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ok, for all you mpg enthusiasts how accurate is the computer average versus calculating by hand (calculator) your mpg (mpg = total miles/gallons of gas)

thanks
 

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:) In my two-year ownership experience, I find the "onboard" computer to average over 97% accurate compared to gallons vs. miles per tank. This is an acceptable variance to me. The "onboard" computer is, therefore, 2-3% optimistic.
 

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The display uses measurements of on time of the #1 injector. This assumes a specific delivery rate, which may be off.

Based on 63K miles, I find the MFD is 1.6 MPG higher than what I calculate the traditional way, at 55 MPG fuel efficiency.

Now if you were to ask me why the MFD shows higher fuel efficiency at the beginning and end of the tank, I believe that is from the evaporative emissions recovery system. Fuel vapors in the tank are sucked into the intake manifold, thus bypassing the injectors. Now the O2 sensor initially detects too rich a mixture in this situation, so it injects less fuel (shorter ON injector time). Because of the shorter ON time, the MFD sees this as more miles per gallon, since it estimates fuel consumption based on injector flow rate, not knowing that some of the fuel is getting to the cylinders via evap recovery. This COULD account for the 1.6 MPG discrepency as well as deviation in flow rate from expected flow rate.
 

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I feel the MFD is more than accurate enough to suit my purposes. If it is off by 2%, so what? The display helps me improve my MPG's so I only need relative numbers. Am I doing good (55 - 60), excellent (> 60), ok (45 - 55), bad (< 45), etc. (These numbers are based on my particular commute, your ranges my be different).

Of course, if I was driving my Prius to the moon, and my displays were a couple of % off, I might miss my target and end up crashing into the sun.
 

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http://autos.groups.yahoo.com/group/toy ... ?id=444783
Question
On average, what is the difference between your mpg calculated by the
car's computer and by the gas station fill-up method?
499 responses:
Choices Votes %
computer 5mpg or more higher 17 3
computer 4mpg higher 38 7
computer 3mpg higher 70 14
computer 2mpg higher 107 21
computer 1mpg higher 12 2
both methods about the same 128 25
computer 1mpg lower 5 1
computer 2mpg lower 9 1
computer 3mpg lower 15 3
computer 4mpg lower 4 0
computer 5mpg or more lower 6 1
varies too much to give meaningful average 88 17
 
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