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:?: I am a new owner. Why is there a difference between mpg computed by the car and the old fashion method of dividing miles by gas used as measured by the pump. Here in OK pumps are checked against a 10 gal standard.
 

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This has been oft discussed... do a search... the car uses known performance of the fuel injector. Most think it is more accurate, at least on a per-trip basis. It certainly should provide a very reliable comparative measure.
 

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I seem to have a practically consistent 3% error between MFD and calculations (averaging a few tanks). Lifetime error certainly is.
 

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Don't forget that you may not be able to fill your tank to the same "full" point at each fillup. And it's also possible that the bladder tank (which is less flexible in the cold of winter, so it can't hold as much) can throw off your calculations as well. Best to average over a couple of tankfuls.
 

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But that bladder tank along with temp still has no effect on the mpg at least here in New England. Of course we don't have temps in the seventies on Monday and the thirties on Tuesday that you get in Texas. I've found that the gas added checked against milage driven has still balanced out. (leading me to believe that the statement about the bladders not receiving a full amount in cold temperatures are negliable.)
 

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Don't be too quick to assume the old method is correct, and the Prius wrong. Fuel pumps at gas stations are allowed a certain error, and surveys reveal as much as a 5% error is common. I would guess the timing and delivery of a (new) fuel injector is more accurtate than this, in order to hold the air-fuel mixture tightly at the correct ratio.

Wanna guess which way the gas station error always is? ;-)
 

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Injectors can be off, the O2 sensor will detect the imbalance and change the injector timing appropriately. That's known as a closed loop system. Closed loop systems are great in correcting for deviations in any variable, including items that shouldn't deviate.

for example, lets say you map says you need 5ms of fuel at 30ml flow rate to mix with 100cm3 of air. But really you get 28ml flow rate, so you are running lean, even though the computer doesn't think so under an open loop. O2 sensor says too much O2 in exhaust, so computer brings it in balance by increasing injection timing to 5.36ms, but still doesn't know that the injector flow rate is low, and thus thinks it injected more fuel than it actually did. It calculates fuel consumption by multiplying 5.36x30 and not 5.36x28.

Now after 23K miles, the MFD is slowly getting closer to the calculated MPG, so it may be recalibrating flow rate based on tank readings.
 
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