What are the reasons that big rims make performance worse. I know beacause of the weight but why else?

You need more torque to spin a larger diameter, even if two wheels are the same weight, such as 15s and 18s, the car will be slower with the 18s.

Weight is the main issue. Take for instance this a lightweight rim will make your suspensioon work better (faster) and react quicker to changing road conditions. Also for every 1lb wheel/tire weight that equals 8lbs static body weight. So a difference between a 20lbs wheel/tire setup compared to a 40lbs wheel/tire is about 160lbs static body weight. That's like carrying an extra person in your car. That's only 1 corner.

Also larger wheels and tires have the weight further out from the center of the rim, which takes more energy to turn. For instance a merry-go-round. Put 20 people close to the axis (middle of the merry-go-round) compared to 20 people hanging off the edge and you get the picture.

Also when you get to 17s and 18s they are a little taller than stock and that will kill your gearing making the car accelerate slower than stock.

but what about if the diameter of the stock wheel+tire equals the diameter of the aftermarket wheel+tire. For example; A 13 inch rim with a 175/70-13 tire and a 17 inch rim with 205/35-17. I know the 205/35 is really hard to find but what if. Wouldnt it be the same?

but what about if the diameter of the stock wheel+tire equals the diameter of the aftermarket wheel+tire. For example; A 13 inch rim with a 175/70-13 tire and a 17 inch rim with 205/35-17. I know the 205/35 is really hard to find but what if. Wouldnt it be the same?

Wouldn't it be the same as what?

if the entire stock wheel/and rubber inclusive diameter was the same as the aftermarket tire/rubber diameter, plus lighter weight, would that be the best?

Wouldn't it be the same as what?

The two set ups wouldnt they be the same in performance if the wieght didnt matter and what if the weight was less like civic d16y5 said. Wouldnt that even be better?

Read my merry-go-round description again.

Read my merry-go-round description again.
Would that be the only difference. And how much of a difference.

He's saying if that the wheel is smaller, the quicker the acceleration and suspension. The bigger, the more torque and power to turn the thing.

Also, the bigger the wheel, the worst the ride quality is. It gets really stiff at times. Say with 13 or 14's.

Would that be the only difference. And how much of a difference.

No one has actually measured it but I know Grassroots Motorsports did a comparison of 15s, 16s, 17s. And the I think the 16s won because they were a bit wider. But that was a track not an autocross.

TO get an idea what the difference between 15s and say 17s would be for example: my Kosei K1 15x7" weigh 14lbs - with 205/50x15" Azenis it comes out to 34lbs. The 17s weigh 17lbs and if I remember right when I weighed my 17s (this was 6 years ago) they weighed 50lbs. That's a difference of 16lbs PER CORNER. Total of 50lbs you are spinning.

And remember my rule for every pound of wheel/tire mass that equals 8lbs static body weight. So....

34*8=272lbs
50*8=400lbs - this would be like carrying one extra person.

And there is no way you can get a 17" wheel and tire to weigh the same or less than a 15" setup. And if you could the weight would be further out from the center, hence more power and torque to turn it anyway.

Are you understanding my merry-go-round example? You can think of it this way too. A figure skater with her arms out will spin slower, when she brings her arms in she is able to spin faster - no change in weight whatsoever.

This ^ might be one of the smartest things I've read this week. Good job being patient too.