A medium gear TT is ridden in a 72inch gear (or lower), with no changing allowed. However, you don't have to ride the events as Medium Gear and can use all of your gears if you like but if you do, remember to inform the timekeeper and your time will be recorded as a private time and won't be counted towards the Championship. It's good fun, social and great training for the season ahead.
There are gear charts online (like this one
) which can help you calculate your closest gear.
700x23 diameter tyre 52 teeth chainring 19 tooth sprocket = 70.88 inches
700x23 diameter tyre 53 teeth chainring 21 tooth sprocket = 66.3 inches
700x23 diameter tyre 50 teeth chainring 19 tooth sprocket = 69.2 inches
700x23 diameter tyre 42 teeth chainring 16 tooth sprocket = 69.0 inches
700x23 diameter tyre 39 teeth chainring 15 tooth sprocket = 68.3 inches
700x23 diameter tyre 34 teeth chainring 13 tooth sprocket = 68.7 inches
700x25 diameter tyre 52 teeth chainring 19 tooth sprocket = 71.98 inches