Daughter needs Gas mileage from 273 commando

my 78 aspen rt came with a 318 4speed od with a 2.96 gear(with the lean burn ign) it got a whole 15 mpg .took it back several times to dealer with fuel mileage complaints was to get 20mpg according to advertising .so after warranty was over I switched the intake to a ld4b/ 650 holly off a 429 ford truck/ 273 solid cam by trw with a .424 lift and 228 deg intake and exh. at .050/ put used 340 valve springs in dropped in a accell duel point race dist with no vacuum adv. and had the local exhaust shop put duels on it. to by surprise the first time I went on a road trip to oc md.( all flat ground) I started cussing at the wife because now the fuel gauge was broke the needle only moved a little off the over fill mark. so when I tried to fill the tank it only took 4 gals. in 120 miles so now I'm under the car trying to find out why I can't get more gas in it. so I get home and its 4 more gallons, so next thing I do is pull the tank to fix a problem . the tank was still full . this car never got less than 30 mpg after that the fuel savings made the car payment /so why didn't they just keep making them like this with the added fuel inj. would probably get 35 mpg. so hot rodding doesn't always hurt fuel mileage.