The time honored way to compare gears is in "inches", ie convert your gearing to the equivalent wheel of the old-fashioned high wheeler. The bigger the wheel, the further you go with one pedal revolution and the faster (up to a point). Hence all high wheel racers were tall (they could straddle bigger wheels).

The math is simple. Divide your chainring teeth by the cog teeth and multiply by 27 (for normal 700c wheels). You can get picky and measure the exact diameter of you wheel. I never do, Life's too short.) So:

47-17: 47/17 X 27 = 75" (No need for decimals here. Your legs can't tell.)
48-17: 48/17 X 27 = 76"
49-17: 49/17 X 27 = 78"

47-15: 47/15 X 27 = 85"
48/15: = 86"
49-15: = 88"

I rarely ride higher than 72". Even in my mid-season racing days many years ago, 76" was about my max. I do have a fixie with a drop long enough to screw on any cog and I have been known to do major descents in 42-12 (= 95"). Now that's fun! But back on the flat? I stop and flip the wheel around. back to the 17t.

Ben