I found a site that gives detailed fuel economy information for vehicles at specific speeds. Here's one for the 3.0 Litre Dodge Caravan I sometimes drive:
2000 Dodge Caravan 3.0L V-6 fuel economy analysis (model for North America U.S.)
And here's one for the 84 Caprice Classic Sedan I drive (With not as much info):
1984 Chevrolet Caprice Classic Sedan 5.0L V-8 automatic overdrive fuel economy analysis (since September 1983 for North America U.S.)
I wasn't sure if I could post the graphs here legally, So I didn't.
Also, If searching for vehicles on that website and you want to get to a page like that you need to click on "see detailed fuel economy analysis" Under the vehicle's description, Not one of the big buttons.
And whether or not this site is quite accurate, I don't know. It might be a bit optimistic.