Pretty much all stock bikes (including 10 speed) come with a lowest cog of 11 or 12 tooth. If all you want is a bigger gear so you can pedal downhill and you have a 12, you can just swap the cassette for one with an 11 tooth cog. Or get a larger chain ring if you have a compact/mid compact. That being said, pedaling downhill where you're already maxing out your speed (35+ mph) is not really an effective use of your energy due to aerodynamic drag.