Currently, I average about 30~31, (mostly city driving, probably 40% city, 60% highway I think, odometer doesn't work lol.) a tank in a 1.6L D16Z6 VTEC engine in a 94 civic. I have the automatic transmission so I know that isn't exactly a world wonder for gas mileage, just recently had the transmission rebuilt. Is there anyway to improve my gas mileage in this vehicle or is that really the average in these cars with that engine and transmission?
Welcome. There are many ways to improve. What do you currently do to save gas?
There are things you can do with driving style (these are where the best improvements come from), minor modifications that are easy, and more difficult modifications.
To start with, you can try increasing your tire pressure -- don't exceed the limit stamped on the side of the tires, and if it affects comfort/handling badly you should back it off until it's comfortable and safe.
For driving style the first thing is to drive like you have no brakes. If you're going to need to stop, get off that gas pedal right away. If you see a red light try to go slow enough that you can cruise through after it turns green without having stopped. Every time you use the brakes, you discard energy that you already bought by burning gas.
Well, I've already pretty much done the part where I can buy a few things to help it out. I have a K&N Cold Air Intake and the car came with 17 inch rims (thinking about getting rid of those and going back to stock tire size/rim). I use 5W-30 Synthetic oil in the car. I am thinking about cleaning it out (I have 2 spare tires, both are in great shape, but last time I checked I only need one in the case of a blowout) and getting random crap out of my back seat. Should I have my car looked over mechanically once more? (I had it looked over by Honda when I originally purchased the car --, it needed a few things done to it like new belts and new CV axles, but other than that, its a good little car.)
Stock tire and rim size will probably help. Low-profile tires tend to have more rolling resistance.
Normally I suggest that cleaning it out and removing extra weight won't help, but most of the credible reports of it helping are with small older Hondas, which seem to be particularly sensitive to weight.
The K&N intake, OTOH, probably is not helping fuel economy unless you always take it up to redline at WOT (and always drive in the city). The difference between an OEM intake and an aftermarket intake is at maximum airlfow, when the engine is making the most power it can. The stock intake has to be able to supply enough air then, so it's always going to flow freely at lower RPM or less throttle.
A modification that works for many people is a WAI (Warm Air Intake) -- they rig it to heat the intake air. This is quite the opposite of a CAI.
Maintenance that hasn't been done can certainly reduce your FE. There have been a few discussions lately about how much cleaning clogged EGR systems in older Hondas helps.
I don't know much about the auto tranny's that come with the EX but I assume, (like the manual EX vs manual DX) they may be on the sportier side than the DX Auto... meaning shorter gearing and less MPG.
Have you ever attempted Pulse and Gliding? I know it sounds ridiculous for a beginner hypermiler but it's not as bad as you think. My P&G sweet spot is 50 to 60 mph. Read this for more info on P&G
Also look into getting a grill block as that will help conserve heat and improve aerodynamics.