Wheel + tire: size vs weight vs confusion

[horrorbiz]

I'm hoping for some input from someone who understands the physics of wheel weight vs size vs tire weight, etc. (Note: I am also aware I am overthinking this)

I have 18x8 Enkei RPF1's on my BRZ w/ Conti ExtremeContact DW tires in 225/40-18. One of the reasons I chose them was due to their light weight.

The total weight for this setup is 38.8lbs... so about 2lbs less than stock.

I've noticed that after switching to this setup my MPG dropped from 30 to 28 (average).

I also have a set of OZ Ultraleggeras in 17x8 with 245/40-17 Dunlop Direzza Zii on my WRX. I was originally thinking about putting this setup on my BRZ, but decided against it due to it being about a pound or two heavier than the stock setup.

Even with the extra weight, would the 17" setup be better for perfomance?

[Admiral Ballsy]

Couple other variables might be involved.

The wider tires will have more friction, including resistance to rolling. And more aero drag.

The larger (diameter) wheels will place the moment of inertia further from the center of the wheel, which would offset some of the weight savings.

Just thinking aloud.

Personally, I want the smallest-diameter wheels I can get that clear my brakes.

[jshadeck]

Stock tires are LRR and skinnier. If your only/main concern is mpg then go with the lightest rim u can find and run stock or other LRR tires on them (u can also go with a taller tire, as this will give u more mpg due to lower rpms at the same mph, but will adversely affect performance)

[Lee358]

Subscribed to see what people say

[Leonardo]

I agree with what everyone is saying, but I also think that the lower mpg, is from the OP's foot. I am always 2 mpg, at least, more excited after I get new wheels.

[Steve]

I'd agree that the MPG loss would be attributed to the slightly wider and higher resistance to rolling (stickier) tires. Your wheel and tire choice is pretty optimal for 18's, I'd rock it. You would probably be able to save a lb or two with the 17 setup, same wheels and tires, but meh.

[horrorbiz]

Thanks everyone! I appreciate the feedback. My biggest concern is not MPG, it's performance. I assumed that maybe since my MPG went down that my car was suffering in acceleration too.
Not sure if that is a reasonable assumption.

Putting lighter tires on the 17's is not an option at the moment because those Dunlops are nearly new.

[horrorbiz]

Originally Posted by Admiral Ballsy

Personally, I want the smallest-diameter wheels I can get that clear my brakes.

Even if the smaller wheel set is slightly heavier? Or are you just comparing the same wheel + tire combo at a smaller size. I got a good deal on my 18" Enkei's, but if I was buying new I would have gotten the 17's.

[Admiral Ballsy]

Originally Posted by horrorbiz

Even if the smaller wheel set is slightly heavier?

I meant like for like. Here's an example, though.

I bought a Corvette last fall. The wheels that came on the model (427 convertible) are the 19x10 and 20x12 set that's used on some versions of the ZR1 and the Z06 with the ceramic brakes. Tires were 285/30 and 335/25 Michelin ZPs.

I see absolutely no reason for a convertible - which will never see track time, by definition - to have 25-series tires. The reason the wheel set exists is that the cars with ceramic brakes need 19s on the front to clear the discs. I guess they just put them on the 427 for 'bling' factor.

I sold them immediately and put on the 18x9.5 and 19x12 set that the standard Z06 uses. I saved something like 1 pound per wheel on the fronts, and maybe 2.5 on the rears.

I bought Bridgestone RE050A tires, 275/35 and 325/30. These were actually a little heavier than the Michelins, for two reasons - the Bridgestones are just heavier tires, size-by-size, and also since the rolling diameter stays the same but the wheel is smaller, the sidewalls gain material (offset a little in this particular case by the 10mm smaller section widths). If I'd stayed with Michelins, I've have saved another few pounds on the tires (but paid about $600 more on the set).

I ended up gaining a pound or two at one end, and losing a pound or two at the other, don't recall exactly. But it rides better, is a little quieter, and the tires in this size are helluva cheaper (there's only one tire in the stock size). And it should be a little quicker, although probably not enough to measure, since the moment of intertia of the smaller wheels is closer to the hub.

[rikdrt1]

I went extremely modest, from stock to 225/45/17.. on 17x7.5 aftermarket rims @ 17.1lbs.. I still lost 3-5mpg.. because of the Michellin SuperSport tires.

sticky tires take more to roll - you would be surprised how much different just the tire makes..