analogkid455 -
I made a spreadsheet where I assumed that the cars have the same Cd :
I ignored the O.D. and engine size because they were within or below 5% of being the same. I used the ratio from the final gears to *extrapolate* RPM and MPG at 75 MPH. Before the aero penalty, your 442 should be getting in the neighborhood of 20 or more MPG at 60 MPH.
Hrmmmm, the 442 weighs 3203 lbs :
1987 Oldsmobile 442 Specifications
The Firebird is over 3200 lbs and seems to have a .333 Cd :
Camaro & Firebird Technical Data - ThirdGen.org
1987 Pontiac Firebird & '87 Pontiac Trans Am - ThirdGen.org
I need to factor in the O.D. because that 4% difference must be a negative multiplier.
CarloSW2