Calculated mileage WAY lower than MFD per tank value.

OK lets try this again. Hand calc relates to 113 gallons. MFD reflects 90 gallons. 23 gallons missing. Could someone have siphoned gas over a period of time or could you have added in the initial fill up gallons by mistake (couldn't account for all of it)?

Edit: Another thought. Did you get the average MFD by calculating gallons used from each tank? There are some spreadsheet floating around that do that. If so, are there any fillups where the difference is large or is it distributed over all the fillups?

 

The averaging of your MFD per-tank numbers will be less accurate than a hand calculation that divides total miles by total gallons. For example, a tank of 7 gallons at 45 MPG will make less impact in your hand calculations than one of 9 gallons at 45 MPG, though they carry equal weight in the MFD averaging.

In addition, as I assume you've discovered in your searching, others have documented that the MFD does not agree 100% with calculated mileage.

Whether these factors would make they much of a difference, I can't say. There are still some unknowns, like how many of your tanks were less than 9 gallons and by how much.

 

butchbs1985, the differences between my calculations and the MFD have been up to 20%. I believe there is a wider margin when it is colder. During the summer I'm off 1 or 2 mpg.

I'm always excited when I gas up and the MFD reads in the mid 50's and disappointed when my calculations come in under 50.

I did not take a mpg hit this winter attributable to using Shell gas exclusively, increasing the tp to 42/40, and blocking the grill.

 

Check this thread that begins with an analysis of my 2006 mileage (and a reference to how it was computed). I showed that computed mileage from a log book was about 3% lower than the MFD and speculated on the reasons. Other posters reported similar results. If you look at the attached spreadsheet, you'll see that the my tank fill point ( I always whent a click or two past the initial click-off) was quite inconsistent from fill-up to fill-up.

 

Now that it is warming up in Central New York, my MFD shows 39.7 mpg; my hand calc on the last tank came out at 40 mpg based on 8.3 gallons over 332 miles. This is about as good as I've ever gotten after 49 mpg during the first 600 miles. Now have 5,400 miles and with typically hot weather up here in summer, I might see a slight improvement. Since MFD and hand calcs are so close, I'm assuming this is about the best my '06 Prius is going to do after Pulse & Glide, cruise control, slow braking, slow acceleration, yadda, yadda.

 

Over 2½ years, my original '04 Prius and my now '07 Prius, I have found the MFD MPG to be somewhat over 98% accurate - "Close enough for government work!" It takes multiple 'tanks' to reduce the hand-calculated MPG errors from tank to tank due to variations in the pumps' calibrations, the bladder's 'stiffness', the temperature of the fuel itself, etc. etc.

Since my original interest in the Prius was its advanced technology, I consider the fuel economy a most welcome added attraction, but definitely do not obsess over MPG.

RELAX... enjoy your unique automobile.