OT: Wheels for cars, aero? Lightweight?

[Likes2ridefar]

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[Ralph]

Originally Posted by yinzerniner

Not necessarily true. While 15% might be in certain best case vs worst case scenarios, a lower percentage change of ~5% wouldn't be considered "minimal" in most cases.

Tesla explicitly states the range differences between their wheel and tire options.
And Car and Driver did a test a while back showing the difference between the different wheel/tire size combos. 225 width tires on 17/18 wheels yielded about 4% savings, so a 2" gap could be more of a percentage difference like the OP is thinking.

Both of the above are discussed here:
https://insideevs.com/news/375165/te...nge-tire-size/

Note on above - the tire width differences could also explain a bit of the range/mpg differences between the different wheel sizes, but the 17/18 comparison had the same width

Smaller diameter wheels with tires of same circumference as stock, even with slightly more aero drag from slightly wider tires or softer rubber with it's extra friction drag....will not be 15% different in fuel economy

Maybe 5% is believable....and that's kinda pushing it.

[Likes2ridefar]

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[charliedid]

As mentioned above I have real hate for my wheels and have thought to get 16" steel or Alum rims. I have a solid set of Conti tires right now, would it be foolish to get 18" steel rims? I'm an auto idiot but it seems like it will save me tire cost up front and I end up with a wheel presumably more durable and less prone to curb and pothole issues. Or is it silly and I just get 16" and be done with it?

[carpediemracing]

Random thoughts.

Steel doesn't change curbing. If you curb a wheel, you curb a wheel. With steel, the big difference is that it's a bit stronger but also it's bendable - a few good taps with an appropriately shaped hammer and you can "true" your rim a bit. And some black spray paint, or a Sharpie, and it looks fine. Alloys announce the curbing to the world, and it's hard to fix cosmetically.

Aftermarket wheels bend pretty easily, unless you get very high quality wheels (typically $300-400 each). I'm talking just driving and hitting the occasional pothole and such. I've road force balanced my wheels and for sure one is out of spec and one is borderline. The car shakes a bit at 70 mph.

As far as gas mileage goes, the tire itself makes a huge difference. A lower rolling resistance tire goes a long, long way to making a car more fuel efficient. They almost always give away traction though (usually not durability, believe it or not).

Smaller wheel sizes usually make for a lighter overall package, specifically because of the tire. You may not think of the tire as the heavy bit, but often it is heavier than the wheel. This is especially true for the bigger tire vehicles, like SUVs.

In fact, the tires that come on cars are usually a lower-tread version of the aftermarket tire. For example, one tire I know of has 7/32" tread when new - as an OEM tire. The same make/model tire, same size, but different SKU, that we sold in the store, came with 10/32" tread (only way to tell was to look up the DOT number - there were no markings that I was aware of that indicated if it was an OEM or aftermarket tire). For fuel efficiency (fed tests the car "as is" so they spec more efficient tires), noise reduction (to meet federal drive-by limits, but also for test drives), ride comfort (for test drives), OEM cars typically have a lower tread version of the aftermarket tire. It's like your printer - it comes with a smaller ink cartridge just to get you going, then you have to go buy the real ones ones that have way more ink.

If the OP has an Accord with 19" rims (Accord sport, I think it's a 245/40-19?) it's a weird tire size, relatively speaking (compared to, say, a 245/40-18, or a 215/45-17). I believe it also requires XL load rating, so it's a "heavy duty" tire. The same size tire without the XL rating will typically have an internal belt failure after a bit of use, and at best will wear prematurely.

Going to an 18" will bring you to a much more common size (225/45-18), be less prone to cut cords (bulging sidewall, usually from hitting a curb or a pothole). The only thing to worry about is brake clearance, but a place like Tire Rack will have all that figured out, and I'm pretty sure you have plenty of clearance for even a 17".

(I figured all this out working as a service writer, and we specifically stocked tires for the Accord Sport because they were expensive and hard to get. For our system they said not to stock them, but I made sure we stocked them. One customer in particular bought, one by one, a new set of tires, damaging one tire at a time.)

A friend of mine got a Civic Type R, which came with 20" rims. Ridiculous, really. He sized down to I think an 18". Now the Type R comes with 19" rims stock.

