mpg on my 340

Our '69 Formula S FB has a stock 340, 727, P/S and manual drum brakes. The original owner had the stock rear gears changed by the dealer to either 2.91 or 2.76, but I have never checked to find out and think they are 2.91. When we first got it 20 years ago it would routinely get 20 mpg on the freeway at steady 60-65 mph and we were absolutely astonished. That was on pump premium before we had ethanol in the PNW. We don't drive it much on the freeway now, but when we do it gets about 18 mpg on the local 92 UL clear non ethanol gas. The change may be due to the different gas formulation now, I don't know.