Join Date: Dec 2004
Location: Middle of da Mitten
Bikes: Trek 7500, RANS V-Rex, Optima Baron, Velokraft NoCom, M-5 Carbon Highracer, homebuilt recumbent
If you only want a lower low without losing your high gear, you could do a couple of things. First, you could replace your rear cassette with an 11-32, which would make your low gear just under 25 inches. Next, if you can breake into the mid-drive, you can try to change the 24-tooth that goes to the front chain to something bigger. To answer your question, a 3-tooth difference shifts fine, 4 teeth still shifts OK, after that shifting starts getting slower. There is no line in the sand, it just depends on your tolerance of possible slow shifting. You might want to replace several cogs so you get a nice progression of gears. Changing either the front chainring or the fixed gear on the mid-drive will give you a lower low but at the expense of your high gear.
For loaded touring, I've heard numbers all the way down to 15 gear inches, although I think it'd be hard to balance at the speeds that would allow.