Find a deserted road, and maintain 40 mph in 4th gear. Now shift into 3rd, and see how many rpm's the engine speed increased. At this speed, the difference won't be very much - maybe only a few hundred rpm. Go back into 4th gear. This time put the clutch in, push the lever to neutral and let the clutch back out. You are now coasting with the clutch out. Raise the engine speed to where it will be in third gear. Quickly push the clutch in, select 3rd gear, and let the clutch out. There should be no perceptible lurch if you accurately matched revs. Try the same thing at higher and higher speeds. As the road speed goes up, the speed difference between gears will go up as well. When going from 5th to 3rd a highway speeds, you may end up within 1000 rpm of redline. The easiest way of determining your maximum downshift speed is to watch the tach and speedo as you are shifting up at redline. If you shift at redline from 2nd to 3rd at 60 mph, subtract 10 mph, and that becomes your effective maximum downshift point for 2nd gear. If are within 10 mph, you are better off staying in the higher gear.
(taken from 1997 Car and Driver)