Borrow a GPS and check your speedo. It doesn't matter if you find (as you probably will) that the speedo reads higher than the actual speed, but you do need a rough idea of where 30, 40, 45 and 50 mph are on your speedo. For example, 40 mph according to my speedo was only 32 mph in the real world, and if you do that speed in top gear, thinking you are doing 40 mph, you could be making the engine labour because you'd be going too slow. All speedos are different, and some are actually accurate, but if you find yours isn't make a few temporary marks on the glass using a marker pen, to indicate where the actual speeds are, just for the running in.
Tom