It's not that you're explaining yourself wrong, this is just a very technical subject that isn't the easiest to grasp - thus why there is so much debate over it.

Almost any wheel is going to have more mass than just about any tire with air in it. You're talking about a difference in mass in any wheel/tire combo and that threshold (whether it's with 17's or 20's) there is a large difference in mass. So, while you are correct that the overall diameter of the tire changes slightly at best (staying within the +1, +2 specs), the threshold point changes and pushes more weight further out on the total diameter of the wheel/tire package thus effectively changing the moment of inertia.

-Big_Ham
'98 SE 5MT
'01 SE 5MT

 

I'll respond to all of this by saying that if the unlikly hapenstance of the wheels weighing less than the 18s is in fact the case, then it is possible that the car could be as fast or faster than on the 18s. This would only be the case if: a) the rim were lighter than the one it replaces as is the tire with that which it replaces; assuming that the tire diameter remained a constant. Because this would mean that the total weight of the new setup is less.
<or>
b) The tire on the original setup weighed more than the rim. In this situation, expanding the size of the rim would lessen weight at the outer portion of the rotational mass by replacing the heavier tire with lighter rim.

I don't think either of these senerios are likly, but to be sure, the density of both the old and new rims could be measured, as well as the density of the old and new tires (both measurments would have to be done with the tires removed frm the wheels.) Then, we could be sure. Until then, we should assume that the wheels did slow the car down... but I wouldn't think it could slow it down by that much. And I'm sure it looks awsome!

 

Thanks 2k2, that was the best, most logical reply to my questions yet. I'm not sure if the scenario is likely but I was just curious if it was a possibility that 20" wheel/tire combos could actually be just as fast or light as 18's.

One observation that I noticed yesterday was that after I removed my 20's and put the 18's back on for winter, I took it for a drive and noticed not that the car was faster but rather the car was quicker to respond in the steering. Obviously this is because of the skinnier tires, but maybe this is the feeling that happens when you put 20's on that makes people think that the car is slower.


Silverstone 6MT Maya DTM 20's

 

Ditto, ditto and ditto.

I was trying to put in as many disclaimers as I could as nothing is ever true ALWAYS. However, based on the theory/science, all your statements are true.

-Big_Ham
'98 SE 5MT
'01 SE 5MT