I have had this happen in the past with cars. You get the smaller powertrain thinking it will be more fuel efficient only to find it just the opposite.
I have a 06 GT Vert with 14k that has averaged 19.3 city/sub driving and normally pulls 26.6 on the highway. Just did about 700 mikes to the shore and back in the lft lane on 79 MPH with the ice on and got 26.33
I have only minor mods: Brenspeed 93 octane XCT2, BBK intake and straight through PYPES
It still wont compete with the mileage I was getting from my 06 C6 (32 highway)
Too many car's, not enough time.