If you are like me, then the gear you choose while racing in a triathlon is going to be the one that allows you to a) pedal fast, and b) go fast! Why pedal fast? It’s usually easier to go up hills or flat out if you can spin vs. mashing the pedals, though I have seen athletes doing just this even when going up hill. Why go fast? That’s the whole point of racing!
So, just how fast can you go in a given gear? This is usually something that any athlete with a GPS or bike computer will know with experience, but it is an easily calculated number, if you know the formula.
First, let me talk about gear inches. Take the number of teeth on the chain ring and divide it by the number of teeth on the cog, then multiply this ratio by the diameter of your wheel (in inches) to arrive at the result. For example, if you have a Cervelo P2 like me, then you probably have a compact chainring – 50 teeth on the big ring, and 34 on the small ring. You may also have a typical cassette with 12 teeth on the smallest cog and 25 on the biggest. So, if you were using the large chainring and the smallest cog, the calculation would be (50 / 12) * 27 = 112.50 inches. This is nice to know, but doesn’t really tell us much.
Gear development on the other hand, is the distance that your bike will travel with one revolution of the pedals. This is much more helpful because we can use it to work out the miles per hour for a given gear inch value. To calculate gear development, simply take the gear inches number and multiply it by Pi (approx. 3.1415).
Using these values, one can easily calculate speed in mph for any chain ring/cog combination. The easy formula then is Gear Development * rpm * 0.0114. Using the same chain ring/cog combination from above (50/12), we find that we will go 30.22 mph at 90 revolutions per minute (rpm).
Of course, the hard part is actually pedaling 90 rpm when using this gear combo. You still need the leg strength to overcome the downward force of your weight. The variable in this calculation is always going to be how many rpm you can pedal.
Here is a list of speeds (mph) based on a 50 tooth chainring, traveling at 90 rpm (the number in parens is the cog size):