Welcome to EM...
The Suzuki likely has an cast-iron block, which can add to the weight of the wagon (which the Sube probably didn't have). It's a cost-saving technique. I rented a Forezna Sedan one humid summer months in Florida. It had a tough time keeping things cool and really robbed power -- that may be another factor in reducing FE.
What kind of driving do you do? Average top speeds on highways, terrain, etc?
Plus, the gearing is probably setup for additional performance per "U.S. Standards".
The wagon will likely be very rewarding for its utility. Check out the advice on the 104+ tips page and given them a try.
Best FE to ya
RH77