Most cars I've had have still measured fuel used whilst stationary, for years now the systems count the injector opening times to determine how much petrol is used.

Vauxhalls actually change to gallons per hour when your stationary... Pete

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2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
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I just remember a few of the cars i've had didn't. Yeah they count the injector opening, but they're also linked to the speedo, so they just stop counting the injector opening when the speed is zero.

Oh it just hit me. I have real time feedback on how well drafting large trucks affects mileage.

My first recorded fill up and long trip. 85% highway at 65mph. Got stuck in traffic 3x for a total of 45 min idling.computer said 18. Tank fill said 17 MPG. I was breaking 22 MPG before traffic. I will have the Sheldon Cooper in monitor it.

Now that I have put about 4 tank fulls of petrol since speedo repair it looks like my consumption computer reads 1 MPG more than fillup vs mileage calculation.

so it's basically accurate then, you'll have to allow a certain amount for differing amounts of fuel when you've filled up, brimming the tank still isn't completely accurate, though not far off. Which is basically what your MPG computer has confirmed.

I used 2 gas stations when I travel and 1 when at home. I use the auto cut-off and don't top anymore. That is about as consistent as I can get. But its electronics with sensors that may or may not have the same calibration from one car to another. Now that I know how the two relate I don't see much need to calculate mileage from miles anymore.

exactly, it's about as accurate as you can do your self, without going to laboratory conditions. There's still a small margin for error with both the computer and brimming the tank. The shut off valves on the two petrol stations may be calibrated very slightly differently too, and the temperature of the fuel can even affect those slightly.

1mpg is a tiny amount really, so it's basically the two methods agreeing with each other.