Chevrolet Repair: 1992 chev full size van-mileage, chevrolet conversion van, cat back exhaust

Question
Hello Van.  I have the same problem with both my 1992 chevrolet conversion van, 59,000 miles, 350ci and my 1997 astro 4.3 awd.  I only get 13mpg with the 92, and 15mpg with the astro.  What can I do to improve mileage, as I do a great deal of over the road driving.  I have tried to find a performance exhaust for the 1992, but have not been successful to date.  Would the 3" cat back exhaust system help?  If no one makes a muffler, I thought I could use a glass pack with an internal 3" diameter tube.  
Are there any computer chips that would help?
What would you recommend regarding air intake?  
Thanks so much Van.  Roger

Answer
Hello Roger,
Most important is keep the tires properly inflated.
Alignment is important also, since out of specs can cause unnecessary drag.
Proper tune ups, clean air filter, make sure the brakes don't drag, and shut the defroster and air conditioner off whenever possible. The defroster runs the A/C compressor.

The cat back could help, but I don't know how long it would take for the difference in milage to pay for the price of the exhaust system.

But with gas going for over $2.00/Gal, anything that helps is great.

I have no experience with chip swaping.

Van