Here is a formula i came across many years ago as i was stopped by a montana state trooper for doing 99mph on interstate 90. With your stopwatch time yourself thru one mile, using the mile markers is the best. Take that number divide it into 3600...example...60 seconds in a mile divided into 3600 as u most know is 60.... 30 seconds in a mile 120...48 seconds 75mph any ways i just thought some of you could use this formula as i have several thousand times over the years...great site