ONe time i lined up at the start line to a race, the announcer said 5 minutes to start, i looked down and felt/ saw a flat rear tire. I started said race on time ish, about ten seconds late. thats with going back to a friends car, trying to find the keys and then reaching through the window to get em. Thats not to say i finished more than a mile of the race before my second flat... but, the first change was an impressive one nonetheless. Probably about 3.5 minutes of actual changing/co2ing and wheel replacing. and it was on a steel bike with semi horizontal, narrow dropouts, which require aot of extra attention for putting the wheel back on.