I always start and do step#1..."stock engine out of a 81 Camero"= do a compression test
my step #2 is mount a manifold vacuum source vac guage in the car, test drive it to confirm approx 25Hg plus on a mild downshift with the motor warm = valves and rings are sealing
is it your size 12 foot or the motor causing the lousy mpg....vac guage will tell you while driving:
just to illustrate....
lead foot "drop the hammer" = guage will show 10Hg or less=carb is full rich (A/F 13:1) and vac advance timing is close to none and mech timing is only 24* or so
"squeeze" the pedal gradually = 15Hg-18Hg= carb is optimum A/F (15:1) ratio and vac advance is full on adding 14* or so to the 24*
squeeze the pedal and Hg still drops to 10....it's the motor
(mounted on the dash like mine...it is a "mpg police"= stay in the green zone or else!)