It depends on how long it's been since the last tune up. You can do most of this yourself for much less than the mechanic will charge. Plugs, wires, oil, these (if worn out) when changed can give you better mileage, but there's no guarantee. Driving style makes the biggest difference.
The drop in K&N made a 1mpg difference in mine, and I never had to buy another filter again. I do have to do the recharge thing once in awhile, but over time, it will save me money. I do have a larger exhaust (2.5" manifold to end) and no cat, so it flows pretty good. All that combined probably made a 1 mpg difference.
You won't see a substancial difference no matter what you do, but you should be able to get 17 depending on your driving habits.
1985 1 ton K-5, Detroit diesel, 36" tires (going bigger when money permits), 4-speed manual swap, Rockwell T-case, Hybrid lift, 12.5K winch.