Difficulty: Moderately Easy
Step1
You'll need to go to your local parts store all load up. This is a full tune up and then some. I done this same thing to my 1988 full size bronco and seen a big change from 8 mpg hwy to about 15 mpg.
Step2
First change the plugs. Go with a name brand you well pay more but save in the long run. I went with Bosch 4s but I wanted to go with E3's there a newer plug and seem to be the way to go. Also upgrade your spark plug wires. I choose MSD wires. You want the bigger wires like 8.8. After your done with that upgrade your cap with a MSD cap. You should also see if they have a coil for your car and a throttle spacer. One main thing you should do is buy a K&N air filter for your car they help a lot and you never need to buy another one just clean it. When you change your oil try a higher grade oil. This is because friction and heat slow you down. I run royal purple mainly cuz i' m a little hard on my truck. That takes care of your spark and lube. You might try changing your o2 sensor as well. You should also see about a set of headers for your car or truck they will help your motor breath better. If you have a new car see about a performances chip or a programmer. Mainly you just want to get away from stock oem stuff.
Step3
Now for the rest of the things you can do. Try a smaller tire . My Bronco had 33s and I went a little smaller than stock. If you don't mind the performance loss try a smaller carb or injector might sound stupid but some how it works buy the higher grade gas. It cost a lot more but you car will get better gas mileage on it. You'll have to run it a few times before you see a change cuz it will need to clean out your cars motor. After you run threw a tank or two then you can get by with the lower grade. For every 2 tanks you'll need to run 1 tank of high grade gas. Lastly always run Lucas gas treatment.
Enthusiast 
Comments
pmead said
on 8/25/2008 I would not change the size of my tires becasue my car's speedometer has factory settings based on this (mph & rpm). What I reccoment is getting a high flow air filter, high flow exhaust (one that does not create loud cabin noise!), plugs, wires, try to coast your stops, pull the car out of gear when coasting down hill, fuel additaves, computer program, throttle body spacers, intake setup- cold/hot, changing gears..... Just a few more that I can think of
KillerKuztomz said
on 7/24/2008 The only problem with that is ,normaly the larger the tire hight wise the more it takes to get the tire in rotation. More force to rotate= more on the gas pedel= less gas mileage.
Sublime865 said
on 7/23/2008 Go with a narrow larger tyre. Taller tire = more distance traveled per RPM. Same RPMs + longer distance traveled = higher MPG. I have 24 inch tyres (TYRES NOT RIMS!!) on my Audi, up from 22, and I saw a jump in economy, and I went with an extra inch of width too! Imagine if I didn't care about performance!