Trip Computer fuel mileage

Have you checked the fuel mileage stated on the trip computer versus actual mileage?
I expected some optimism, but it varies considerably. Tonight's check for
271 miles of mostly highway showed a difference of 0.9 mpg. Computer stated 29.2 but calculated was 28.3 mpg Last week the difference was 3 mpg. others were in the 2 mpg range of differential.
I'm not sure what should be expected on that type of metering, nor do I know how the fuel consumption is measured. Is there a flow meter of sorts?
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Ed Pawlowski wrote:

I've read that the trip meter mpg is based on injector time open. It knows how much fuel can be squirted out assuming rail pressure is normal.
I reset the trip meters often so have not been able to average a trip using it. Between fill ups on the rental vehicles it seems quite accurate.
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Thanks.
The readout on my Sonata is better than previous models. Two trips, miles to empty, mpg since last fill, instant mpg while driving, hours, average speed. Large crisp numbers too.
The 4 cylinder turbo has about 25 more HP than my previous V-6 but gets about 3 mpg better mileage.
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Ed Pawlowski wrote:

Also, keep in mind that you seldom fill the vehicle to the exact same "full" level each time, if the car is parked a little off level, or the pump kicks off more of less aggressively, etc., you can easily vary half a gallon or more from one fill-up to the next.
Now, if you keep a long-term running MPG as I do, that comes out in the wash as the shortage on one tank becomes the overage on the next and gets factored in. However, if you only check your mileage on one tank and do so every so often, then substantial error can creep in.
Matt
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True. I did a bunch of them sequentially and only one was off considerably, but that did not correct in the next fill up as you might expect. Every one was lower than the readout, as I would expect.
It is more difficult to be accurate to four decimal places like you could on older cars. No more filling to the top of the filler pipe. After the shut off I only round off to the nearest nickel. At today's prices, that pump handle is a hair trigger on the $$$ dial.
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