Why is the fuel gauge in my Vauxhall Zafira not accurate?

I recently bought a 2006 Zafira 1.6, fully serviced. It idles perfectly and runs smoothly. I filled half a tank and at the first notch on the gauge it had done 50 miles, which meant with a total of four notches it would do 200 miles. However, I only managed 125 miles in total (that was driving with the fuel light flashing - so getting it to empty as possible). The driving routine/journeys was exactly the same all the way, so how come I only managed an extra 75 miles instead of 150?

Asked on 7 December 2013 by TA, Birmingham

Answered by Honest John
Because this is a nonsensical way to measure fuel economy. Dashboard fuel gauges are never accurate because the float sender sits in a flat fuel tank. The only way to measure reasonably accurately is brim to brim.
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