German guy needs help...

fuel economy stuff can be found at... http://www.fueleconomy.gov/feg/2008selemodelf.jsp?year=1986&make=Ford Didn't know if you have a 2wd or 4wd, either way you should be getting about 14mpg +... so thats what... ~16.5 L/100km? However it's an older car so I'd expect less than that just due to it likely needing various maintenace items... plugs, wires, etc, etc...

rust can be an issue with older trucks. If the oxygen sensor has 3 wires, try the following: Weld on a stud onto the exhaust pipe near the oxygen sensor. Mount a ground strap from this stud to battery ground side. Also, due to design, the injectors are subject to a great deal of heat. They tend to gum up badly. Try flushing the injectors. Another thing to check: the brakes aren't seized on, are they?

Actually, the inline 6 from Ford is pretty bad on gas. They didn't get awesome fuel mileage beacause they weren't made for that. The old inline(s) were made for work on the ranch or farm. You could pull just about anything with that truck, but you won't be happy with the fuel mileage. Sure you could give it a badass tune-up or rebuild the engine, but you're not gonna get aweome mileage with it. Also, you didn't say if it's 2WD or 4WD, as that could play a big part as well. Hope this answer is to your satisfaction. Enjoy your new/old truck.

the 300-6cylender in the 80es were more for work than a plesure vehicle.the only advice i can give you is. 1-if it has the big tires on it i know they look good,go to the factory 15 inch tire and wheel. 2-give it a complete tune-up,air filter and fuel filter included 3-you probley have a low geared rearend,change the gearing or replace it with a auto with OD rearend,allof the above will help your gas milage.