MPG - Pump versus MFD

It seems to me that the calculations on the MFD are done in a specific way measuring fuel injected against wheel rotations, and thus should be fairly accurate and at least consistent. Set against this, each fill up has significant capability of resulting in a different 'full' level, making each tank give a variable and unpredictable result. As such, while manual calculation will tend to even out over a number of fill ups, the car is likely to give a reasonably accurate calculation.

 

I track both, but rely on the hand calculations since no matter what the MFD says if you put so much gas in it and travel so far that is the bottom line MPG. That is unless the pump is inaccurate, which some say.

Off the top of my head, I can have as much as 8 MPG difference between the hand calc and the MFD.

 

I have 7 years of Prius data recorded!


2001 Prius - 59,827 miles
---------------------------------
tank = 45.4
computer = 47.5


2004 Prius - 81,217 miles
---------------------------------
tank = 48.3
computer = 49.6


Details are available in my graphs & spreadsheets.

 

<div class='quotetop'>QUOTE(scottynettles @ Jul 27 2007, 04:00 AM) [snapback]486012[/snapback]</div>

I've just started, and there is a variation which has been discussed frequently on this forum, with various theories as to why.

For anybody who is really interested in maintaining an independent measure of consumption, I strongly recommend this neat resource...

http://mymilemarker.com/

It will also provide historical data over time, as to how consumption is changes over longer periods of time, which isn't possible with the MFD.

 

duplicate post deleted

 

<div class='quotetop'>QUOTE(markgunn @ Jul 27 2007, 09:25 AM) [snapback]486054[/snapback]</div>

Why isn't it possible with the MFD? You can reset the MFD every fill up and monitor use that way in the same way as by manually calculating from fill ups, or reset it monthly, or indeed in any pattern of use that seems worthwhile to track. It seems to me a matter of whether the MFD is accurate or not, and I suspect it's reasonably close, and at least not prone to the sort of errors one can get from variable fill levels at the pump.

 

The problem with hand calculation is that while the odometer gives a relatively accurate indication of how far you just travelled, for any given tank you are trying to guess how much fuel you just finished using based on how much fuel you just added. The bladder in the fuel tank means that your guess can be rather inaccurate at times.

Example:
There is still a bit of gas in the tank and you fill it up by adding 9.5 gallons of fuel.

How much fuel is in the tank?
You don't really know how expanded the bladder is, how much was sitting in the bottom of the tank before you filled it, or precisely how close to the filler tube you are.

Ok, so you decide that you just purchased the car and filled the tank with fuel before you left the lot. This gives you a starting point.

You drive untin the guess guage indicates that the tank is getting low on fuel. You now fill up with 9.5 gallons of fuel.

You assume that the tank was full to start with, and now is full again so therefore you've used up 9.5 gallons of fuel that you just replaced.

Meanwhile the bladder had expanded a bit since your last fill up allowing the tank to hold about 0.5 gallons more this time and the previous fill up was not quite as close to the filler tube as this one meaning you got about .2 gallons closer to the filler tube.

If the bladder held 10.2 gallons when you dorve off the lot, you used 8.8 gallons of fuel and then added 9.5 gallons, and your tank now has 10.9 gallons in it.

If you drove 450 miles, you calculate 47.37 MPG from the 9.5 gallons, while you actually got 51.14 MPG.

Obviously if the bladder contracts a bit, or you don't get as close to the filler tube, hand calculation will be higer than the actual.

So for any given tank the MFD is likely to be more accurate than hand calculations.

On the other hand, over a large number of tanks for a lifetime MPG, hand calculation is likely to be pretty accurate. Some tanks you'll be high, some tanks you'll be low, but the total amount of fuel you put into the car over 25,000 miles will still be the total amount of fuel you put in and if the amount used in the current tank is off by a gollon or so it won't significantly affect the over all MPG.

Example:

After 25K miles, your records indicate you added 490.2 gallons giving you a calulated 51 MPG.

Most of the errors have canceled each other out by now. If you have actually used 1 gallon less in this tank than the fill-up indicates, your total fuel used so far is actually 489.2 and your actual MPG is 51.1 MPG.

So for any given tank, the MFD is probably the best way to go, but for lifetime totals, hand calculation works just fine.

By the way there is room for error in the MFD. As I understand it, it doesn't measure the amount of fuel injected. It measures the amount of time the injector is open, and then makes assumptions about how much fuel is likely to pass through the injector in that amount of time. Furthermore, as tires wear down, or slip, or skid, the distance travelled can be off a bit. These are all pretty small variations, and I'm not sure how much the cumulative affect is over time.

 

I find my car's MFD is normally about 1 to 2 MPG higher than the actual gas I pump into the vehicle. This is only after 4 tanks of gas since I am a new owner.

cheers Mike B)

 

The MFD on the '04 and '05 resets automatically after each fillup (of over about 3 gallons). I have kept a spreadsheet ever since I bought the car, and my calculated lifetime MFD mileage is about 2% (i.e. 1 mpg) higher than the lifetime calculated pump mileage.

Because of the bladder and pump variations, you can never get an accurate MPG from just one or two fillups. The MFD is the most accurate thing you can rely on. And fill up before you run out of gas! There really is a reason the last bar blinks and the screen says "add fuel"!!!!!!!!!!!!!!!!!!!!!

 

scottynettles, welcome to the site. The MFD calculation is close enough for government work. Usually it is optimistic and within a few mpg. Sometimes it can be off more than 10%. You may want to start tracking your mileage at greenhybrid.com.

 

I trust the MFD and not the pump.

Pumps are calibrated to be accurate only when going at full flow. If you top off, if others turn their pump handles on or off using the same grade while you're pumping, even when you first turn the pump on... none of these events are included in the calibration measurements. And pumps only have to be measured every few months or years or whatever. I think most pumps are only calibrated when there is a complaint... and I think customers will only complain when it's reading excessively high, while station owners will "complain" anytime it's reading even a tiny bit low.

Topping off especially can make a huge difference. I have found topping off to be off by 2x-4x. Once just for grins I really "truly" topped off at a favorite station that always seemed to have slightly lower prices, but always seemed to end up costing maybe a tad bit more per tank... just a hunch mind you... I managed to get 13.5 gallons "into" the tank!