Okay I just competed a full trip of 340 miles @65 mph the whole way. the XB 5-speed only averaged 28.5 mpg, w 18" wheels w 35 series tires big and heavy, and intake and exhaust. even if I had the stock 15"wheels there is no way im going to get claimed mileage.
My matrix 180 hp XRS gets better MPH on the frwy ?? proabably due to 6-speed. Go figure. But the XRS I am forced to use 92 octane even in cold weather Ill ping with even mid-grade
I guess 11.5:1 compression will do it every time

 

Does the matrix have 18's too? The mpg is about right it seems.

 

yeah those eighteens are going to throw off your milage. In actuallity i'll bet its better than 28.5 did you use that calculator from the saturn site?

 

oh and just to be helpfull and stuff heres that calculator.
http://www.saturnperformance.com/tech/tiresize/

 

With my stock tires on an xA, I'm getting 34-35 mpg at around 65 to 70mph. When I first got it it was getting about 28mpg after about 1000 miles. Also in California with the crap they add to gasoline in the winter months, it will screw up your milage a little bit.

 

with my xB i'm getting 30 mpg mostly city, and thats with an auto

 

I'm getting 30-31 city and highway combined with mid 30's for highway only. And, I have not added ground wires or a cold air intake (These additions may or may not make a difference in mileage.). Auto trans xB.