Emissions compliance is always the bug-a-boo. Maybe a chip manufacturer can be of help.
Be sure your mileage figures are GPS-corrected. Weigh the vehicle with full fuel, driver and usual load. Chalk-mark a tire, roll one revolution to find exact tire height, and figure overall gear ratio (not just axle gearing). There are calculators on the Net to figure rolling resistance and wind resistance.
For a "best scenario" mpg run, get out on level Interstate, traveling at a GPS-corrected speed (say, 60 mph) after topping tank to first click-off; run 30 miles out and 30 miles back to same station, same pump; fill same way. (Don't top tank until you're 25-30 miles out to be certain of complete warmup.) Ease back onto road, ease off the same way. This makes a good baseline, especially if you have done it more than once. It will also give you info on tuning information.
Bottom-line: Horsepower equals fuel consumption. With those numbers you're looking at big block torque, and, in days of old, a 5200-lb cruiser with electronic ignition and vac-sec carb was 10-12 mpg city and 14-16 hwy. I've seen sharp tuning bring them to 18 mpg, but this was on a high compression (perfect quench head), high vacuum engine where teeny-tiny changes brought it about.
Look to perfect the alignment, keep shocks/springs/bushings new, use synthetic fluids, clutch-operated fan, 195F or higher thermostat temp, underbody air-dam, tonneau cover, etc. Obviously, headers and mandrel-bent full-length exhaust with crossover. A whole lot of changes to inch things upwards.
Best gauge: the less you use your brakes to move through any kind of traffic (smooth, graceful driving) the better driver you are; the longer your vehicle will last; the better the fuel economy.
A dash-mount high quality vacuum-guage (AutoMeter) would be a good tool. Learning to feather the throttle is different in an FI engine over a carb'd one, but it makes a difference.
With that HP number, 15 is pretty good. Achieving 17-18 is spending a lot for a little. Run the figures as a cost percentage over time/miles.
[ March 07, 2004, 09:31 PM: Message edited by: TheTanSedan ]