I have 2005 Denali when driving at frwy speeds my speedometer start dropping eventually it goes all the way to 0 speed if I shut the vehical off it will start working again.

Speedometer
The instrument panel cluster (IPC) displays the vehicle speed based on the information from the body control module (BCM). The engine control module (ECM) calculates the vehicle speed based on the controller area network (CAN) serial data message indicating the wheel reference speed from the electronic brake control module (EBCM). The ECM sends a CAN serial data message to the BCM indicating the vehicle speed. The BCM sends a class 2 message to the IPC indicating the vehicle speed.
The speedometer defaults to 0 km/h (0 mph) when any of the following occur:
• The EBCM detects a malfunction in the vehicle speed sensor circuit.
• The ECM detects a loss of CAN communications with the EBCM.
• The BCM detects a loss of CAN communications with the ECM.
• The IPC detects a loss of class 2 communications with the BCM.

Having the vehicle scanned for codes would probably point you in a correct diagnostic direction .

I think they're probably correct.

You have to check speed sensor,could be worn out.

ck. for a speed sensor code

I suspect that your sender is malfunctioning. Todays speedometers and RPM is sent from a sensor/sender near the drive shaft. A magnet on the drive shaft is sensed and the revolutions are converted to RPM and speed.

For information see:

http://wiki.answers.com/Q/Speedometer_on_2003_Yukon_XL_doesn%27t_work_most_of_the_time_is_this_a_common_problem_how_much_to_fix_it

Sounds like you need a new speed sensor in the transmission,both the speedometer and the transmission rely on a signal from it to work properly.

You need to replace the sending unit ,speed senser,in the transmission housing or repair any shorted wireing comeing from it to the dash.

HI, the speed sensor is damaged. replacing the sensor will fix this issue.

you have a stripped and or binding speedometer cable or gear :)

get your speed sensor checked its probibly dirty and could use a good cleaning or you would need to drop a max of $69 at napa for a new one