In this British Review I read:
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So if those numbers are Imperial gallons, and I'm converting to US gallons correctly, that's 53.5 mpg (US) they read off the display and 61.9 mpg (US) advertised.
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There’s no real excitement about it, and the jury is out on how it will ride on British roads, but our car posted an indicated 64.2mpg on our hour-long real-world route.
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Economy: 74.3mpg (combined)
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Economy: 74.3mpg (combined)
So if those numbers are Imperial gallons, and I'm converting to US gallons correctly, that's 53.5 mpg (US) they read off the display and 61.9 mpg (US) advertised.