As you put more and more miles on your vehicle, it will be harder and harder to change your average.
For example, I live 3 miles from my job. Considering that this short trip is on 30 mph roads with plenty of traffic lights, I probably average about 15 mph if I'm lucky (accounting for the traffic lights)
My 4Runner doesn't even have 2,000 miles on it yet, but it has already been on a 7 1/2 hour trip to Pittsburgh and back, and I was doing about 70-75 mph on the highways. Also, I've been driving every day about 120 miles round trip for a two week training seminar and averaging around 70 mph on the highways. Yet my average speed readout is 33 mph.
The point is that as your mileage goes up, the average will start to stay more constant, regardless of your driving speeds.