You'll need to come with a little more info than that.
For starters, what is "old"? Is a 90s truck old? Or are you looking even older?
Do you need full size or is a Ranger/Courier acceptable?
Does it have to be 4wd or is 2wd acceptable?
Will the engine be modified or are we talking 100% stock?
I clarify all this because otherwise I'd say the best fuel economy from a Ford truck is going to be a 2wd Ranger with a 2.3L 4 cylinder. The 2.3L is about as small as the Rangers ever came with although I've heard rumors of a 2.0L diesel Ranger even though I've never seen one myself. And if you go back pre-83, the Ford Couriers came with engines as small as 1.8Ls. In stock form, I'm sure their fuel economy was far better than ANY of them are going to do today unless the engine's been rebuilt. So unless you are talking about a 10 year or newer vehicle, a discussion of fuel economy is likely to be more academic than practical without more specifics.
Now if you are talking full size, about the best fuel economy you'll find is F150s with the 300 L6 and a manual T18 transmission...in stock form. I don't recall if F150s ever came with anything less here in the US. And if they did, I would question how safe they are to actually drive on the road. I recall riding in a family member's 90s F150 with a 300 L6 and it didn't have power for nuthin', but it drank gas with the best of'em. Best fuel economies I've ever heard from older F150s was in the 17-18MPG range with most being 16 or less. The new ecoBoost V6 F150s are getting over 20s on the highway, but I don't think that was even close to reasonable expectations with anything made in the 90s or back.
89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA