I usually ride my new car back and forth to school which basically a straight shot about 60 miles a day. I do not do a lot of driving around town at all. Well today I did a lot of riding around town for Christmas shopping. After about 2 1/2 hours of riding my car made one beep and all of a sudden the gas milage dropped down. Now I had been paying attention to how many miles I had until empty because it was running low and it dropped from 93 to 66. So I decided I better feel up and when I do it usually goes back to around 600 miles until empty but today it only went to 500. I don't know if that beep had anything to do with all of this milage stuff but I was just worried if it meant something more.
The reason the mileage is different is because the DTE is based on the average MPG. When you drive on the highway, you get much better mileage. In town, the average will drop. Thus, it won't go as far on a tank of gas. The Beep you heard was most likely a warning that you were low on fuel. I feel confident that when you get back into driving to school all the time, the mileage per tank will increase again.
I hope this makes sense. Let me know if you have other questions.