I recently installed 18" chrome Iroc rims on my iroc and at some point I've noticed a drop in gas mileage (I'm not sure when, it may have been when I installed the rims, I just don't remember). The overall diameter is real close to the stock diameter. Using calculators on the web, my speedometer should be like .16 to .18 mph off. I for some reason have gone from getting 18 mpg to 14 or 15. I don't believe wheels that are 3 or 4/16 of and inch larger in total diameter could cause that, and they didn't feel any heavier, but then again I've never changed rims. So, is my odometer that far off or did these rims somehow kill my mileage?

bigger wheels, and they being chrome added alot more unsprung weight,

As for the odometer, one of the best ways to check accuracy is on a measured mile. Some highways have areas with a "speedometer check" over a few mile stretch. I-5 in SW Washington has a couple of them at 5 miles long. My odometer gains a tenth over the 5 miles or about 5%. For that matter, just use the mile posts on the freeway.

Ok thanks. I still have the old wheels, so I think I'm gonna weigh an old one and weigh a new one and see how much heavier they are. They didn't feel heavier when I put them on, but I guess that could be. I think the maf could be bad as well so maybe that's causing it ( I can't really tell yet). It just seems weird to me that two inch larger rims would make that big of a difference.

So, if the wheels are causing more drag, making the engine eat horsepower, does that mean I just need more power ? Of course, that'll just bring me back to stock level .

Wait wait wait.... so, if the calculator says that with these wheels I'm actually traveling at 60.18 when the speedometer says 60, is that an extra .18 for every 60 miles, or .18 for every mile at 60? Because if that was the case, I'd be losing ten miles or so every sixty miles traveled. If I was to fill up at 180, which is now about all I can get down to around 1/8th a tank on the gauge now(use to be like 210 or 215 at times), I really would have done like 210, which would put me right back on the money. Sorry if that's wrong and/or stupid, I'm just stumped and I suck at math lol.

EDIT: Never mind I'm an idiot. I checked another calculator and it said that after 10K miles I'd be off a whole 28 miles. I'm gonna do what Al Hasse said anyway just to be safe.

and don't forget the tires are wider, contact patch is wider than stock, more rolling resistance, more fuel used, but better traction. Its a trade. I use ROH 17 with 245/45's and it did cost some fuel.

it's mostly the weight... I've run 215, 235, 255, and 275's on the same 15" wheels and I don't know that I could document an MPG change, but all the combinations were under 40#. At the same time it's not uncommon for the pontiac 'hi-tech" wheels to end up in teh 56-58# range with tires, and I've noticed a significant loss in MPG with them. I have a set of 18" Z06 wheels that are fairly light weight for the size but if I remember right they still weigh 52# with the tires and they get about the same MPG as the hi-tech wheels.

Well ok thanks everyone. Yeah it does handle quite a bit better and it looks a whole lot better. I keep getting comments on how nice the car looks. I guess these wheels way 10 lbs more per wheel than the stock ones. That doesn't seem like a whole lot, but I guess that's a lot in unsprung weight?

SorryI know this is an old thread, but I was wondering something else. What have these wheels done to my performance? It doesn't really feel different, but then again maybe I wouldn't feel it. I'm wanting to install a cam which should bump this thing up to around 220, maaaaybe a little higher, but still. I hope I'll be able to break the 15 sec mark without having to port polish. What do you guys think?

Edit: Yeah I don't think a cam will make me break the 15 sec mark, even with the hooker 2055s it has on it. Btw this is a 1986 Iroc, so it has the 190 hp 305 tpi. I don't know what it would run with the cam, but this comp cam has more lift than the 1985 engine, which had 215 due to a hotter cam.

to figure out the exact change the weight has made you'd have to find the radius of gyration (the equivalent radius that you can treat the mass of the wheel as rotating at) for both combinations, and then calculate the torques...

a rule of thumb that most people use is that rotating weight has 4x the effect of stationary weight on acceleration, but it also increases unsprung weight meaning that you need that much better dampening in the suspension to control it's motion accurately.