bolt a block to the back of the gas peddal so you cant go over a 1/4 throttle is the best advice...lol of course if you need more it could be a problem. your driving habbits will effect everything the most. my 85 i can get between 10-12 mpg highway if i dont try and drive 80mph. headers is a good place to start. a good clean air filter will help. recurving the distibutor ive heard helps a lot. good plugs and wires also. basicall open the exhust as much as you can and a really good tune up.

hope this helps
tony