I had a 318 87 150 longbed pickup with 2.94 gears and I got 16 in town. It had taller tires so it was more like a car would with 2.20 or 2.45 gears and more weight. And for the highway it would get 19-20 and it only had a 3spd 904 and worse aerodynamics. With a more optimized engine and drivetrain I really don't think 20 in town and even 30+ on the highway is too much to ask for(with sane driving habits and a few other car tricks).