First fill-up since Dealer - MFD says 45.0 MPG

Well, that depends on what you consider "actual"...

The MFD is capable of measuring the exact (or nearly so) amount of fuel that is injected into the engine, as well as the exact (or nearly so) distance traveled. Calculating, on the other hand, introduces a number of potential problems. First, you're using the same distance calculation the MFD is using (except maybe not quite as precise - you can only go down to 1/10 of a mile). Next, you don't know if the bladder has contracted or expanded at all between the start of the previous tank and this fill up. Third, you don't know if the pump you're using automatically shuts off at the exact same level as the previous pump used (and even the same pump can have variations from week to week as it gets abused by customers). And finally, you don't know for certain the accuracy of the pump. it may read 9 gallons but given you 9.02 gallons.

So, in conclusion, drive happy, keep your logs, and fret over those "missing" 1-2 mpg like the rest of us

 

Monguire,

I have been seeing about 5-7% higher values on the MFD versus what I calculate.

What part of VA?

 

The general consensus here is that although the MFD may be a bit higher than calculated values, it is at least consistantly so.

So, if you accecpt the assumption that the MFD is at least consistant, if not 100% accurate, then you have to ask yourself what you are really trying to track with your tank averages...

Is it the actual MPG value that is important to you or is it the trend that is important to you? If what you actually care about is whether you got better gas milage this tank than last tank, or this month than last month, then just use the MFD and be happy. It will provide you this trend quite well with very little effort.

If for some reason it's really, really important to you to know that you really got 53.2 MPG instead of the MFD's 54.8 MPG this tank, and 52.5 MPG instead of the MFD's 54.1 MPG last tank... Well, then you have a problem, because you can't get that kind of accuracy with manual calculations... Ever.

With MPG numbers around 54 MPG, a mere quarter of a gallon inaccuracy in your fillup gallons will cause a 1.5 MPG inacuracy in your calculated value. A quarter of a gallon is just 2.7 percent of a 9 gallon fillup. So you have to ask yourself... Just how accurate was that pump. Is the fill level this fillup the same as the fill level from last time? How do I really know how much gas was used to drive the miles I've registered for this tank? And anyhow, how do I really know I traveled x miles when tire size and tread wear can affect the odometer reading?

You can increase the accuracy by averagung your manual calculations over 4 or so fillups. This will reduce the error caused by fill level inaccuracy (but not pump inaccuracy or odometer inaccuracy), but it doesn't give you a definitive "I got 53.2 MPG this tank" result.

So... My opinion is "Why bother?". People are just fooling themselves if they think their tank by tank calculations are any more accurate than the MFD. Every single number you use in your calculation has some level of inaccuracy in it, so why expect a calculation using them to be anything other than an approximate value. And it's the trend that's more important anyway.

 

Over two years & 22,000 miles, I found that the difference between "hand calculated" and "MFD" Miles/Gallon was less than 2% [MFD being 'optimistic'] - "close enough for government work!" For the "hand calculated" to have meaning, it must take place over many tank fillings since there are variables due to the fuel bladder and station pump accuracies which will 'work out' over numerous fillings.

I keep an Excel spreadsheet to track my "calculated" MPGs compared to the MFD numbers, not so much as a compulsion since I did the same on my previous cars: Toyota Camry, Cadillac diesel and Jaguar XJ6.

I have only ~350 miles on my new 2007 Touring Edition Prius, so it's much too early for meaningful numbers.

 

<div class='quotetop'>QUOTE(Marlin @ Nov 28 2006, 08:34 AM) [snapback]354781[/snapback]</div>

Single tank calculations aren't (can't be) very accurate, but average mileage over a large number of fills will be. I don't know what kind of measurement techniques are being used in the car for liquid flow, it could be 10 ppthv but I somehow doubt a commercial product would contain anything with high sensitivity/cost, so I'd trust running averages of tank fills over the MFD (if I cared for high accuracy).