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Old 12-24-2012, 11:13 PM   #1 (permalink)
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Default MPG Calculation Accuracy

Is anyone seeing totally off-the-mark instrument calculations? I have an '01 5-speed, and on a recent 800+ mile trip across the country the MPG gauge for the trip was registering on the order of 57.5. Actual manual calculations done with an ordinary calculator at fill-ups were more on the order of 48.

Yes, the manual calculations were done beginning with a full tank, and calculating at subsequent fill-ups.

Now, I'm not necessarily crying at 48 mpg. It's a far cry from the 22 - 25 I see with my other vehicles. But 57+ would certainly be more desirable.

No lights are lit on the dash. Tires seem properly inflated.

Where might I start to get my manual calculations more in line with the computers'? I am new to Insighting. Perhaps there's a thread in here somewhere which the rest of you are more familiar that might read off a list of 10 or 15 things the beginner can check on his own.
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Old 12-25-2012, 01:15 AM   #2 (permalink)
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I don't think you are using the trip buttons right.
Explain more.
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Old 12-25-2012, 01:58 AM   #3 (permalink)
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just a hunch. you may be looking at the lifetime mpg. Like Willie said. more info would help (what mpg screen/mode ect are you veiwing)
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Old 12-25-2012, 11:20 AM   #4 (permalink)
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Well, I was using the range MPG function- the function that shows a double-sided arrow, as such: <=====> I simply reset the mpg which, as I understand it, calculates an mpg over a specific distance, from point A to point B. I reset it to 0 when I filled up, and after having driven 600+ miles over a 800+ mile course, the dash was registering 57.5, but manual calculation was more on the order of 48. By the time I was 600+ miles away from my starting point, I had stopped twice to fill up. Each fill-up was for about a half tank. I simply added up the two fill-up gallon purchases, and miles driven up to that point / gallons purchased = close to 48 mpg- a considerable discrepancy over the dash calculation.
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Old 12-25-2012, 12:13 PM   #5 (permalink)
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If you are continuing to fill your tank after the filler nozzle clicks off, you can start to fill the charcoal evaporative control canister with liquid gasoline. Gasoline from this canister, normally only vapors from the tank unless the tank is overfilled, is sucked into the air intake and, therefore, doesn't pass through the injectors. This fuel vapor-rich air causes the amount of fuel injected by the injectors to be reduced to maintain the proper air-fuel ratio. The amount of fuel that passes through the injectors is used to calculate fuel efficiency. Therefore, the reported fuel efficiency can be higher than it actually is if you overfill your tank. Might this explain your fuel efficiency discrepancy?
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Old 12-25-2012, 12:51 PM   #6 (permalink)
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To get a better (closer) reading, you should have reset the mileage at each fill up. Then divide the amount to fill up into the mileage shown.

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Use: 346K mi. @ 58.0 LMPG


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Old 12-26-2012, 05:32 PM   #7 (permalink)
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Quote:
Originally Posted by Florinsight View Post
Well, I was using the range MPG function- the function that shows a double-sided arrow, as such: <=====> I simply reset the mpg which, as I understand it, calculates an mpg over a specific distance, from point A to point B. I reset it to 0 when I filled up, and after having driven 600+ miles over a 800+ mile course, the dash was registering 57.5, but manual calculation was more on the order of 48. By the time I was 600+ miles away from my starting point, I had stopped twice to fill up. Each fill-up was for about a half tank. I simply added up the two fill-up gallon purchases, and miles driven up to that point / gallons purchased = close to 48 mpg- a considerable discrepancy over the dash calculation.
The double arrow you refer to <====> along with an mpg indication is likely the lifetime mpg (lmpg) and the number of miles the car has been driven with that lpmg. (readable by activating the fcd button?, left side of dash) As soon as my lmpg updates itself with a new figure the mileage resets itself to zero. (about 400 miles ago my lmpg improved to and reset itself to 50.0 mpg, and the mileage next to the arrow symbol reset itself to zero.)

Use your trip button, right side of your instrument panel (left of the kph button), to get accurate indications of mileage and mpg per trip / tank, dependent on how you use it. I find my 'tank fill' (actual) mileage to be very near identical to my trip meter (indicated) mileage.

Do you still have an owners manual ? Good descriptions are in the om for trip meters.

..Bob
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Old 12-26-2012, 06:16 PM   #8 (permalink)
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The double arrow <====> indicates segment mode... the FCD button mode.

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Old 12-26-2012, 06:24 PM   #9 (permalink)
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I think the dash MPG calculation is a fairly good approximation ... if you want to do better by all means ... but that is much harder than most people think.

Most gasoline Pumps when tested by your state department of weights and measures only has to be within 2% to pass with no fine ... ie get that little date sticker on it... when it reads 10.000 gallons you might have 9.8 gallons or 10.2 gallons and still pass and get a sticker in some states.

Depending on your state some pumps that fail only get a fine and are not taken out of service ... while other states take the pump out of service ... but even if a given pump were say 10% off and taken out of service ... the gas station doesn't give anyone a refund ... etc ... the best that happens is that they get fined and the pump gets taken out of service until its error % is small enough for your state standards.

Also what provides the energy is the molecules ... sense a given number of molecules has a set mass but not a set volume ... volume of the same number of molecules varies with temperature and pressure.

So , if you want to get a very accurate MPG count the 1st thing ... is a highly accurate method of measuring the amount of fuel put in ... the best method is a precision scale ... weigh the empty container ... weigh the combined ... ___ milligrams of fuel were added.

As other have said there is also a potential issue with gallons until the tank is full ... officially the I1 Gas tank is 10.6 Gallons ... but there are people who have pumped over 13 gallons in.... so is pumping 10 gallons full ... or 13 gallons full ... well it turns out they are both full... another good reason not to use a more abstract concept like 'full' and instead a more precise measurement like ___ Milligrams.

Also the distance counter is based on an assumed circumference of a tire ... but a newer tire will cover more inches per rotation than a worn tire for the same number of rotations... so the best way to know the distance is with a more accurate method ... like a measured mile... GPS can be good , but don't make the mistake to think it is 100% ... every form of measurement has a +/- margin or error / resolution limit.

But that's just my two bits.
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Old 12-26-2012, 08:09 PM   #10 (permalink)
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My gas calculations are usually very close to what the display shows. Typically the display is less than 2% off from what I calculate. My calculations are typically a little lower than the fcd, but sometimes the fcd is slightly lower. I have also read that temperature affects the actual amount of gas, that the pumps are calculated at a given temp (72?). I believe I read an article that people in Canada have complained that ov a year time frame the average temp is less than the cal temp, so in the long run they are getting jipped. People in warm states might be getting more than they are paying for.
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