I have a 2006 prius with 125k miles. No warning lights or any other "things" I can see. The question/problem I have is with a rather large difference between what the Multi Function Display claims for MPG and what I calculate the old fashioned way. The car's display is showing 46-49mpg for a tank of fuel ( I have reset it after each fill ) while the actual miles driven divided by the gallons used have shown 4-5 MPG less. This has been consistent over the past 10-15 fill-ups so it is not a question of under filling/over filling at the pump. If that were the case, after a few fills things would even out. The miles driven are accurate as the GPS I am using also has an "At The Pump" feature which shows miles driven since last fill and allows me to input gallons of fuel used. The mileage driven as shown in the GPS is always with in a mile of the Prius's display miles after 300-350 miles. Anyone have any idea what is going on? This is a pretty large difference.
As you have found, the MFD is not 100% accurate. It is a calculated estimate based on the amount of fuel being injected. Most Prius drivers have documented anywhere from 2 to 4 MPG difference. Also, due to the bladder in the US made Gen II Prius the MFD will vary depending on how the bladder is reacting to ambient temperature. Take your figures from your Garmin (At The Pump) and plug them into a fuelly account online. With that you can get even more information to track your mileage. What you are seeing is normal. And what you are doing is the correct way. A good average is based on several tanks of fuel being calculated manually by number of miles driven divided by gallons of fuel. Good luck to you and enjoy your Prius. Ron
My experience with 80,000 miles quickly approaching is that the computer will "overestimate" MPG by an average of 3%. See the attached graphic.
I get wild swings too I think the bladder is causing it I have observed sometimes as plus or minus 10 mpg on display vs fill on some top offs not all f course. Example: display 57 mpg fill calculates 47 mpg Example: display 59 mpg fill calculates 68 mpg These are partial tank top offs of 7 gallon range.
