best mileage with my 5th aves (had an '86 and '88) I got was when they were stock with lean burn. I'd get 22-25mpg on the highway, 13 around town, and about 18 mixed.

when I modded the '86 with a mild 360 (stock long block, eddie 600, eddie air gap, comp XE262, headers), & 3.55 rear I got about 11 in town, 15 highway, 13 mixed. I replaced the 998 tranny with an A500 (OD), and that brought it to 11 in town, and about 17-18 highway, 15 mixed.

then I replaced the 360 with the original reringed 318 shortblock wearing magnum heads, the same intake and headers/exhaust as the 360, and had the factory roller cam reground to 259adv/208@.050/127@.2"/.316" lobe lift/.506"@ the valve, and mileage increased to about 13-14 in town, 20 highway, and 17.5 mixed. power felt about the same as the 360....either would hang with a 2005 300C AWD (so, realistically mid 14's car)

tires were either 205/75R15 or 255/50R17 (both 27" tall).

then I sold it and bought a 2011 charger R/T, which is faster, more comfortable, and I've been averaging 23mpg in mixed driving....

as an aside, I honestly think I coulda gotten the 5th ave's mileage up another 2-3mpg had I swapped the air gap/eddie AFB combo (which I tuned with an LM1 wide band O2 for optimal mixure...was way too rich OOTB) with a small t-quad on a modded LD340 intake, or had tried one of the new thermoquad style demon carbs

http://www.demoncarbs.com/Types/Street%20Demon.asp