Performance mixed with fuel economy!?

Our family's then new '69 4 door 318 Belvidere got 21 mpg North Jersey to Fla panhandle. Factory w/ A/C, using American 'Premium' with the timing slightly advanced (like 2*). Automatic transmission. Plain Jane family car. Reasonably snappy thottle response.

It's 65 mph up full throttle acceleration where a 4 bbl and more hp will really shine.
Off the line, its the gearing, traction and torque that will launch it.
Sitting at stop signs, traffic lights, and put putting in parades (or traffic) the bigger engines eat more fuel per minute.
Its all trade offs.

The current 340 in my '67 Barracuda gets anywhere from 6.5 to 13.5 mpg depending on what I'm doing. Longer interstate drives its averaged 15.3 mpg. That's with a 727, 3.23 gears and 215/70r14 tires. Quarter mile here

Tuning process burns a fair amount of fuel too!