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Originally posted by mbkennel
1) the UK uses Imperial gallons which are bigger than standard U.S. gallons.
2) Furthermore the specific test procedures used in the US versus EU for measuring 'highway' fuel efficiency are not identical.
3) US vehicles are indeed slightly less fuel efficient on average, because the US regulations favor lowering smog (excluding CO2) heavier than in the EU and that favors some engine tuning procedures and designs which lead to less fuel efficiency. For instance, at cold startup, US cars dump lots of unburnt fuel into the exhaust---intentionally. The reason is that this will hit the catalytic converter, which will burn it quickly and heat up quickly, and with a warm converter, then the (much harder to deal with) nitrogen oxides will be dealt with more efficiently.