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I trust the fuel consumption in my car, yeah. If it says I've been using 12l/100km on average and I've travelled 500km, you can bet I'm filling up exactly 60l at the pump.

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That really depends on the car. I don't own a car so often use rental cars. Most cars' consumption is within 10% of the shown values but at least every fifth car I drive has vastly different numbers.

I think it is not necessarily due to the car but because of the calculation methods they use. If you drive motorway and city, consumption is accurate. But as soon as you start driving in the mountains or a mix of different terrains, data often vastly differs.

If the car has a Mass Airflow Sensor (MAF), then it can more accurately calculate the fuel usage. The majority of older / cheaper cars use a Manifold Absolute Pressure Sensor (MAP) and have to make some guesses at what the actual fuel usage is.

I've had a car that consistently showed 0.5 l/100 km less than the real consumption.

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