I've just come from a trip around the US national parks, and noticed something similar.

I rented the cheapest, smaller car that I could, which was a 2015 Dodge Dart which would probably be about mid-range in the UK. It claimed 30 miles / gallon for optimal driving. Converting, that's 36 miles / UK gallon.

I don't actually own a car at the moment, but for a few years I owned a 1995 Peugeot 106 which normally got just under 50 MPG, say about 40 miles per US gallon.

Makes for a pretty disappointing comparison. 20 years later and still 25% behind?