Carb and the ratio of your rear end can make a difference. Higher ratio rear end will get you higher RPM's at cruising speeds thus could use more gas. If you have a high cfm carb that will be part of it as well. If it's a Holley carb it loves gas like no other.
I have a 350HO 4sp with a 850cfm carter thermoquad with small primaries and large secondaries. I also have a 3:73 (or close to that) safety-track rear end. I have been able to get about 25mi/imp gal on the highway at 70mph @3000 rpm. imp Gals are slightly larger than US Gal.
But if you hammer it you can watch the gauge move to Empty in real time!
The carb is not stock but he has more power and better mpg than the Quadrajet. However, I'm moving it all back to stock and just drive slower.
Engine Test Stand Playlist: https://www.youtube.com/playlist?list=PLwoxyUwptUcdqEb-o2ArqyiUaHW0G_C88 restoring my 1968 Firebird 400 HO convertible (Firedawg) 1965 Pontiac Catalina Safari Wagon 389 TriPower (Catwagon) 1999 JD AWS LX Lawn tractor 17hp (my daily driver) 2006 Sequoia 2017 Murano (wife's car) 202? Electric car 203? 68 Firebird /w electric engine 2007 Bayliner 175 runabout /w 3.0L Mercuiser__________________________________________________________
