18mpg would generally be a pickup truck. You'd be hard-pressed to find a regular sedan that has mileage that bad.

Large SUVs with overcooked engines to meet emissions requirements (lower PM2.5 emissions at the cost of slightly worse fuel economy) get bad mileage.

Having all wheel drive or 4 wheel drive, high ground clearance, a poor aerodynamic design (driving 70mph or 80mph sounds bad in many of these cars) also hurts efficiency.

Even when it comes to cars, often the US base model has a larger engine than the EU base model.

No idea why: speed limits are often higher in EU and often with more hills.

It's because in addition to fuel being cheaper in the US, many EU countries tax you on engine displacement, CO2 emissions, fuel economy etc.

Well yes, but watching American car reviews(as European) leaves me puzzled - reviewers often describing 3.0L V6, 200bhp engine as small and low power, and discussing if it's not dangerous to merge on the highway with it, while over here you get a 1.0L 110bhp engine in a large family SUV and it isn't even brought up as an issue. In my last car the base option was 90bhp, and the "high performance" model was 160bhp. I guess in US that would be described as a horse cart. That's despite the fact that even the 90bhp model had no issue keeping up with 80mph traffic.