You checked with what? GPS is the only way to tell at this point, if you didn't change out your speedo gear in the trans. With a GPS unit you can check actual miles driven and your speed.
If you know the percentage of error, you can correct the calculation. However it's one more thing to fudge up. Not to mention different brands of the same size tire measure differently new, and of course the diameter changes (albeit very little) as the tire wears. I fill until it shuts off, drive (using my GPS to measure miles), refill until the same shutoff, preferably at the same pump, and divide. You can do a partial tank, since you are only measuring the fuel consumed, but the more miles, the more accurate the test will be. Which is a great point to average three tankfuls, as suggested.
