change in guess gauge: problem or anomaly?

Has anyone run into this: three fillups ago the gauge started acting differently, and has since. No change that I can detect in the other systems, the way it runs; no change in the way I drive or fill up. And I have a couple year's experience and nearing 40k miles.

The gauge seems fine at first, slipping down a pip at 110-140 miles depending on how well I filled up. After that they go down every 25 or 30 miles. I never paid such close attention to these in the 'middle' that I have hard data, but I was always above 1/2 at around 300 miles (or more) on a tank when I got a good fillup this time of year and now it is always down to 1/2 at 240 to 250. The pips continue going down quickly until I get nervous and fill up. I started calulating mileage after I noticed the issue.

Mileage has been way below the screen once, about the same once, above once. I recognize the issue of differences due to the way the tank bladder affects fillups. Average over three tanks is 51+, and the average is consistent with the computer generated mileage.

Since I only calculated mileage for the first 1/2 year or so of use, I don't really know if this has changed, or only the guage. Is there a check that Toyota can do to see if there is a real problem or only a change in the guage?

thanks
Jim