Well this is a late entry, but I'll add my two cents anyway. My basically stock GMC 270 with a granny gear 4-speed and 3.08 gears in a 3500 pound Suburban gives me 14 to 16. About the same as my basically stock 350/350 with 3.70 gears in a 1/2-ton pickup.
I have a 64 chevy biscayne that I drive every day. I ran the original motor w/ the weber 2 bbl and split manifolds. That motor was burning oil so I pulled a low miles one from a 69 nova. It has the mono jet carb(rebuilt) and the stock exhaust manifold and is getting better mileage the the other set up. 18mpg.
Don't care as long as the gauge keeps working. I burn gas for the fun of it. Out of curiosity I checked my truck (weighs 2800) with 250 chivvy a year or so ago. Rochester B pot bellies-twice on Offy manifold, single straight pipe, 403 rear, mostly around town, calculated 17.
Ford 300 CID in van got about 14~15 highway with the original Holley 1 barrel. Upgraded to a 400 CFM Carter 4 barrel and highway mileage jumped to 22-23. Then we got E-10 in Missouri and mileage dropped below 20. Engine was pretty much bullet proof. in 420 K miles, no significant problems other than normal maintenance and ignition modules. Changed the *&^%$# ignition module enough times to carry a spare. Head was never removed from engine in 420 K. Finally scraped after it rusted out the third time (don't you just love the idiots that salt the roadways in the winter?)! Replaced it with a vehicle with points and condenser! Points and condenser last as long as the *&^%$# electronic ignitions, and are a bunch cheaper! Jon.
Back in the days of the 55 mph speed limit, I had a Gremlin 232 3 speed with a high mileage cam and stock carb. It would get 28 mpg at 55-60 mph. The Gremlins had 22 gallon gas tanks, that 550+ miles/tank would bust dang near any bladder. Bill
The Jalopy Journal
