Bill Lum's formula is a real time saver (thank you, Bill!). Of course, what he and Microsoft don't tell you is that the reason you have to divide by 86,400 is because Excel expects you to provide the time in days (!), and there are 86,400 seconds in a day. So it's
(3600 secs/hour) / (Target Speed mph) = Seconds/mile at your target speed
Then divide by 86,400 seconds per day to convert the time in seconds to days so Excel can give you the hours:min:secs.tenths. For example at 125 mph:
3600/125 = 28.8 seconds/mile
28.8 / 86400 = 0.00033333 days to go 1 mile.
That's 0.03 days to go 90 miles at 125 mph, or 2,592 seconds, which is 43.2 minutes or 43:12.0000 for your perfect time.
Now if Bill hadn't genetically passed his scientist's preoccupation with detail onto his wonderful daughter, Megan, this would have worked just fine for them. But, no, Megan had to have her 1/100ths seconds. Well, they did win the 100 mph class their first time out, so Bill shouldn't complain too much ;-)