I regularly get 56+ mpg out of a stock non-hybrid diesel 2015 Ford Focus with a 1499cc engine (and it has a turbo on it). It's not hard, it's just conscientious driving: gentler acceleration, coasting to lights a little more, driving at optimal speeds rather than the fastest speeds the speed limit allows, using cruise control when you can (it'll coast more than you do on gentle declines when able), using speed limiters when you can't.

I feel like if I had time in my life for a new hobby, hyper-mile driving like this could be it. :)