Does anyone have any thoughts on how much difference there would be in the gas mileage between a full size car and a full size pick-up truck if both vehicles weighed the same, had the same drivetrain, the same gear ratios, the same tire size, etc.

Probably a mile or two difference with the car doing better because of aerodynamics.

I put the complete drivetrain from a '77 Gran Fury cop car (440, 727, 3.23 rear) into a '76 Club Cab.

Only changes when I did the swap was using (stock) electronic ignition to replace the Lean Burn the car had and I used headers.

Engine ran better with those two changes, but, as Stumpy said, lack of aerodynamics killed any real increases in MPG so it stayed about the same as what I was getting in the car.

HOWEVER, strangely enough, the 440 got better mileage all day, everyday, than the 360 2bbl it replaced. If I remember correctly, I never got above 12 MPG with the 360 no matter what I did to it, or how I drove and it was real easy to get it to drop into the single digits. The 440 averaged 14 MPG and even got close to 17 on a cross country jaunt. The only time the MPG dipped down to 10-11 was when I enjoyed the wail of the TQ too much.

Depends on Era of Fullsize car. My old 5th ave got about 20mpg after lean burn delete. Just did it to my current one and have some more tuning to do.It was getting 13-14 with the crappy lean burn. My trucks get 10 or less

I've had multiple tire sizes on 2 and 4wds and result were "Still sucks!

If the pickup has 4wd with lockable hubs the MPG penalty is slight, but the "Center Axle Disconnect" Dodge Ram system hurts by at least 1.5 MPG at 70 mph highway speeds.

If you stand directly in front of both the car and truck and take a digital photograph at the same distance, then blow this photo up onto 11 by 17 inch "Ledger" size paper, you can cut out the outline of each vehicle and weigh the paper, from which you can then get an accurate estimate of the square feet of the frontal area, which is used in calculating the aerodynamic drag.

Most pickup trucks have at least 25 percent more frontal area than full size cars of the same weight, and their "Coefficient of Drag" commonly abbreviated as Cd is also about 15 to 20 % worse too.

1994 to 2001 Dodge Rams are more aerodynamic than other trucks of their age
and have Cd of about 0.42 for a 2wd Clubcab with frontal area of 34.5 square feet
up to about Cd of 0.50 for a 2500 4wd regular cab Ram with frontal area of about 38 square feet.

For comparison a 2012 Toyota Prius has one of the lowest Cd of about 0.26

A Kenworth T600 had a Cd of about 0.35

if you're talking drag, it must be noted that the underside of the vehicle has drag as well, and pickups seem to have a much messier underside than cars. Also it's up there a bit which lets more air get under (negative ground effects).

R.

A little hijack here but in the same vein... I drive my 01 3500 Cummins/auto/quad cab instead of my Chry Pacifica due to better fuel costs
Pacifica = $80.00 every 4 days - Full tank
Dually = 130.00 every eight days - full tank
same amount of miles