I don't get it? I only got 3 more MPG???

[rmcelwee]

1991 Miata with a supercharger. Yeah, I know it isn't the perfect car to experiment with but I did it anyway. The car is normally driven VERY hard and I keep it in boost as much as possible. Over the past two weeks I kept it out of boost, shifted around 2500 rpm (redline is 7500 in a Miata), and drove around 45-50 MPH to work instead of my normal 65 MPH. After 213 miles (not a great experiment but again I was just playing) I check my MPG and saw it had risen from 20 to 23.9. I figured that I would have been close to 30 or so. Any idea why my mileage didn't jump more than that? The car runs pretty rich but I figured staying off the supercharger would have at least given me +5MPG.