Speedometer/odometer Failure problems of the 2003 Chevrolet Trailblazer

This is an intermittent problem with this vehicle. I am 3rd owner. 2nd owner had same problem. The speedometer fails to register correct speed. I am currently using a automotive scan tool to verify actual speed. The most interesting thing about this is it does not occur all of the time. Just when I am getting 19. 5 mph or better with a lot of traffic. 1 week ago I had 20 vehicles in back of me. Turns out my speedometer was 20 miles an hour off. I was going 35 in a 55 mph zone. Good thing I had my scan tool with me. This is a problem gm has recalled other vehicles for. My question is why the NHTSA is allowing this. These are the same clusters that were recalled for the silverado. Why even have an agency as important as this one if it does not do anything about all of these complaints. There are many speedometer complaints. Gm says replace the cluster . It still works some of the time so what is the real issue. It should have completely failed by now if it were electrical or mechanical. The owner should not have to pay for this. Why discriminate against tailblazer owners. Not even an investigation.