I filled up my '70 with gas this morning.

[dochawk]

a note on gas quality . . . "Top Tier" is from the car manufacturers, not the oil & gas companies.  It's a standard for levels of detergent and such that Cadillac, the Japanese, and German manufacturers came up with as compression was climbing (i.e., Northstar).   Anyway, it's an indicator that you have quality cleaners.  I'm not aware of any brand that saves the best cleaners for premium anymore (but there might be).

As for mileage, I once got 16mpg in an Impala 400.  It at a two barrel on the 400cid engine and turbo-400 transmission.  We drove a steady 60 from San Jose to San Diego.

[MeToo]

I can get 12-13 MPG highway in a '76 FWB cruising at 65mph with 98 octane fuel and moderate AC use

["Cadillac Kid" Greg Surfas 15364]

Keeping "out of" the secondaries and maintaining a steady speed I usually get 14-16 MPG in my 76 CdV at a steady 80 between San Antonio and Corpus Christi, a 150 mile jaunt on nearly all flat roads.
Greg Surfas

[The Tassie Devil(le)]

When I was driving my '72 around USA and Canada in 2008, the best I ever got was 18 MPG, and the car seemed to be really nice at around 75 to 80 MPH on the Massachusetts Turnpike, not using Cruise Control.

And this was accomplished with an engine that had the nylon missing from the cam gear as it turned out.

Bruce.

[James Landi]

Bruce,

You must have had a good tail wind, it certainly helps to get you those extra miles per gallon.  With more modern Cadillacs with 4100's out of the early '80's, a headwind made a significant negative difference in gas mileage (as well as one's ability to keep up with highway traffic)!  Happy day, James

[Cadman-iac]

  The best I ever got out of the 69 Deville I had back in 86-89, was right at 16 mpg at 75-80 mph between Tucson and El Paso and back. That's going over the continental divide near Deming New Mexico.

[Barry M Wheeler #2189]

I use non-ethanol Marathon Premium for my 4.9 '91 Seville. It has new injectors, and yesterday, turned over 31,000. I haven't punched the button lately to see what my mileage is now as we drive quite a bit less since the Pandemic hit and also, the past year, we had fewer health issues to make 50 mile round trips to "town."

While it's nice to get "better" gas mileage, I agree that when you need fuel, simply go fill it up and don't worry too much about it.