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 *&^%$# ignition module enough times to carry a spare. Head was never removed from engine in 420 K. Finally s****ed after it rusted out the third time (don't you just love the idiots that salt the roadways in the winter?)! Replaced it with a vehicle with points and condenser! Points and condenser last as long as the *&^%$# 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
