The camaro rental I've been driving has 245/55/18 and MAN is the ride nicer than my G8 w/ 40 sidewall, I doubt the wheelwells of the G are big enough to accomodate that, but I think a 45 would soften bumps up a bit w/o sacrificing much if any grip compared to a 40

 

I went from the stock 19s down to aftermarket 18s and love the difference. You will get a bit more squeal in the corners but the grab off the line seems to be better and the hwy ride is vastly improved. I did go up to 39psi on all corners due to the softer/tally sidewalk and that did reduce some of that squeal.

 

Not sure I understand the problem you are trying to solve?

Based on adding more rubber to the taller sidewall of the tire I only imagine the ride improving. Since both the 18" and 19" wheel versions of the G8 get exactly the same suspension, its not as if the suspension is precisely tuned for the factory 19" tires. In hard driving you might notice a small change in responsiveness to steering input, but I doubt you would ever notice it in regular driving.

The biggest drawback you should look into is if you will run into any rubbing as the wheels move through their full range of motion with the front wheels turned.

You may want to consider more than just the aspect ratio / tire size in trying to determine what tire to buy. Again it depends on the problem you have with the factory tires. If they ride too rough, realize that different brands and types of tires in the same exact size are going to ride / handle differently due to differences in tire construction. If you problem with with ride quality, you should research tires that have less rigid side wall construction (more of a touring tire). If your problem is the gap between the fender and the tire, you should consider lowering the car. If done just to correct the gap, you can select springs that will not negatively impact ride quality and not have to deal with the speedo error of the larger tire (unless of course you will also plan to correct the speedo). If your problem is just not liking the look of the low profile tire, then you might also consider moving to an 18" wheel that allows a taller side wall to maintain the same tire diameter.

It all comes back to the problem you are trying to solve. I generically mean problem in this case, not that it has to be a functional problem. As an example, you not liking the looks of it is a "problem" to you. If there is no problem, then why mess with it?

 

Thank you for your thoughts. The OEM wheels an existing ride height will always stay the same. My eyes tell me that when tires are needed, that changing from 245/40/19 to 245/45/19 will not cause any rubbing or contact issues. Not being a tire smart type person, my thinking was that some of you would know from you vast knowledge, what I might expect to feel in curving turns or cornering at a speed exceeding the posted MPH by a few knots. I have no problems with these OEM tire. But they -don't- sell this model in a 45 series at this time, an probably never will. Therefore I would have to switch to those (S-04 model). When driving straight, I would expect a little softer feel (maybe not possible), but when taking those curves or corners at some higher than normal speed, I would hope the car would have little to no change in behavior. NO road racing done here. If you agree, an if I like the looks of that tire, then I might change to it. But I'm happy enough with the stock tires, if opinions are to much on the negative side. Thanks again.

 

When I switch from my 19" (255/40 front 275/35 rear) to my winter 225/55 R17 the ride is softer and high speed corners feel squishy (like the sidewall is flexing). Since you are only going with a 45 tire, you might not feel it flex.

 

I do not think the G8 has an 19" and 18" rim suspension tune, for what I see is the same and makes no manufacturing sense to have both (except for the GXP for more obvious reasons). Except for the choice of rims.

What I know is a very low profile tire might do more harm than good depending of road surface.

IMHO, 18 inch rims are adequate for this car for as the best all around size.

I drove cars with really low profile rims, they feel like you are all over the road.

 

What people don't realize ... is that the larger the rim, the harsher the ride quality, and LOWER THE PERFORMANCE.

I opted for stock 18" rims for this very reason.

Check out this video

Specifically start paying attention from 8:41 onwards.... the Walkinshaw 21" rims make the car actually HANDLE WORSE, RIDE HARSHER, and LOWER ET's, as compared to the stock 19" HSV rims, in this comparison.

That said, going from a 245-40 to a 245-45 in the 19 size will be negligable at best, as you're not changing the 245 part, and tire sizes are more of a ratio than a real individual measurable number.

That is to say, just a "5 point sidewall change" with the "width being constant" will not make a noticable difference in the car, from what I have seen.

Now, when 1meand swaps between summer and winter tires, he's going a much more drastic of a change: Summer 19" (255/40 front 275/35 rear) to winter 225/55 R17

• 255 and 275 width on his 19" summers, 225 width on his 17" winters

• 40/35 sidewall height on his 19" summers, 55 sidewall height on his 17" winters

That above is about as drastic of a change as you can do on a car of this size and weight, and yes, winter compound tires are also optimized to "grip" meaning they're softer by design and therefore will be softer and squishier in cornering. Anyone who has driven hard-compound-summer-tires-in-winter will tell you they don't grip at all, again, the right tire for the job.

I'd say to the OP that keeping the width the same and going up one small increment in sidewall height will be barely noticable and non-ride-effecting.