Is 40 mph very fast for a 20 inch mountain bike down a steep slope?

Today I rode my 20.0 x 2.0 inch bike down a steep slope in my neighborhood. I'm just an amateur biker. Recently I asked about what circ. size to enter in the speedometer and he said 160 cm. I did that and it said that I almost got up to 40 mph riding downhill after a lot of pedaling and stuff. Is my thing wrong or something because that's really fast. I mean I'm just an average rider. It isn't even my sport. On flat grounds it said I was going about 20-25 mph.