A friend had a 455 in a '68 Olds Delmont 88,with a factory 2 bbl,TH400, and 3:08 gears---he got almost 16 mpg with it putting around town and some highway runs going 55-65 mph back when gas was still leaded..that engine had 10:75:1 compression too though,that makes an engine much more efficient..
The main reason gas mileage decreased in the 70's and onward was they lowered the compression ratio on most engines to less than 9:1 and added EGR ,and retarded the ignition timing and cam timing-- the leaded gasoline also didn't burn as fast as today's oxogenated ethanol gas,it had more BTU's per gallon...it was not uncommon in the 60's to hear of some high compression 327's in Corvettes getting 15+ mpg even in highway driving..
The main reason gas mileage decreased in the 70's and onward was they lowered the compression ratio on most engines to less than 9:1 and added EGR ,and retarded the ignition timing and cam timing-- the leaded gasoline also didn't burn as fast as today's oxogenated ethanol gas,it had more BTU's per gallon...it was not uncommon in the 60's to hear of some high compression 327's in Corvettes getting 15+ mpg even in highway driving..
