LOL, yeah you could have reset it by pulling the battery cables for a minute. Or pulling a fuse but I don't know which on a '96. If not reset it takes several driving cycles before clearing itself.

And Sherwood's right. You need to calculate MPG properly. 270 miles but HOW MANY GALLONS? Don't trust what you think is the size of the tank, when you can just read the actual gallons off the pump at the gas station.