Since I first became interested in my gear ratios (30 years ago?), I've learned to think of them in traditional terms: gear inches. Gear inches translates like this - imagine an old pennyfarthing, or 'highwheeler' bicycle. The way to go faster on a bike like that is to make the wheel bigger. So the longer your legs, the faster you could go, right? Well, once bicycles got gears, it was still convenient to think of our gearing that way, 'cause we could visualize a wheel with a diameter of X inches to think of the gear we're riding.
There are other ways to do this, too - especially one developed by the late great Sheldon Brown, called 'gain ratios'. Very sound and logical, but I don't think about my gear that way because of old habit. So I'm going to continue to talk about my gear in 'inches' meaning the theoretical diameter of my wheel, based upon my actual gear ratio. I do this by dividing the number of teeth on my chain-wheel by the number of teeth on my sprocket, then multiplying the product by my wheel diameter (actual measured diameter).
So, I did this the other night when trying to plot a new gear ratio for the bike and MIS-MEASURED my wheel diameter. Realizing this later, I re-measured the wheel and re-calculated my gearing.
So, my 3-speed is currently running a 44x18 gear with 27" wheels, making my 'N' (or normal) middle gear 66 inches. This is a pretty good gear, although still a bit low. I'm going to move it up to 46x18 soon, giving me a 69" 'N'.
The Sturmey-Archer hub on my bike has Low and High ratios of 75% (low) and 133% (high), which means my low gear will be 51" and high will be 91.8" with a 46x18. A 51" low is about equal to a 40x21 ratio - about normal for a racing road bike (a little high for a bail-out gear for an old guy, but way better than 69" for climbing).
So if you check the M.P.H. chart in the last post, I should be able to do about 22MPH in my 'N' gear and about 28MPH in high (assuming I can turn the pedals over in high!). A lower 'high' gear than you'll get on a modern road bike, but plenty high for a roadster-style 3-speed under this old guy.