Bad highway MPG, Rebuilt stock 7a-fe Options

[PaukST]

So I just took a trip a few weeks ago from South Carolina to visit a friend in Virginia. Everything went well, nothing broke. But when I did the math for how much gas I used, apparently I only got 25 miles per gallon on the trip. Re-filled, calculated again after the return trip, 25 mpg again. This seems like way worse than I should be getting, even considering how many hills were involved in the trip. I was getting 28 mpg at the time delivering pizzas, lots of lights, traffic, stop signs, everything. After the trip I replaced my plugs, wires, dist. cap and rotor. I'm still getting about the same mileage as I was in my daily driving, so I'm not assuming my highway mpg has changed. So, here are the only other culprits I can think of:

I do have an exhaust leak, from a decent sized crack in the manifold before the first O2 sensor.
Aerodynamics? I haven't gotten my front end fixed since that accident in september, and the bumper cover is at least an inch or two off where it should be. Also, I don't have the fender well plastics, nor do I have the plastics under the engine bay on the car.
Gearing? Tires? I have the 16" 7gc 5-spoke wheels with 215/55 tires, and I'm not sure where those put me in relation to stock final gearing. My speedometer seems to be pretty accurate, so it doesn't seem likely that they changed my final drive a lot, but they are pretty fat, so I don't know about weight or higher drag.

So does anyone have any suggestions as to where my problem is coming from? I'd really like to get better than 25 mpg on the 600 mile drive I'll be taking to visit my family for christmas. I'm probably going to get an ebay header and magnaflow cat (and hope the new header doesn't bring any leaks of its own) to fix my exhaust leak, and I'm hoping to get the front end damage fixed soon too.

[Box]

Well, going from 70 down to 55 is a 15% increase in mpg.