The tires are most likely the cause of your reduced mileage, a set of
235s is the ideal size for a 3 speed auto, I don't know how they would
affect a 16V auto if it were a 4 speed.
I do know that a turbo setup on the 16V makes them run down the road
at better speeds and improved mileage and emissions, Bentparts reported
close to 25 MPG and a big grin doing it
Wild