Fuel mileage is something I've always paid attention to since we drive our classics daily. They have to be at least somewhat respectable on mileage or they'd drive me into the poor house.
These classics can be pretty darn good on mileage, and you don't necessarily need an overdrive to do it. As long as the engine is built to run efficiently, tuned properly and the rest of the cars details are looked at.
My 70 RA Formula, with a 400 RAIII, 400 turbo and 3.31 gears will consistently get 17 mpg highway cruising at 70 mph and 2800 rpm. Gets right between 13-14 around town driving. About as good as any new SUV with it's fancy 6 speed overdrive and fuel injection, cylinder deactivation etc...
I'm pretty happy with it. A simple overdrive would easily put that car at 20 mpg or better.
Our 69 Z/28 with a 302 running a fat 780 holley, M-20 and 3.55's out back gets 14 mpg around town and a solid 17 on the highway cruising at 65 mph which is 2900 rpm. I just recently installed a TKO600 in this one. 70 mph cruising is now 2100 rpm but have not checked the mileage on the highway yet. I fully expect this one to top 20. Around town it's already improved to 15.5 mpg.
The 79 1 ton pickup with a 454 and 3.73's gears out back runs a 400 turbo. This thing weighs 5300 lbs. and is shaped like a brick. 65 mph cruising is 2800 rpm and it gets a solid 14 mpg.
We have a ton of examples here, they are all pretty respectable on mileage.
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