Here's the thing... The gear ratios listed for any IGH are based upon the input... That is...a 1:1 first is one to one based upon the sprocket turning... Not your crank.
ergo... You change the cog or chainring, you change the real level of that first gear... But not by much, because 1:1 doesn't multiply the effect of a lower ratio.

however... If you have an 18t cog, and a 48t chainring, that's a 2.6:1 ratio at the rear cog. If the IGH has a high gear of 3:1, you are multiplying 2.6 times 3.... So the effect of changing the rear cog is far more dramatic the higher the chainring/cog ratio is to begin with.


on my bike I installed the Bafang 750 watt Bottom Bracket drive. I tried it with a 23t cog and 49t chainring... And got a very low gear... But in 8th I couldn't spin my legs fast enough to get any more speed.
i swapped out the 23 for an 18, and I ended up with a very slightly harder 1st... But a much faster 8th.

The motor, however, makes the low gear feel just as low in terms of rider Input.


as to top speed, the Bafang can be user programmed to a 50kph limit... About 31 mph. But I find that the throttle control does NOT really give you full motor power. Throttle seems like it offers up about 75% of full power. It may be that the controller doesn,t allow the throttle to make full use of the 48v my battery can output.
I can cruise without pedaling on level ground at around 28 mph, throttle only... But in 8th gear, with just very easy pedal input, I can get the thing up to 35, no sweat.

however... Its a city bike geometry. Carbon fork, aluminum frame. I,ve larded it up with a heavy kickstand, racks, and a brooks flyer ( I could easily strip 5 lbs from this machine with lighter components)
i have had to up to 40 plus on downhills and that's just plain scary.

my experience is that bikes, inherently, are unstable at speeds over 30-35. You need something a lot heavier, and well sprung to go faster with any sense of security... That's more like motorcycle territory.

i wanted a machine I could easily carry upstairs... And, of the power ran out, still muscle uphill back home.

i find the Bafang 750 to provide the perfect balance of power and speed for anyone who lives in hilly terrain.
being able to easily pedal at 25-28 mph is perfect.