If you drive 60mph for 20 minutes, you will go 20 miles. If you drove 65mph, then you would travel the same distance 1 minute 32 seconds sooner. At lower speeds, the effect is bigger, e.g. going 45 instead of 40 saves 2:13, and 35 instead of 30 saves 2:51. If my math is right, to only lose 30 seconds by going 5 under for 20 minutes, you would have to be going 195 in a 200.