In 1993 I drove a 65 Catalina 2DR 389 2V TH400 sedan from San Diego Ca. to Springfield Ma. and back. I averaged 17 MPG. My Dad (R.I.P.) went with me on the first leg of the trip and I let him drive for a bit. I took a nap and woke up to him doing 100 MPH! I said why are you driving so fast??? He says: This thing has to be off, it doesn't feel like 100 MPH.
The only upgrades to the car were an HEI and a one wire alternator, halogen headlights. Not one bit of trouble. Gas was around $1.60 then.