I installed an intake, exhaust and 10.9lb wheels with lightweight tires within the first week of ownership of my 2004 MCS. My average fuel econmy slowly increased as the car broke in and leveled off at about 12k miles to 28.5 mpg. About a month ago, I replaced the stock intake, exhaust & wheels/tires since I'm selling the car. The car definitely felt more sluggish, especially from a stop with the boat anchor wheels & tires. However, I wasn't expecting the nearly 3mpg drop in my average fuel econmy in my last 3 tanks of gas!

Maybe the computer just needs more time to adjust, but I doubt it...

 

funny isn't it . i get 30 mpg when driving normally (define normal in an s ) before the mods i got about 27 .

 

That is almost certainly related to the weight of the wheels and tires.

Hope that helps!
Randy