You are correct in that the diameter is the same, but the rims generally outweigh the tire by a significant ammount.

Note: Mechanical advantage = length from center to outter most diameter, by installing larger wheels that move a heavier mass farther from the center you are increasing the resistance to acceleration. Actually it is the resistance to change - but you probably don't care about semantics.

Your hypothosis would be correct IF the weight of the rim and tire were evenly distributed along the entire radius, it isn't. Rims weigh more and especially on light rims, most of the weight is at the farthest point away from the hub as it can be.

If you get the chance to drive two identical cars one with say 15" wheels and one with 18" wheels, this becomes very apparent, as long as they are both good rims and not super heavy like steelies. Lots of Honda and BMW Mini drivers find this out.

good luck.

 

If looks were all that mattered, I'd have some 18x8/8.5 Volk CE28Ns on my STi.

For the money it pays to have 17s, as the tires are much cheaper.

Plus my car is stiff enough on the street, I don't need it to be much rougher.

I hate that my STi wheels are only 7.5" wide, so I will get some CE28Ns in 17x8/8.5 in the future.

 

I'm thinking about getting the 17's now. Seems like it's cheaper and just about as good as the 18's. Although, Im not going to race my car anytime soon, It's good to know that there are all these options. thanks guys I'll try to get 17" x 11.5 hehe I was just kidding.

 

Alrighty... I did a little tire shopping... Yeah.. they can get pretty expensive... but the ones I've seen were about 100 bux at tirerack... They're only about 10 dollars more for the 18s.. so what gives?