I'm sorry if you don't believe it, but I did do it. Emissions stuff really takes its toll on FE, especially the early emissions stuff. In the 74 Chevy I'd do 27 - 28 on the highway, and I'd usually average 20 in daily driving. The older cars got pretty decent mileage. Even my old 1980 Bonneville wagon regularly got 16 MPG with a 4bbl carb, TH350 tranny,a big block V-8, and a 16 year old driving it. When I bought Rusty I thought Wow, this truck has dual tanks. Imagine how far I can go on dual tanks... Turned out that it needed dual tanks to go as far as the 74 went on one...
In addition to emissions, there's the sheer amount of power available in newer vehicles, required for these purposes:
- decent acceleration with the much larger amount of weight in modern vehicles
- 2wd regular cab 1970 C10: 3,737 pounds of heavy iron
- 2wd regular cab 2008 Silverado: 4,453 pounds of thin sheet metal, hollow aluminum driveshaft, and hydroformed frame
- additional performance for consumers with higher standards
- additional other parasitic loads
Also, consider that modern emissions stuff, while more numerous, is less parasitic. For example, modern catalytic converters flow well...old ones, not so much.
Now, on the weight issue, at least with trucks the extra weight comes with extra capacity, so maybe you could compare a modern 1500 to a classic C20...
If my memory of late 70's GM cars serves me correctly that car came with a 25 gallon tank? My grandfather had pretty much the same car (84 Delta 88 Brougham). Dad & I were fighting each other for who would get to have grandpa's Delta when he was ready for a new one, but then he just traded it in...
Anyway, back on track here... for a 20 gallon fillup on your Delta 88 would mean you got 37.5 MPG. Not too shabby. Even 750 miles over 25 gallons is 30 MPG. All I have to say about that is WoW!
The Mercury Grand Marquis that my dad has will get about 27-28 highway running 65-75 mph. It has the 4.6L with O/D transmission. Of course it is nothing other than a dressed up Crown Victoria. I'd like to take it on a longer highway drive just to see what I could get out of it. I think I could easily break 30 mpg.
Edit: It will get about 24-25 mpg in just normal shorter trip driving, trips of 50 miles or less driving 50-60 mph.