That is about right I think. I noticed that my Strida copy has a lower ratio than the mid ratio in my Merc which I have calculated to be 60". This puzzled me a bit because of the claims on various websites that the Strida ratio is 56". I was sure it felt lower myself since I ride both bikes.
I measured the circumference of my wheel with 16x1.75 tyres at 1220. I made the measurement three times by doing a roll out and got an identical figure. I make my ratio at a shade under 51" (50.956). Exact ratio depends on tyres like you say. It is way lower than 56" though.