Difference in MPG between a car and truck

[Inventive1]

I'm interested in an older model vehicle for a summer time daily driver. I've considered a 67-72 pick-up or a full size Chevy from the same period. Keeping it stock would not be important, so an overdrive transmission swap, up dated brakes, etc. would be done to improve drivability and safety.

Trucks generally use more gas than cars, presumably because they usually have lower gears and more air resistance. Lets say the car and the truck have the same engine, transmission, AND rear axle ratio. Assuming these vehicles are equal as far as possible, does anyone have any idea how the gas mileage would compare?