Poor fuel economy

Hi all, I've had my 1995 GT convertible for almost a year now and I am trying to find out why the fuel economy is always pretty terrible. It's an automatic and while I could normally attribute it's around town mileage to my driving style (pretty heavy footed) I did do a round trip drive from NY to LA last summer and I averaged 18mpg at 70-80mph, using cruise control on the flat parts of Texas and oklahoma. The car is off the road for the winter but IIRC the best tank I ever got was almost 200 miles, I don't think I've ever made it past 200 miles, no matter how hard I've tried. I have a friend that has a 1994 coupe and his gf drives it every day in the summer- he claims it gets 28ish all highway at high speed, similar miles on both cars (125k) both cars are bone stock so far as I know, my car now has wheels and Pirelli tires from a 2008 GT on it, but same exact mileage numbers. His is stock wheels and OEM style tires..

Any ideas why I would get such poor economy when cruising?

Thanks