1 mile = 5280 feet.

If it takes you 1 hour to drive 5280 feet, then you drove 1 mph.

1 hour = 60 minutes = 3600 seconds

3600 seconds / 5280 feet = .68181818181818

So, however many feet divided by however many seconds times .68 will give you MPH.

I suggest you do it over 100 feet for several reasons. The accuracy on the person timing the run for one thing. Anything less then 100 feet will be hard to time accurately. And anything over 100 feet, well, I would want to hold my throttle wide open that long.