I used to have a Workcycles bakfiets cargo bike with a Shimano Nexus 8 rear hub. Here are some notes on the original and lower gearing. The change was great and I kept it for years, even taking the heavy bike on a multi-day tour. My only regret is that I didn't capture the size of the front change ring in the post to calculate the overall gearing.
This week I lowered the gear on my bakfiets to prepare it for use on longer trips with steeper hills and bigger loads. It was shipped to me with a 17 tooth (17t) rear cog. Lowering the gear range involved purchasing and installing a 20t rear cog.
I found the 20 tooth cog online through Niagara Cycles, referred to as the “Shimano Nexus 20 tooth cog”. The product doesn’t seem to be listed on the site now. Perhaps it is temporarily out of stock. The part was about $6 plus shipping. My local bike shop charged me about $25 to install it for me, which seems like a good deal.
I was quite concerned that I wouldn’t like the change, that it would be too drastic. I had read online that people made this modification for “hilly areas”, almost as if there would be no good gears to use on level ground. My experience has been the change is no compromise at all. In fact, I think it would be sensible to sell them like this in the first place. On flat ground, I am more likely to be able to use the most efficient direct-drive gear. Before, the direct drive gear was set to high for my common use. The lower gearing is welcome on hills, allowing me to spin at a higher cadence. I doubt I’ll miss the lack of gears at the top end of the range. I rarely used them. As a cargo and kid bike, getting up to 20 mph sometimes is plenty, and the adjusted gearing still allows me to do that.
While I’ve only had a few days to test the new gearing, it already seems like a clear upgrade from the 17 tooth cog the bike shipped with. Already this spring I’ve made a successful 20 mile trip with my 13 month old daughter, and she seems to love bike rides, even as long as that two hour trip. This summer I hope to try full day tours, with 50 or 60 mile distances. At this point, my primary concern is working out a shade solution for her.