You should still be a ble ot use a conservative number like 18mpg to determine when you should refuel. So after 200 miles, you'll want to look for some gas. On the freeway, you should still be a able to go 250 miles.

For improving your mileage. The usal things such as air filter, spark plugs and clogged injectors can cause this. Be sure to use regular 87 octane.

Most of all how you drive makes a big difference. Look down the road, don't race up to lights, and use the gas pedal very smoothly. In most cities the traffic lights are timed so no matter how fast you go, you'll get there in the same amount of time... just drive the speed limit. Make only very small motions when holding a steady speed, let you car slow down a little going up hills and speed up on downhills. Keep you speed when possible over 35 mph to get it into 4th gear, and stay under 70mph. Follow all of these closely and you can imporve yoru mileage by at least 10%.