I bought my 2008 Prius new. About 8 months ago, I started having occasional "fill up" problems when the pump would click off and the gauge would not show all bumps (dots, bars, whatever) lit. I would then go back and put in another 1/2 to 1 gallon. I can attest to the bladder issue as the cause due to temperature variations. However, on my most recent trip, things got weird.

# Miles Gals MPG (Calculated)
1 339 8.5 39.9
2 382 6.2 61.6
3 208 3.25 64.0
4 209 7.9 26.5

The temperatures for all fill ups were between 75 and 95 degrees. This much discrepancy cannot be explained by the bladder/temperature relationship alone. However, the age of the bladder may have something to do with it and I will be talking to my dealer about it.

Just a couple of thoughts:
I always calculate my mileage and I never use the display.

I run my tires at 90% of tire pressure rating (40-41PSI). The "recommended" tire pressure is to give a softer ride, but I want better gas and tire mileage.

Be aware that most new cars have "harder" rubber on them when you get the car from the dealer. They will get better mileage, but when you replace them (I prefer Michelin), you will get a drop of about 2-4 MPG!

It IS disconcerting not to know your range. So, I don't usually let the gauge go below 2 bumps.

With careful driving (NTE 60 MPH), new "dealer" tires and 90% inflation, a few Prius owners have gotten a consistent 48-50 mpg. My Prius has been a pretty consistent 42-44 with the Michelins.

My longest trip between fill ups in my records was 447 miles. 8.7 gallons for 54.0 MPG. Previous fill up was 357/5.6/42.5 and the following fill up was 361/7.5/48.1