As the last review said - these pinions are indeed the best! I've been running Kawada pinions and spur gears in my car for ages and they last and last and last! I swear by then now and recommend them to everyone!
The manufacturing quality of these gears really is second to none. They are very precisely engineered as easily meet or exceed the quality of any other gears I have run in the past.
I also like the fact that they are clearly marked with the number of teeth - this makes them easier to indentify and ensure you are running a good ratio for the track conditions.
The only reason I buy more now is to give me different gearing options for different track conditions - although it's always good to carry a couple of spares of your favourite gear combination as it's not good to have to miss out on racing action due to damaged gears!
48DP gears are a good compromise between pitch (size/spacing of teeth) and smoothness of drive-train. I don't always run my car in perfect track conditions, often running it in car-parks with a fair amount of grit and dirt. My Tamiya TRF415MSXX has all the gears exposed, and not once have I had any problems at all with these gears - they really are SUPER TOUGH!