2005 Ext. Speedometer sometime works sometimes it doesn't. Most of the time is inaccurate.

May be vehicle speed sensor. Any codes?

sounds like a faulty speed sensor check you electrical connections at the sensors as well as the transmission is the check engine lite on

Scan the system for fault codes, you could have a defective speed sensor or a defective speedo both are possible.

The speedometer sensors on these cars can go bad sometimes and often, and also low power steering fluid can effect their readings. Check your power steering fluid level and see if that is the cause, If not, the sensor is probably worn out a bit.

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 want to do the repair yourself.
Trailblazer Speedometer

For speedometer related issues,the help links are provided below.Click the link below to go through its details :----

How to trouble shoot speedometer?

http://schematicsdiagram.blogspot.com/2011/06/speedometer-not-working.html

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why speedometer not working? http://technoanswers.blogspot.com/2012/01/1996-fiat-bravo-speedometer-not-working.html

------------ Speedometer works but odometer not working? http://technoanswers.blogspot.in/2012/01/speedometer-works-but-odometer-not.html----------- Speedometer runs backward?

http://technoanswers.blogspot.com/2012/01/speedometer-runs-backward.html

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Warning lights on and speedometer died?

http://howtobyme.blogspot.com/2011/08/warning-lights-on-and-speedometer-died.html

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ABS & Brake lights warning indicator steady on dash and speedometer not working?

http://howtobyme.blogspot.com/2011/11/abs-brake-lights-warning-indicator.html

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Vehicles Cluster panel Not reading Miles?

http://schematicsdiagram.blogspot.com/2011/12/vehicles-cluster-panel-not-reading.html

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Odometer works properly but speedometer works intermittently? http://technoanswers.blogspot.com/2012/01/odometer-works-properly-but-speedometer.html

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How odometer works in cars?

http://technoanswers.blogspot.com/2011/11/how-odometer-works-in-cars.html

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Speedometer not working on Ford?

http://repairhelpcenter.blogspot.in/2012/02/speedometer-not-working-on-1993-ford.html

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This will help.

Thanks

try this link might help you.. speedometer doesn work Google Search

maby the cluster smack the dashborard with your hand while your moving and see if it reads ok

Pretty common problem with these. Here's a guide to learn how to fix GM instrument clusters yourself, I used it to fix mine and it worked great.How to guide for Impala instrument cluster repair

i have a 2004 buick lasabre gauges are not working can anyone give me an answer of how to fix this