I regularly ride with an older friend called Ken who always goes on about how many inches he is riding on his single speed!! This basically is a figure to measure the gear ratio you are riding, the higher the number the harder the gear.
Now I know that I currently ride a 48 x 17 which works out at a 74inch gear but never quite understood how this is calculated so Ken has kindly explained this in his own words, hopefully you can follow it…..
“In short once understood it is a far more accurate way than saying I think I have a 52 teeth front chain wheel & 13t sprocket rear which does not mean a lot. 52 front & 13 is 108 inch.
Let me explain more the above is worked out by dividing the chain wheel 52t by the driven rear sprocket 13t = 4 x 27 the old imperial wheel size = 108 inch. There is a abnormally here because we all ride 700c wheels now, take 2″ off for 700c = 106″ but cyclists tend to ignore this small error. The inch scale can runs from 20″ very low gear to 130″ very high.
Once you have the inch reading you times by pi 3.142 which will give you distance travelled in inches. 20″ for one turn of crank will move you 5ft 2″ forward.130″ for one turn will move you 34ft forward a little bit harder, the 108″ = 28ft.
Now for some history until late 1950 senior club riders max gear allowed for Time Trials was 84″ fixed wheel one gear.
A 90″ gear at 90rpm cadence = 25 mph a 100″ gear at 100rpm cadence = 30mph. Now you know what you have to do.”
Hopefully thats clear as mud!