Yea, unless you live in a very flat area or somewhere that you can always travel downhill, don't expect over 21 mpg even with a 4-cyl around town. The problem is compounded by the fact that they put ethanol in our gas now and that stations are authorized to put up to 15% ethanol in the gas. Ethanol is not as effective as gas for powering a vehicle, it tends to separate from the gas (and lay at the bottom of the tank where your pump can suck it up) and it attracts water.
In my '00 Ranger I was getting 21-23 mpg around town out of a 6-cyl and up to 31 on the highway, pre-ethanol. Despite my truck being billed as a "flex-fuel" that can run ethanol, when stations started putting ethanol in the gas my fuel economy dropped to 17-19 mpg around town and around 24 on the highway.
For an older truck (like an 88), it might help to give it a good tune-up, maybe run some double platinum plugs, new fuel filter, new air filter, clean the idle air control valve, new o2 sensor, and have your TFI module tested (Autozone I know can test them). I went through all of that on my F-150 just recently, got me another mile or two per gallon and a bit more go. My truck ran, but the TFI module failed something like 4 out of 8 tests.
Those e-3 plugs? I wouldn't use them on anything except maybe a lawn tractor. No joke.