Here, you dropped these:

. , . : ; , .

The bel air probably doesn't have any emissions stuff. That's why it gets better gas milage than you expect, whereas the newer stuff absolutely does. Plus trucks are geared for torque, unlike a car, which reduces mpg. A more apt comparison would be to older ('50-'70s) trucks, where you were probably talking more around 9-10mpg without emissions.

Also remember that those trucks are both heavier and less aerodynamic than your Bel-Air. A well-equipped Tundra or Silverado is pretty close to the weight of your Bel-Air and Z3, combined.

I guarantee that your 70s motor isn't really all that eco-friendly. Once all the emissions stuff is warmed up, those trucks are honestly pretty "clean" in regards to tail pipe emissions compared to even a car 20 years ago. Remember that CO2 and water aren't the only things emitted, and while they're worse on the CO2 front, anything without a catalytic converter is going to emit some pretty nasty stuff in addition to all all the CO2.

I used to own a 2012 Jeep Patriot. I swapped the air filter, fuel filter, spark plugs, plug wires, and coil pack. Chrysler sent technicians to me to verify that, yes, as long as I kept the tach between 1000 and 2000, I was getting 35/50 mpg.

I figured this out because I was a delivery driver at the time, and managed to go from Lexington, KY to a town in northern South Carolina that I cannot remember the name of, on one tank of fuel.