Which is Better Calculated or Screen?

Well calculated life time average is the best. I really don't trust calculated tank figures but will be willing to claim the first one I get over 60! 8) I am toying with a 3 tank average as being the most accurate for your recent driving that changes season to season because of the problems of filling the tank and the pump, and the temperature, and etc. etc. etc.. The car computer is close most of the time but off on other occasions. An average of the car tank m.p.g. and calculated life time average converge. I was up all night last night does this make any sense?

 

I personally don't trust that the gas pump delivers 100% of what it tells you it's delivering. Since the car can calculate instantaneous gas-usage figures, I feel the car's calculation is more accurate. I don't bother dividing total miles over total gallons...

That's just my opinion though.

 

For me, over more than 20K miles, the Screen Weighted Average is 0.3 MPG higher than the actual calculated by total real gallons pumped and miles traveled.

That's a difference of less than 1/2 of a percent.

As far as I'm concerned my "Real" mileage is based on real miles and real gallons, but since that's only accurate over multiple tanks, the screen is pretty darned close for an individual tank.

 

miles driven divided by fuel used is the only valid number. The more miles you drive the better the accuacy of the figure. At about 10,000 miles you should have a pretty fair idea of your true LT miles per gallon. Yes the bladder comes into play as does the pump volume of which most are temperature corrected, and the inaccuracy of the MFD. My life time for 16701Km's is 5.52 Liter per hundred Km. the MFD indicates 5.2 My life time is 5.52Lperhundred. It can't be anything else. Fuel burnt to Km travelled. This is on a Classic which the MFD reads life time.

 

I compare lifetime mileage (miles driven divided by total fuel pumped into the tank) and also guage mileage on the MFD. For the last 4,500 miles the calculated mpg is 1.2 mpg better than the average of the guage mpg.

 

The fuel pumps are at least as suspect of inaccuracy as the cars own method of calculation so I keep lifetime for grins but I use the display for my tank by tank comparisons. Currently 196 miles at 62.8 MPG.

 

I was wondering the same thing. Just filled up my new Prius yesterday for the first time. MFD read 50.4 mpg. Calculation came to a much lower total: 44.3! From all the posts I've read so far, the discrepancy is usually very minimal, so I'm wondering why the big difference in this case. I'll definitely be paying attention over the next couple of fill-ups.

 

Personally, I would use only the screen and use it only to observe trends. It is likely to be accurate enough for any reasonable need.

Come on people, do you really need to know that your mpg was really 48.8 instead of 48.4? Assuming that the on-screen calculation is consistent, isn't it more usefull to observe the trend?

 

I have kept a record of every fill-up since I got the car. The MFD has averaged 52.62. Calculated is 51.59. I have just over 15,000 miles on the car.

 

I have about a 2% difference between calculated and MFD, with the MFD being higher.

 

At 41,076 miles on my '04, I have used 823.05 gallons = 49.91 mpg.
Using a weighted average of the MFD mpg display for all my fillups = 50.07. Not a huge difference. In fact, no real difference at all 50.07-49.98=0.16 mpg

I also keep a +/- record for each tank to compare the MFD with my calculated MPG. The MFD mpg has ranged from 7.5 mpg higher than calculated to 19.1 mpg lower than calculated (I believe this is due to the bladder effect). HOWEVER, after 41,000 miles, the overall difference between the MFD and the calculated is: 0.09 mpg.

Take the easy way and go with the MFD for each tankful. Don't obsess over the difference between the MFD and calculated, because there is none - it all evens out!

 

I'm still testing mine, but for sure it is within 2%.

And my individual fill ups vary a lot, with temperature and how patient I am with the car taking the last gallon or two in 0.05 gallon gulps.

 

Look at my graph at http://enerjazz.com/prius/ or my data at http://enerjazz.com/prius/index_gas.htm. The MFD is more consistently accurate, but has a slight bias of about 1.4 mpg for my car (2.7%).

The calculated tank fill up has wild mood swings based on the bladder, gas pump, temperature, etc.

On my graph I applied the bias to the MFD display figure to indicate the probable actual mileage (thick black line on chart).

My two low tanks in Nov were a road trip with my family and luggage (extra weight) and a darn 20 mph headwind - in both directions (a few days apart)!

 

i will echo what most have said here. i keep track of both readings and i think it helped me to determine that gas pumps are inaccurate and taught me that a certain station seemed to always give me lower gas mileage figures. i have since stopped going there and have increased my LT mileage. as far as difference goes, you can see by my signature that there is no difference. also, my MFD mileage was about 1.5 mpg higher because of the 3 fillups at the station that shorted me. ever since then, it has been a slow climb going the other way because i now only use the pumps that gave me the best mileage.

moral of story, some pumps are inaccurate but not always in favor of the station...

hehehe

 

This all assumes, of course, that you fill the tank the same way every time. I personally fill until the pump clicks off the first time. I have heard of people "burping" the bladder to get a complete fill every time, but this doesn't make much sense to me

Mark

 

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If you are pretty sure that a station's pumps are giving you a short gallon, you owe it to your community to notify your state's bureau of measures. Most state's require that pumps be checked and sealed; and then have a sticker indicating the date that the last inspection was performed.

This bureau is tasked with ensuring that the pumps are accurate and the station is not cheating its customers. If the station is reported and is found to be cheating the fines can be pretty stiff.