We have a normal Civic, it came with 17" rims. I stuck snow tires on the 17" (stock size, not skinnier, because we don't often drive in really deep snow), and for the summer tires got 18" rims (aftermarket, cheap/soft, two are no longer very round).

Often when you go up in rim size there isn't a lot of tire width difference, unless you want it to be there. For example, on our Honda the tire sizes were:

215/50-17
215/45-18
225/35-19

When you go from a 17 to an 18, it's considered a "+1". 17 to 19, "+2". If you don't change the diameter, it's a "+0"

In case you didn't know, tire sizes are:
(width in mm / height as a % of width - rim diameter)
(215mm / 50% of 215mm = sidewall height - 17" rim)

When I put the summer tires on, I wanted more width for better handling / braking (and it seems that the mileage really didn't suffer with good quality summers). So we went with 245/40-18s (+1), which have effectively the same circumference (1% off, compared to the factory 215/45-18 which is 0.6% off). I try to stay at or under 1%. Due to tire availability the car now rides on 215/45-18s (same tire model, just the 215 size). It's still hard to really push the limits with the (very good) 215 tires. The 245s were ridiculously good.

Another car I had came with 225/45-18s fronts, 245/40-18 rear. I got wider wheels and had 285/35-18 and 315/30-18 tires (+0, but two steps up in width). Same sidewall height (same rolling circumference, same rim diameter) so it was similar to the factory wheel in ride and "pinch resistance", but it was much wider and had better traction. Too much traction, actually, as the car's limits were a higher than I felt comfortable exploring.

Incidentally, if you slam your brakes on and the ABS kicks in, it means your brakes are outperforming your tires. Better brake pads and such won't do much for your braking because you're already exceeding your tire's traction limits (hence the anti lock brake system releases the brake to let the tire regain traction).

The best way to improve your first stop is to put similar but wider tires on your car. There was a car shootout between some "tuners" on the same car that I had that had the 225/45 and 245/40 tires. Two "tuners" were brake companies so they had their top of the line kits on, along with the standard +0 set up for those tire sizes (245/40-18 and 275/40-18). So the tire width went up 20mm for the fronts, 30mm for the rears.

One tuner company showed up with a real street fighter - lower bucks but better performance. Stock brakes with aftermarket pads but for tires they went +0 two steps up - the 285/35 and 315/30 set up (on wider rims, but they were still 18s). That car absolutely stomped the others in the braking test because the tires had much more traction. Their competitors with multi thousand dollar brake upgrades didn't stop better because they didn't go up enough in tire size.

(However, those big brakes are great for dissipating heat - so if you're doing multiple instances of heavy braking, like at a track, they'll maintain their performance, whereas the smaller stock brakes will lose stopping power if you repeatedly ask them to brake hard.)

[charliedid]

That^ is an amazing amount of info...I guess I am still inclined to go with 16" wheels and ditch the 18" for my purposes.

[joshatsilca]

I'm way late to the party here, but the 17" wheel in question is a fairly aero optimized wheel with directional cooling which has been shown to reduce pressure build up in the wheel wells and can also reduce pressure wake at the rear end of the car due to increased vorticity over the sides of the rear quarter panels onto the rear bumper area.. These are not quite as advanced as what we see on the new Tesla model 3 or some others, but they apperar to have side area coverage on the order of 80-85% compared to the 19" sport and touring rims which are not aero optimized and are rather low percentage coverage (~40%) and no clear directional cooling or shaping type features. So combined the much greater wheel area with the much lower side area coverage and I'd say that the vast majority of the difference here is aero not tire (though the 10mm wider tire is most definitely adding to the 19" wheel problem).

I can't see online what tires are spec'd between them, that can play a big part here too.. lots of lower profile and 'sport' tires for mid-market cars get stiffer/reinforced sidewalls for rim impact protection making them higher rolling resistance as well as heavier than higher profile tires of similar type/rating.

All in all, yes, the 19" option here is going to be way less aero, way heavier in both rim and tire, and almost certainly way higher rolling resistance.. but the aero is likely the larger hit here which is why the highway mileage is hit harder than the city mileage.