How fast are you looking to go, and at what RPM? I ride 46x16 as well and it's good for a solid 30mph, while still being able to cruise over rollers and mash up steep hills.

The ratio that's good for you is up to you. Usually mid-to-high 70's are what people recommend for city riding, and your 46x16 is right in that sweet spot. If you're strong enough to push 46x13, go for it. Otherwise, 46x15 might be the next logical step.