Adding to the rotating weight on a car is the worst place to do it, on the order of ten times worse. The HCH comes with very lightweight rims and LRR (Low Rolling Resistance) tires. Adding 10 lbs per wheel has been known to lose 10 mpg overall on the HCH. Loss of the LRR tires will lose about 3 mpg I hear. Don't know how much more the stock EX tires/rims weigh, but it is a safe bet you will lose FE if you switch.