Average miles/gallon reading is inaccurate

The avg mpg display would have been also been factoring in all the miles you put on from the time you last filled you tank.

 

Welcome to PriusChat, wmckeown! :welcome:
Bohous statement isn't completely accurate. The average MPG displayed is the average since the reset screen button was pushed. If you want to get your average for a particular half-hour or trip, press the reset button and you'll get the average. When you fill up, the number of miles resets to zero, but the average MPG continues to use the old values since the last reset. On 2004 and 2005 Prius, filling the tank would reset both miles and MPG.

 

+1 on Bill Merchant's welcome to PriusChat.

When you stop and think about the "fat yellow bar" display, you will see that
while it looks like a wonderous thing, it turns out to be all show and no go,
all hat and no cattle.

The fat bars are for 5 min. increments, but the display is wholly insensitive
to how far tthe vehicle traveled in the interval. It could have been 100 ft at a
crawl, or 5 miles on a down-mountain run. So there is no way to visually
average the bars, or make a calculation based on the info presented.

This thread I started has my thoughts, and those of others here:

http://priuschat.com/forums/fuel-ec...-graph-mpg-h-better-way.html?highlight=rokeby

This active thread also suggests that pictures of "All yellow," solid 100+ MPG
bar screens can be easily, if artificially achieved:

http://priuschat.com/forums/fuel-economy/47913-mfd-tricks.html

As to a fix, there's nothing in the OEM display to adjust. On the other hand,
if you have a ScanGuage, one of the available guages is for trip MPG. You
can reset this on-the-fly with a touch of a button. You can fiddle with this all
you want. In the background, the SG is also runnning an MPG tally for the
day as well.

You can get a ScanGuage at the PriusChat store.

 

Ah, sorry for the bad info. Bohous drives an '04, hence the confusion.

 

In fact, the average MPG on the display is VERY accurate, far more so than any other method of calculating it. The only other thing that comes close is keeping track of the gallons used over a long trip. The one time I did that (for a 2000+ mile trip), I came out with 49.9 MPG that way and 49.8 from the MFD screen. I thought it was pretty good to come with 0.1, and still consider the MFD to be the "standard".

The bars are not completely useless; they can give you an idea of how you have been doing in the past 30 minutes. But as others have said, you have no idea how many miles are represented by each bar, so you know nothing about the average FE based on them. The average MPG is a WEIGHTED average, where the weights are the number of miles in each interval.

 

Over 145,000 I have tracked the MPG average based on the MFD display and with calculations based off of the current tank and the previous tank of fuel.

The calculated MPGs are nearly identical but about 1 MPG lower than the MFD displayed MPG.