Well, I've been lurking for about a week or two, but I thought I might as well introduce myself. I'm from SF bay area in California, and drive an 87 Toyota 4runner. Everyone I know thinks I'm crazy for trying to make it more efficient, but I'm pretty stubborn, so I continue to work on new ways of getting better mileage, although most are still in the thought stage. Anyhow, great site, glad to see there are other people concerned like me. I plan to stick around for a while.<!--break-->
Well, my mileage (with my 22RE 4 cyl engine) as it stands now is around 19-20 mpg combined city/hwy. '87 4runners were still the original two door, If you're looking at '90 or later you're looking at the newer 4-doors, I think those average probably 17-20 combined, depending on whether you have the V6 or 4 cyl. I'd recommend the 4 cyl, the V6 from those years are notorious for weak headgaskets and a variety of other problems attributed to fairly poor engineering. If you want something '96 or newer, I believe those hover around 16-18 mpg. Any of the engine options on those years are good bets reliability and quality wise.
As for my own high-mileage experiements, I purchased an EFIE from Eagle Research a while back, still haven't applied it to this truck (I bought this 4runner in August, I owned two toyota pickups prior to this.) I also tried building his HyCO- hydrocarbon oxygenator- but the application of that got stifled by some unrelated mechanical issues that arose. I've toyed around with water injection and a few other mods but have not had any results- again partly because I need to refine my applications. On my current truck though I've added acetone for about 2 tanks now, with no mileage gain, and I installed side-gapped plugs last week. We'll see how it goes. I'd like to get that EFIE installed and maybe get some water injection in the truck sometime soon, and continue to test little things here and there.
you should be able to get some narrower tires to start with, then I would do a full tune up, index spark plugs, check compression, find and fix any oil leaks, if it doesn't leak any oil then you might try synthetics, if it leaks oil, and has low compression, then you might try some additive for worn engines.