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If you have GPS to verify it, then just use the GPS as your odometer. If you can't do that, then use the GPS to give you a scale adjustment for your speedometer. If you can't do that, maybe try using the odometer calibration signs they have on certain parts of highways.
If you're not using the stock tire size, you need to scale the odometer for the actual tire size. Automotive engineers refer to the "rolling radius" of the tire, of which the speedometer and odometer are directly driven by. The rolling radius changes with many things -- load transfer to and from the driven wheels under braking and acceleration, the pressure of the air in the tires, and the tread depth, to name a few. The speedo and odo are calbrated linearly to the rolling radius of the stock tire and thus are only as accurate as the rolling radius is constant. Unless you're using a GPS odometer (which I don't know about their accuracy) I would say it's unreasonable to expect better than 3-4 percent accuracy from any speedometer.
Even your highest reading odometer at 30.3 miles is only about 4.5% off of the "actual" distance, if that is indeed the actual distance. If you actually get 25mpg, you will be reading 26.1mpg. The measurement system is really not accurate enough to do better than that.
On the flip side, the measurement system for gasoline use (the pump) is really accurate to a thousandth of a gallon. The gas companies invest a lot to make sure they don't give you too much gas.
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blackbird 
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