I'm in the UK, so it is UK gallons ... 50 mpg UK is around 42 mpg US. Divide by 1.2 to convert from UK to US figures, multiply by 1.2 to go from US to UK :)
For local driving on our windy country lanes and brief bits of 60mph single-carriageway roads, my mum gets around 50 mpg in her 1.5L turbo diesel Mitsubishi Colt. Diesels do get better mileage than petrol engines, but my mum isn't a very efficient driver. I drive my dad's Peugot 406 estate 2.1L turbo diesel, which is about eleven years old; it gets more efficient as time goes by. I'm not nearly as efficient a driver as my dad (I'm learning) but he typically gets around 60 mpg (50 mpg US), which is good.
Obviously when we're towing the caravan that goes way down, and the car is most efficient on straight level roads at about 50 mph - not much of that round here! Because of the conversion factor, there isn't so much of a difference between our figures as you might think - but with prices currently at around 122.9 p/L for diesel, fuel economy is important.
If you believe everything you read, better not read.