When these cars were new they were known to get about 15 miles per gallon
The only time I checked mine after a complete tune up and a new Edelbrock carb I was getting just that
I would think this is what SB expected from a good running well tuned engine in my case a 64 429
Just throwing this out so you might get an idea of how your own car may be performing only if you have taken the time to be checking your miles per gallon which could be totally irrelevant to many
Just kind of bench mark as to how your own 390 - 429 engine might be performing if and when you take the time to be checking this stat or not
Could be a good indication of how your engine is performing over all and if it might be time for some basic engine maintenance to be done to help improve your gas mileage
MPG is going to be very subjective to the type of driving --> hwy or city or both, flat terrain or hilly, how do you accelerate (do you do jack rabbit starts) and do you tend to speed a bit more than the average driver (I usually go 5 to 7 mph over posted speed but see many that seem to keep their cars a consistent 10 to 15 or more over speed limit). My two Escalades get about 12 mpg overall (I only know this because they tell me), but if I lived a few hrs west of here in the Bue Ridge Mountains or towed cars more than 3-4 times a year it would be worse, but if I did a lot of traveling the MPG would be quite a bit different. When I am freeway driving on flat terrain with either the 2002 or 2013 I can seem from the instant MPG my overall MPG would be much better if I did more highway traveling. I also like to do those jackrabbit starts ... especially next to some your whipper-snapper with their new etro Dodge Charger. I especially have fun when they assume I am going to be a slow poke.