Unfortunately, these trucks were not designed with fuel economy in mind. What I have to say below, considers that you use your truck primarily as a road vehicle and not primarily as an off road vehicle and the engine is in good operating condition. An off road vehicle doesn't really have much if anything to do with fuel economy. Everything changes when you are setting up a real off road vehicle. The Bronco was designed as a compromise between the two functions, a daily driver and a family fun vehicle on the weekend. Modifying it one way or the other has a direct effect on it's marginal gas mileage. Also, keep in mind it was designed when the cost of gas was not a problem for most people and it was comparable to other vehicles of it's time.
1. Converting to an electronically controlled electric fan will take some drag off the engine. The least expensive conversion is getting a fan from a junkyard. You should be able to get a Lincoln Mark VIII fan for about $30.00 and a control will cost you another$70 - 80 bucks. You can see how I did mine by clicking on my supermotors link at the bottom of the page and then click on fan conversion.
2. Reduce the weight of the vehicle: If you don't really need to carry a bunch of stuff in the back, take it out. If you don't use the rear seat, take it out, put it in the garage, and only reinstall it when you need it. Anything that you leave in there for weeks on end without moving except to push it out of the way. I realize that these two may not be possible for some people. This is especially helpful if you live in hill country, dragging around all that extra weight costs when you are starting out and while going up hill. I also remove the rock guard from the transfer case. (weighs around 50 lbs), it is easily re-installed when needed.
3. On the highway let your cruise control do the work, it is less spastic than a human driver who accelerates and decelerates all the time.
4. Turn that A/C off and use the (3/60 AC). I personally don't have A/C in my Bronco and I've found you can survive without it. Again that is a personal thing, but it keeps the drag of the A/C system off the engine.
5. Keep your right foot off the floor board and stop racing the guy beside you to the next stop light.
6. Use the proper weight oil for the conditions you are operating in.
7. Don't use those 12" tires. The wider the tire the more rolling resistance your engine has to overcome. If you gotta be "COOL" you ain't saving "FUEL". The factory size wheels will most likely give you the best mileage.
8. The 302 and 351 have well known optimum "gearing vs tire size combinations" that produces a cruise RPM that gets the best mileage. You can find that chart in the Bronco Graveyard catalog. The only problem with their chart is it is calibrated to a 50 MPH cruise. However, they give the formula so you can figure the combination that works best for the speed you normally cruise at. If you go up in wheel diameter, then you need to adjust the gearing to get you near to the best cruse RPM.
9. If you have increased the size of the tires, did you remember to recalibrate your speedo so that you get accurate numbers when calculating the gas mileage.
10. That C6 is not a very economic tranny. When I have to change mine, I'm going for an AOD.
DO NOT waste your money on fuel additives. Some have a small effect on carbed engines but controlled and practical testing has shown they have no or even detrimental effects on EFI engines.
Keep in mind that anything you do to boost the HP, costs you in mileage. The two things mentioned above (fan and A/C) don't boost HP, they reduce the total HP required for a given set of driving conditions. It still requires "X" amount of HP to push your truck along at a given speed. It is just that you don't have to develop any extra HP to operate them which reduces the over all amount of fuel required. They may seem to increase the performance of the engine because what was being used on them is now available for the drive train. But if you are not easy on the peddle you may just negate any savings they provide.
I bought an '83 XLT new with a 302/C6 running 31x10.50x15s and the best mileage that thing ever got was 13 mpg. I used to joke that it got 13 mpg climbing up the Sierras and 13 mpg coming back down. You might consider a higher set of gears. 4.56s are pretty tall gear sets and have to impact your mileage. Particularly with a 3 speed automatic like the C6. If you aren't doing any rock climbing a set of 3.56s might be more appropriate highway gears.