Skyline, it is the TIRES circumfrance, not the rims that determine the change in the speedo reading. Go to Advanced tire or similar web site that has a calculator that shows the different speedo readouts between two different sized tires.

What sized tire did you go to?

One of the guys here drag races - and didn't he go to smaller rims/tires to save weight? Seems familiar. Was it Bigjacksauto about two years ago?

 

i went stock LS size to stock SS size..215/55r16 to 225/45r18 i think they are

 

i checked those speedo sites and there was hardly a change between the two..im just shocked that it brings down the engine power that much

 

I had 215/50/17 tires, & up-sized the tires to 225/50/17

But the tires I chose are 2 lbs lighter per tire.. No regrets here.

 

this is the change i made as well that is why i'm saying i'm not sure how performance would be improved with 20's. I was on tire rack comparing wheel weights and about in any brand of wheel the 20's were about 5-6 pounds heavier than the same wheel in an 18" version.

 

Tire tread has a lot more to do with it than you yourself seem to think. My old tires on my 16's were non directional, well actually they look half directional until you find that you have to run them in the opposite direction on one side vs the other. They were sidewall specific insted of tread direction specific lol. But my new wheels are directional. Less rolling resistance makes a huge difference. Also 5-6 pounds makes huge difference to humans (think bicycle) but to a car it make less of a difference. But yes it does still make a difference but then rolling resistance can end up making a bigger difference.

 

i see what you are saying...I went form stock 16" wheels and factory equipped tires to 18" cobalt SS wheels with really fancy Michelin Pilot Sports (directional) still lost mileage. I think it might be outlandish to think that tread pattern with make a difference 2-4 mph. I do fuel mileage testing for a living (6 years) and we have not done any tread pattern studies. Pretty sure it is safe to say that weight is the key factor because we are not changing the aerodynamics of our cars.

 

So you are saying that because you haven't tested them it means that they can't make any difference? LOL. Also I am talking about normal driving stop and go situations here not driving at consitant speeds lol.

When you compare the same wheel of coarse the larger wheel will be heavier. But I am talking about different brands or models having different weights. Many times yes the 20's will be heavier but between 18's and 16's of the same or atleast close width there are many on both sides of the fence. Bottom line for what my point has been here is that just the 16" vs 17" vs 18" does not automatically make the larger rim heavier. Especially when the tires may weigh less for the same overall height tire. But I can even find some 20's that weigh less then some 16's not to mention 18's.

 

Extreme examples here but here is a 18" that weighs 17.8 http://www.tirerack.com/wheels/Wheel...19&sort=Weight

and here is a 16" that weighs 23.7 http://www.tirerack.com/wheels/Wheel...25&sort=Weight

 

now you are comparing apples and oranges for sure 400 dollar wheels vs 100 dollars wheel I sure hope if I pay 400 dollars for a wheel it is light