Here are my sugustions for spending 500 $ to get better milage.
1. Good exhaust should be your first purchase.
Single 3" (like stock) with a good lowback pressure muff and a high flow cat. The stock Cat is pretty restrictive and plug up as they get older.3"
Flowmaster 50 or 70 series muffs are good or for less money Check out a 3" Summit or dynomax muff.
2.MSD blaster 2 coil(for 87 &up GM). These will bolt in and plug into your wireing harness like stock. The hotter spark will allow you to open up the spark plug gap to .050-.060" that will help milage a little.
3.Go to
http://www.turbocity.com(their website is down right now but should be up soon Ph# 714 639 4933)
They have a TBI performance page. Get the Air flow enhancer and throttle body spacer spacer. These improve throttle response and boost air velocity at low speeds They will help with around town and highway gas milage. If you install a TB spacer you may need to re- adjust the TV cable. raising the TB more than 1/2 inch will throw it out of wack.
3.Air filter: K&N. or for paper go with AC delco. Stay away from Fram or Wix they are the worst flowing filters on the market.
4.Keep your tires aired up and check regurlarly. Do not excede max pressure. properly aired tires reduce rolling risistence= better gas milage.
5.Change all the fluids...Engine, tranny, Xfercase, and diffs, and keep them clean and changed regularly use synthetics if you can afford them.
6.Use at least 87 octaine gas. TBI doesn't like less than 87. 89 octaine is the best. Also a can of injector cleaner once a month or two is a good idea.
Tune it up!!!! Ac Delco plugs, 8mm plug wires, AC delco or aftermarket performance Cap and rotor. Check all vacume lines and throttle body gasket for vacume leaks.
Doing these sugustions and proper driving will increase the fuel milage significantly. you will also get a nice boost in performance.
One of the reasons I like my K5 is it gets better fuel milage than any fullsize 4x4 I have ever owned.