Terminology suggestion: Rather than express those as ratios, or gear-sequence numbers, it is more common, and more understandable to road cyclists, to use something called "gear inches," which is the diameter of an equivalent direct-drive wheel. Divide ring teeth by cog teeth, and multiply times wheel diameter in inches (about 26.3 inches for the common 700/23C wheel size).
So your 53x15 is about 93 inches, 53x16 about 87 inches.
Unless you're a very strong rider who goes quite fast, those are big gears, and you are correct that your cadence is slow. For example, if you are going 18 mph with the 53x16, you're turning about 70 rpm. You might try using some lower gears and see if you can get used to spinning a little faster. It works better for most riders. In a 53x21 gear at 18 mph, you'd be at 90 rpm.
I haven’t really paid attention to that but now that I’m thinking about it, I guess I am probably in the mid 70’s, maybe the low 70’s actually – yeah, I’m gonna go with low 70’s. That’s a solo cruising speed (comfortable but not beach cruiser effort) on flat ground with little wind. Probably seems slow to all you Interweb strongmen but also know that I like to spin so as not to put too much stress on my delicate and oh so supple walking appendages.
And anyone who thinks it too slow, I got a finger to show you.
I run 71 gear inches on the fixie and when on flats with the geared bike I'll settle into 77 gear inches. On the geared bike I can always downshift if the wind picks up. On the fixie I run a little less gear for the reserve it offers for strong head winds.
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