So I've ridden the Heybike Mars several times since I installed the 56-tooth crankset (or sprocket as we once referred to them). My theory about bike riding on streets is to maintain the same pedal speed for level or uphill terrain. With the stock 48-tooth crankset, my pedal speed was always too fast even on level ground. Besides being uncomfortable, there was the occasional clicking sound when my legs didn't keep up at the top of the cycle. The upside was that riding on streets even with inclines could be done leaving the Shimano shifter in 7th gear. My wife prefers this on her identical bike.
A particular downside on the stock crankset was that going downhill, at the point where the motor no longer helps and you're exceeding the motor's top speed, your legs are pedaling like crazy. Looks childish to onlookers if nothing else, but my wife somehow makes it look cute. With the 56-tooth, I can pedal downhill at a reasonable speed which also matches my willingness or ability to apply leg power to each stroke. With it being easier to apply power to each stroke going downhill, my downhill speed is faster than with the 48-tooth (about 23 mph) with reasonable effort.
If there is a downside to the 56-tooth, it is that I'm now shifting the 7-speed gears to 5 or 6 on pretty much any incline. I expect there exist hills that would warrant even lower gears, but not on my usual route. If that's the case, the 56 may be the most appropriate size for this bike.
A happy medium would be the 52-tooth sprocket I tried before the 56. The difference was pretty subtle. It slowed my pedal speed somewhat on level ground but not as successfully as the 56, and I didn't need to make gear chances for inclines. Still I'm wondering why you would avoid making gear changes on a perfectly good 7-speed bike. The shifter on the Heybike Mars works pretty smoothly, by the way.
Hope this helps.
BTW, for you older guys I clearly recall buying my first multi-speed bike, a two-speed Schwinn, in 1965 or thereabouts. I was the only kid in town who had one, mostly because I used it on my newspaper route and therefore was employed, so I didn't need parental approval. For months I could out drag-race any kid in school. Unfortunately, Honda came out with their 50cc motorcycle line shortly thereafter which I jumped on, both literally and figuratively. Sold the two-speed to a friend.
