I had a 94 LT1 Trans Am that got 27 on the highway during the day (it lost 3 mpg at night when the lights were up). With some custom tuning and lower gears, it would have seen 30 mpg with the LT1. Where the mileage will show up with that 4bt Firebird will be around town. People in my family have owned 3 Firebirds and none got better than 14 in town driving.
As far as fit, isn't the 4bt a bit narrower than a V8? You've got to have some tiny, flexible hands to change all those spark plugs. Another thing you wouldn't have to worry about with a 4bt
