If You Do Sport: a Gap Calculator
If you do racesport, like bike racing, it seems like a gap calculator is a handy thing. I think it would be possible to develop your own simplified method to get useful info. You have to be able to do it while in oxygen debt. Or have a coach or trailside helper who can do it.
The idea is that if you're behind someone by a certain distance or amount of time, how much faster do you have to go and for how long to catch them. Or, if you're ahead of them, how fast do you have to go to hold them off for some desired distance/amount of time (to the finish-line, perhaps).
We had a coach for the Ann Arbor Velo Club in the late 80's who was into this tool. I recall him teaching it to us, but then I forgot it. I don't race anymore and it's not useful to me, really, but someone might find it to be handy.
Seems like it could be applied to anything, even car driving where you're trying to safely catch up to someone to tell them something (like they're lost or you're in a caravan of vehicles and have had a change of plans and they don't have a cell). Running, skiing, mtbiking, road racing, swimming, canoeing -- anything that involves a gap and a need to close it or maintain it.
I googled it and came across ONE discussion about it in 2009.
Here's the handy-dandy shorthand: 1 feet per second per mph. That'll work for a rough estimate. It's quite conservative though.
So if you have to close a 100 yard gap and you're riding 22mph and they're going 21mph, it'll take 3.5 minutes to close the gap. The 1 foot rule will tell you 5 minutes but you'll be pleasantly surprised. The actual stat is 1.4 feet -- which if you're quick you could round up to 1.5 and get a good estimate.
Here's a link to the Bikeforums.net discussion: http://www.bikeforums.net/archive/index.php/t-562478.html