't roll or round "under". My OE 225/40 (the PS4) wore on the outer edge: especially the LF outer edge, which caught the brunt of my "skid pad" (a cloverleaf interchange), despite the half inch wider wheel (TSW "Watkins" 8.5" wide). This second set of front tires (PS4S) are near the end of life: and so far the outer edges are c. even with the center tread: the difference apparently has been my decision to run the psi cold at 40 (all four corners, not just the front).

 

[RM7]

When I race, I and other racers will mark tire sidewalls with chalk or a marker or one of several different methods. We want to run the minimum PSI where our sidewalls hold up and don't roll over. When the marking starts disappearing, we know we are running PSI too low. When you run wider tires on skinnier rims (even if still within the tire manufacturer's recommended width range), you'll find you have to start using a lot more pressure to keep your tire from rolling over. This decreases your traction, but rolling over also decreases your traction, so the best traction comes from the skinnier tire on wider rims. But again, this isn't an issue unless you are pushing a car to 10/10ths.

The 225s that come stock on many of these are IMO too skinny overall for the mass of car. You also simply need enough tire under foot. I noticed during hard braking that I was starting to lock and skip a little and I attributed it to the narrow small contact patch of the 225s (and I'm not having that problem anymore on 255). A guy was racing a CTS-V wagon yesterday with 255s front and back. Stock out back is something like 285. Similar issue, tires too small for how heavy the car is...

Based on my BMW 4 that was a similar config and weight, I'd guess around 245 is a much better min-width for the car. If you start going wider than 255 you are probably going to start getting a lot more tire and wheel mass. If you start autocrossing, it's going to be a hard thing to figure, since yeah, you want narrower tires on wider wheels, but the car comes with real skinny wheels and real skinny tires stock, so any way you hack it, you are going to have a fairly big disadvantage from what would be "optimal" for the weight of the car. That's why I set mine up for wet autocross as a backup to my primary car, since wet traction actually improves with more sidewall as long as width is the same. But realistically it's not going to be very competitive any way I hack it due to the chassis weight and wheelbase.

 

[Redleg94]

Stretching the side wall makes the tire stiffer when cornering. That's the only change. The patch is the same size, but on a stretched tire the outer edge doesn't roll or round "under". My OE 225/40 (the PS4) wore on the outer edge: especially the LF outer edge, which caught the brunt of my "skid pad" (a cloverleaf interchange), despite the half inch wider wheel (TSW "Watkins" 8.5" wide). This second set of front tires (PS4S) are near the end of life: and so far the outer edges are c. even with the center tread: the difference apparently has been my decision to run the psi cold at 40 (all four corners, not just the front).

Thank you. Are you running something wider in the rear as well?

 

[Redleg94]

When I race, I and other racers will mark tire sidewalls with chalk or a marker or one of several different methods. We want to run the minimum PSI where our sidewalls hold up and don't roll over. When the marking starts disappearing, we know we are running PSI too low. When you run wider tires on skinnier rims (even if still within the tire manufacturer's recommended width range), you'll find you have to start using a lot more pressure to keep your tire from rolling over. This decreases your traction, but rolling over also decreases your traction, so the best traction comes from the skinnier tire on wider rims. But again, this isn't an issue unless you are pushing a car to 10/10ths.

The 225s that come stock on many of these are IMO too skinny overall for the mass of car. You also simply need enough tire under foot. I noticed during hard braking that I was starting to lock and skip a little and I attributed it to the narrow small contact patch of the 225s (and I'm not having that problem anymore on 255). A guy was racing a CTS-V wagon yesterday with 255s front and back. Stock out back is something like 285. Similar issue, tires too small for how heavy the car is...

I'm not going to be racing the car, or even pushing it to 9/10ths so, in theory, the stock tire sizes would be good. Wider tires would simply be about aesthetics. Most of the rims that I am looking at don't come in a 19x8 with the right offset and/or bolt pattern, they start at 190x8.5, hence the question. It also seems like it is hard to do just a "little wider" on the tires while keeping the same height, thus going much wider front and rear

 

[RM7]

NM.

I know what a redleg is. I was one.

 

[Redleg94]

NM.

I know what a redleg is. I was one.

I'll only be one for a few more months, and haven't been an actual working one for almost 10 years now. I picked it as a standard forum name about 25 years ago, have just continued to use it.

 

[JSolo]

@JamesNoBrakes Wouldn't a wider tire also have a higher load index, thus requiring lower pressure to support the same weight?

Stock 225/45/18 re97as02 had a load index of 95, while 245/40/18 michelin PS AS4 has load index of 97. 18x8.0" wheels falls in the recommended range for both sizes.

 

[MerlintheMad]

Thank you. Are you running something wider in the rear as well?

Yes. The rear rims are 9.5".

 

[Redleg94]

Yes. The rear rims are 9.5".

Thank you, that's what I'm now thinking, 8.5 and 9.5

 

[MerlintheMad]

Works for me.

Thank you, that's what I'm now thinking, 8.5 and 9.5