Originally Posted By: 92saturnsl2
I'm curious as to why/how manufacturers specify a certain OEM tire size, and the resulting priorities. I've noticed a trend in last decade or two, that widths have increased and aspect ratio (the formula for height) has decreased. Rim diameters have increase in similar fashion, but why? Looks alone?
As far as I know, there are exactly two reasons to go for a bigger wheel with a lower profile tire:
1. Looks
2. Brakes
That's it.
The looks part is important these days because cars all have high waistlines, and small rims would look stupid (see the Pontiac Aztec for an extreme example). I personally don't care, but I'm not most people.
Brakes these days are pretty big, too; some (especially on sports cars and large upscale cars) just wouldn't fit under a reasonably sized wheel.
And of course, the downsides of bigger rims with lower profile tires are:
1. Wheels are heavier
2. Wheel mass is farther out from the wheel hub
3. Wheel hubs and suspension parts see more strain
4. Everything is more vulnerable to damage
5. Everything is more expensive
Originally Posted By: 92saturnsl2
Seems to me that ride quality would suffer with less sidewall height, although handling would be improved as the sidewall would flex less. Narrower tire = greater traction, and vice versa is what I always understood (within reason and factoring in vehicle weight.)
Ride quality definitely suffers, all else equal.
Less sidewall flex... not always, but usually. But higher profile tires can provide the same benefit with stiffer sidewalls. At least with a higher profile tire, you have more freedom to choose a harder or softer sidewall as you please (assuming a range of tires is offered in that size).
I don't think sidewall height affects traction much.
Originally Posted By: 92saturnsl2
On a vehicle I'm buying tires for, if I have a choice between say, P225/75/16 and P235/70/16, which quality (handling, ride quality, traction) would suffer?
I don't think those sizes are different enough to make a huge difference, but here are a few things to consider in general:
1. On a dry surface, wider means more lateral grip (cornering) but maybe slightly less longitudinal traction (accelerating and braking). If you want to improve both, you need a larger overall diameter as well -- but that'll affect your speedometer, odometer, and ABS.
2. Narrower tires (e.g. 205 vs. 225) have better hydroplaning resistance and lower rolling resistance.
3. A tire that is bigger in any way will usually weigh more.
4. Lower profile means more NVH.
5. All of the above is "all else equal." Different tire models in the same size can vary more than different sizes of the same tire.
6. Tire mass makes more of a difference than any other kind of mass on your car. It's unsprung, and it's rotating at the farthest distance from the wheel hubs. Saving a few pounds in each tire can help as much as having one less person in your car. The less your car weighs, the bigger the impact of reducing tire mass.
7. How the tire fits on the rim can be important. Look up the "measuring rim width" on Tire Rack and use that as a guide. Ideally, you want the tire sidewalls to be pretty upright. If you go wider than that, the tire will squirm more. If you go narrower, it's just bad.