You're averaging over distance, but to calculate the average speed you need to average over time. If you drove the first mile at 30 MPH and the second at 90 MPH, you'd spend 120 seconds on the first mile and 40 seconds on the second mile, thus giving an average speed of

    (120*30 + 40*90) / (120 + 40) = 45 MPH

Helps to think about it like this: if you averaged 60 MPH on a two mile drive, your trip would last two minutes. Since your first mile was driven at 30 MPH you've already been driving for exactly two minutes. The only way to hit an average of 60 MPH for the whole trip is if the second mile is instantaneous.

Put it this way: If you were to average 60 mph over 2 miles, you would travel those two miles in 2 minutes.

However, if you've already driven the first mile at 30 mph, it's taken you two minutes to drive it. In order to then average 60 mph over the full two miles, you'd have to travel the remaining mile in 0 seconds -- in zero time.

There is a good explanation of this here: http://brainden.com/forum/topic/39-speeding-up/

For your method to work you need to add the time spent at speed, not the miles covered.

In your proposed solution, the first mile takes two minutes, and the second mile forty seconds. That's 2:40 to cover two miles, or an average speed of 45 MPH.

At an average of 30mph, the first mile took 2 minutes (120 seconds).

At an average of 90mph, the second mile took 40 seconds.

So your average speed was 2 miles / 160 seconds, or 45 mph.