While it doesn't hold true for the later designs of steam locos, the general rule of thumb for many years was maximum speed in MPH was roughly the same as the number of inches of driving wheel diameter i.e. having 63" drivers meant roughly 63mph top speed - and a caveat before someone starts trotting out statistics; this is the maximum that a loco could run at comfortably, with a train under normal conditions....I am well aware that they could be pushed to do a lot more. Also, the wheel arrangement played a major role in dictating top speed - trying to do 50mph in an 0-8-0 with 51" drivers would probably be a wild ride, if not downright dangerous due to its lack of guiding wheels up front.
When you do the math, this works out to be 336rpm at the wheels, but I am more interested in low-speed performance so I am aiming for something around 200rpm. Using those slow-revving motors out of printers I mentioned above, the ratio of gearbox I need is in the range of 10:1 up to 18:1 depending on which motor I can fit. This has come as quite the surprise, because I was expecting a figure much higher. I am so glad my teenage son talked me into buying a digital tachometer; it has saved me a lot of bother with false starts on powering my Rivarossi 0-8-0 kits!
