Seems low.

In my last Challenger it would pull an easy 17+ mpg (US Gal) in town or on the highway and it wasn't fully broken in.

Rebuilt 71 440-6 with 10:1 compression
Comp Cams XE262 (not enough cam IMHO)
Factory 6 pack intake w/stock Holleys
3.23 SG axle
4-speed

Electronic ignition - I forget what I set the vacuum advance at, but as much as it would stand.

Carbs were pretty lean. Per the other guy's post, they were even worse for economy with the 'as delivered' jets & power valve and the car ran too hot. Fattening up the mixture helped.

If you got into it, yes you could see the fuel gauge move, but with normal driving it was quite decent on gas.