Played 
I've been doing a reasonable amount of training on the road bike lately and given that the weather here in Melbourne is temperemental at best coming into winter I decided to get another indoor wind trainer. Because I didn't want to break the bank I didn't buy one that shows you your speed or distance. Apart from the fact that effective training should be based off heart rate and cadence anyway (or power output if you're REALLY serious) you're looking at AUD$800+ for that sort of thing, money I'm not prepared to spend on a trainer at the moment.
Anyway, I still wanted to know how far I'd gone and what my average speed was so I set about figuring out how to calculate these figures given the information I do have - cadence, time, wheel size and gear sizes.
Required Info
A bit of research came up with a few different ways of calculating the distance and average speed when you know the following:
- Wheel circumference
- Average cadence for the training session
- Chainring sizes (measured in number of teeth)
- Sprocket/cassette cog sizes (measured in number of teeth)
- Training session time
The method I found that worked best for me came from a Wikipedia page entitled Bicycle Gearing. It contains a whole load of useful information, including how to calculate what is referred to as "metres of development" i.e. the distance travelled for a single crank revolution. This, of course, is dependant on which gear you're in. Another thing to note before carrying on is that it's often recommended to train for certain intervals at certain levels of intensity and cadence. The only problem I can see with the calculations below is if you change gears a LOT during a trainer session you'd need to know how long you were in each gear for (in order to calculate the metres of development for each gear used). I'm not a doctor either so please be responsible and don't train before consulting a physician if you're not sure what you're doing. However, carrying on ...