Not sure where you got the city riding part, but the OP wasn't asking about the city/urban scene. Just the mere fact that he spins out on the flats. FWIW, I usually go 48x16 in the city (flat) and is about perfect. And just a disclaimer that I'm not a strong rider at all.

Also, some mash, some spin, and your gearing should reflect your riding style, so telling him to just spin faster isn't the best advise.