Both speedometer & tachometer not working, both reading wrong.

There was a recent class action law suit settled with GM for this exact problem. You can google gm speedometer lawsuit and get lots of information. Unforutunately for most people I think the warranty was limited to 7 years and 70,000 miles. Which isn't a lot of miles for most people.

If you car isn't covered by the warranty there is a way to "reset" the speedometer.

You can turn the ignition slowly like you are going to start the engine. There is a very small "dead" space where you will see your guage lights go off.

What you want to do is cycle the ignition switch from off to that dead space. Each time you do it your stuck speedometer will move towards zero about 10 mph. So if its stuck on 40 you will have to cycle it at least 4 times.

At first you will probably actually hit the ignition and the engine will try to start.....just release it and try it again.

I know this because I have a 04 avalanche with 80,0000 miles and it has this problem about twice a month.

Had the similar problem with my 2005 Silverado. Instrument Cluster ribbon board replaced. @ 55K Miles. GM did it at no cost to me. Here are a couple links that might help you research http://drspeedometer.com/silverado-i...cluster-recall http://www.gmgaugerepair.com

This is a pretty common issue for Trailblazers to have. Here's a link with more information about these GM gauge problems. And where you can purchase replacement parts on Amazon if you are a DIYer.
How To Repair My Instrument Cluster