If its carbureted, and you do not have the heat riser from the manifold working, it won't run worth dodo.
You need the air cleaner to be intact, and have the thermal sensor working in the air cleaner snorkel, with the high temp hose connected to the appropriate flange on the exhaust manifold.
Runs like crap, terrible mileage, poor atomization due to the temperature drop through the carburetor venturi creating icing conditions, even in temperatures up to 50 degrees and above if its set up lean enough to get real mileage.
All of this is based on it being carbureted, and I am almost positive Ford had not gone to fuel injection on that model in 86.
'86 Bronco II should have the 2.9L V6 fuel injected motor in it. All Ford trucks were FI by 1986. I had the same motor in a '90 ranger 4x4 with the 5 sp manual tranny. Regularly got 19-21 mpg hiway with the 30x9.5x15 tires on it. I suggest checking all the plug wires and distributer cap. If they are still stock Ford items they have aluminum contacts in them and corrode to a nice white dust. Mine ran and idled smoothly with over 40k on them but the fuel mileage dropped badly. Replaced plugs, wires, cap & rotor and economy came back up.
There is no such thing as a 2.9L carb V6. 2.8 is carb and 2.9 is efi. The spedo is calibrated, seeing as it reads the same as a police radar, so I've been told by the police. and as far as the tires go, I don't see how stiffer tires would promote lower gas mileage. The tires are all terrain with lite tread. I would never ask for basic advice, I've tried everything remedial and common. I'm looking for specific information about the 2.9 EFI cologne motor that would negativly influence gas mileage but not affect performance.
One thing that may not come to mind when troubleshooting bad fuel economy is wheel bearings. The usual test is to wiggle the wheel and see if there's play, but sometimes wheel bearings fail without play or noise, they just drag a lot instead. Jack it up and see if the wheels spin freely and keep spinning.
Could the transfer case or hubs be broken and not totally disengaging like they should?
I don't remember seeing anyone on here with the same engine.
Ok then get some measurement of the water coolent temperature. From what I am seeing on my upgraded ScangaugeII (V3.17) my engine burns less and less fuel at idle as the engine warms up more and more and likes to run at 182-186 degrees normally. Rolling test in neutral is a good bearing test as well as rolling it on a level paved surface - my xB moves really easy with only one foot on the ground while sitting in the driver seat. Spinning the wheels is a great test but even better is looking for heat buildup - friction = drag = heat be it from the tires, brakes or the lube or the bearings to a smaller degree. Usually a IR Heat gun is good for testing for heat rises. Easy way to check is also the tail pipe - soot means burning oil or rich mixture which would give you good performance while also getting low mileage.