Small Block Stroker ... Gas mileage?

67 b-body satellite, 408 stroker, 20 miles pr. gal. has run a best of 11.93 @ 111.4 mph in the quarter.
91 octane, with a half can of lucas octane booster.
Revs. 3000rpm @ 60mph on the hwy. with 4.30 rear gear, M&H 275/60/15 rears.
j-heads ported polished, extreme energy solid lift cam, .502/.511. Crane 1.6 aluminum roller rockers.
M-1 single plan, 2" open spacer, 750 HP/DP. MSD 6AL, 1 5/8 headers.
9 1/2" dynamic converter that stalls at 4200 yet is totally streetable with a large trans. cooler, @ electric cooling fans on the rad.
9.5 compression keith black pistons, eagle I beams, eagle internal balanced crank, internal rotating weight only 1800 grams.
This same set up in my 360 got 18 mpg, best 360 et was 12.56@107mph.
In the end the 408 is obviously more efficient at cruising speed and quicker in the 1/4 by 6 tenths..