Time Gaps and Race Strategy In a race this weekend I found myself in a breakaway with another rider.  We were getting updates on the size of the gap we had opened from the race official who was on a motorcycle.  This was extremely useful but I found myself wondering if the information could be more useful if I understood the math of the timing a bit more.

So, for a gap of a given time, say 30 seconds, is there a good rule of thumb for how the speed of those in the breakaway relates to the speed of the chase group?  

ie - If I'm riding at 25 mph and have a 30 second gap, and my speed drops to 20 mph, assuming the chase group maintains their speed (assuming they are cooperating and working together with a larger group) how long will it take the chase group to reel me back in?  

Is there a good way to figure this out quickly while on the bike?
Rounding slightly to make the math easier (And all paces taken from the conversion machine at the Cool Running site:

At 20 mph, you are covering 100m every 11 seconds. At 25mph, you are covering 100m every 9 seconds. (And yes, I'm mixing mph and meters). So the one going faster is gaining 2 seconds every 100m, so a 30 second gap will be bridged in 1500m.

Other than either memorizing the differences per 100m, the only other real way to do it is be able to do the math in your head. However, the problem is that you won't always get the speed of the peloton with the time update. The best thing that I've found is to watch the mile markers or other signals on the side of the road, and if they bring the gap from 30 to 20 seconds in a mile, they're gaining 10 seconds a mile.

Conversely, if you can do the math in your head, and you know how much they are gaining per mile, then you can work out the speed of the peloton compared to your speed. Or, you can also calculate it against how far you have to go. If you have a 30 second gap, and they are gaining 5 seconds a mile with 10 miles to go, you're going to get caught if you can't speed up. Then you have to gauge how much you have left against what they are doing, etc etc. It gets complicated fast towards the end of a race. :)