When we first got married, I had a 65 Olds Delta 88 with a 425 and 12 to 1 compression ration. She bought a brand new 73 Olds Cutlass with a 350.
We drove together over a thousand miles to our new home. Everytime we stopped to get gas the old 65 took less. It did have to have premium tho. The 73 took regular.
The weights of the 65 88 and the 73 Cutlass were similar... the low compression 73 had poorer mileage because of the low compression , a rich mixture to avoid oxides of nitrogen and a low performance cam , very similar to what would have been used in a pre-70 2bbl olds 350... but ground with a few degrees of retardation....