My best hour in 2015 on my stock 2010/11 Giant Roam 2 with 700x32 Kenda Kwest tires is 16.47 mph (26.5 kph) for 1 hour. By comparison my best hour in 2015 on my road bike with 700x23 tires is 17.5 mph (28.16 kmh). Both were done around the same loop, which is very flat.

I'm not in my best shape right now, but it's still not too shabby for an overweight 55 year old with a bad back that hasn't been riding for several years until this spring.