So here's the story. I bought a 1986 Eddie Bauer Bronco II(I know, first mistake. Just listen). I was not lifted, but had 30 inch tires, and was gutless as all things unholy. So I was getting 10- Miles to the gallon. I decided to take a closer look into things. The previous owner had removed the factory air box, and slapped a K & N filter on the end of the tube, obviously he had to wrinkle up the tube to fit the filter on. I went ahead and got another factory air box, but cut the top half of it off so I can have a little more aspiration in addition to the cold air from the Factory set up. I then used a larger fitting to eleminate the obstruction in intake. All tires are inflated to just under 40 PSI. Transfer case is not engaged, and hubs remain unlocked. Engine runs great, and is properly cooled and lubricated. How could I possibly be getting 12.6 miles to the gallon?
I think the problem is your tires. There is no 30" tire setup for fuel economy. Put the size tires that's stated on the label inside the driver's door jamb.
Also, your economy could be better than what you think. Many times people do not change their speedometer gear when changing their tire size. It's early in the morning here, but if my brain is properly engauged I'm thinking that your speedometer and odometer would read low with oversize tires, which will translate into you entering less miles than you actually traveled into your mileage calculations.
To test your calibrations use your GPS. Get on the highway and set the cruise at 60. Look at your GPS. I'll bet your GPS will say you're going faster. In any event, it usually doesn't cost much to have a speedometer calibrated if you don't want to replace the tires.
Also note that I don't think that unless that vehicle is running in perfect condition that you'll get much more than 15 or 16 MPG highway without some serious mods or extreme driving techniques.
HolyCow can chime in on this one when he wakes up, he's better at all the mental tire math than I am.
Jay's tire math is correct. If the tires are larger than those that came on the vehicle and the speedometer/odometer were never adjusted, you are going more miles than your odometer says.
I'm usually the first guy to suggest higher tire pressure, but 40psi in those big tires on that little SUV might be too much. How's the ride and handling?
I'm pretty sure you're equipped with a carburetor. Is your mechanic good at tuning carburetors properly? That'd be the first place I would look for missing power and terrible FE (fuel economy). Then there's everything else. That's a 23 year old vehicle, anything and everything could be wrong.
I bought one of those new years ago and it got much better fuel economy than that. I think it was about 20 on the highway but most of my driving with it was highway and not city. I bought it new and kept it for a long time as it was a really good truck. It had 140,000 miles or so on it when I sold it and it was still running fine. Mine was all stock with stock size tires though. I'm sure that made a difference. I also did not do any modifications to it. It sounds like the problem is not with Ford, but with what the prior owner did to it.
Another test would be to drive 60mph and see how far you travel in 1 minute or go 30mph and see if you go a mile in 2 minutes. That will test the odometer pretty well. You could be a few tens of a mile off in a mile distance. Basically you have a fairly inefficient engine so don't expect too much out of it unless your cam timing is off. Coasting test and idle fuel consumption test would be a good start to figure out how much it burns doing nothing. Then maybe some idle ignition timing work could help a lot - get a faster idle by advancing the timing at idle and cut back on the throttle stop or air bypass screw which will help FE - just don't expect factory timing to be where the best miles per gallon is going to occur.