Computer mpg versus calculations?

On a 407 mile round trip, tankful to tankful, mostly cruising at 80+ the car burned 36.07 ltr, equating to 51.3mpg. The trip computer stated 59.1 mpg, over 13% difference. Is this level of metering inaccuracy normal or acceptable or should I take it up with Vauxhall? The car is new.

Asked on 10 September 2012 by Earthling

Answered by Honest John
Standard. My BMW is telling me 72.6. Brim to brim calculations show 65.
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