Speedometer doesn't work

Knowing how these gauges work , I'd say you have an instrument cluster problem not oil sender unit. 2003 - 2006 GM had bad stepper motors in the instrument cluster . If there was a problem with the oil sender the gauge would default to zero .
Engine Oil Pressure Gage
The instrument panel cluster (IPC) displays the engine oil pressure as determined by the powertrain control module (PCM). The IPC receives a class 2 message from the PCM indicating the engine oil pressure. The engine oil pressure gage defaults to 0 kPa (0 psi) or below if:
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The PCM detects a malfunction in the engine oil pressure sensor circuit.

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The IPC detects a loss of class 2 communications with the PCM.

If you don't know what class 2 serial data communications are ,google it. How to repair GM Instrument Cluster Speedometer Gauge Stepper

You could have anyone of a number of different problems . I know your year vehicle falls in the range of GM instrument culster problems . There are little stepper motors ( electric motors ) that make the gauges move . There could be DTC'S - diagnostic trouble codes stored in the instrument cluster . An you have to know how the instrument cluster knows how much to move the needles on the gauges . All the input sensors to the PCM/ECM - engine computer vehicle speed sensor , fuel pressure , oil pressure , etc... All the modules on the truck are hooked together on a serial data communication network .The PCM/ECM sends this information out on the data bus for the other moduels that need the info .
Engine Oil Pressure (EOP) Gage
The IPC displays the engine oil pressure on the analog EOP gage as determined by the PCM. The PCM monitors the signal circuit of the EOP sensor. When the EOP sensor resistance is high, the engine oil pressure is high. When the EOP sensor resistance is low, the engine oil pressure is low. The IPC receives a class 2 message from the PCM indicating the engine oil pressure. The EOP gage defaults to 0 kPa (0 psi) when the following conditions occur:
• The PCM detects a malfunction in the engine oil pressure sensor signal circuit. Refer to Diagnostic System Check - Instrument Cluster .
• The IPC detects a loss of class 2 communication with the PCM.
Speedometer
The IPC displays the vehicle speed on the analog speedometer based on the vehicle speed signal from the PCM. The PCM converts the data from the vehicle speed sensor to a 4000 pulses/mile signal. The IPC uses the vehicle speed signal circuit from the PCM in order to calculate the vehicle speed.
The powertrain control module (PCM) creates the vehicle speed signal by pulsing the signal circuit to ground at a rate of 4K PPM. The instrument panel cluster (IPC) converts the 4K PPM signal to a speedometer position. The IPC also receives a class 2 message from the PCM indicating the vehicle speed. The IPC uses this class 2 message for diagnostic purposes.
There could be up to twenty different DTC'S stored ,here is an a for instance
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DTC B1365
The instrument panel cluster (IPC) uses the ignition 0 voltage circuit in order to supply voltage to various indicators and gages. The IPC also receives a class 2 message from the serial data gateway module (SDGM) indicating the current power mode. Your best bet would be to take your vehicle to the Dealer or an ASE certified independant repair shop .

Hi, this is a pretty common problem on these Silverado instrument clusters. We can definitely help you out with that. Click here to learn more about these Silverado instrument cluster problems: Dr Speedometer Instrument Cluster Repair Speedometer Repair Guage...

You need the sensors of those functions, which you don't have installed. I would go back to the correct part.

possible instrument cluster bad or has bad connections or corrosion inside the cluster

check for the instrument cluster fuse

Hi, this is a pretty common problem on these Silverado instrument clusters. We can repair it for you or provide you with the parts to do it yourself. Click here to learn more about these Silverado instrument cluster problems: Dr Speedometer Instrument Cluster Repair Speedometer Repair Guage...

Hi, this is a pretty common problem on these Silverado instrument clusters. We can repair it for you or provide you with the parts to do it yourself. Click here to learn more about these silverado instrument cluster problems:
Dr Speedometer Instrument Cluster Repair Speedometer Repair Guage...

Your Speedometer Needle is driven by a "Stepper Motor" which is an Electronic Device. There are many GM Vehicles that were factory equipped with these and some if not most have already gone bad. Most reputable Speedometer Repair facilities can repair it for you. You might also check with a Chevrolet Dealer. If the vehicle has LOW MILEAGE it may still be covered under warranty. The entire Instrument Cluster must be removed and disassembled to gain access and replace the Stepper Motor(s). I would strongly suggest having ALL of them replaced when you have it fixed. These are Fuel Gauge, Temperature Gauge, Oil Pressure Gauge, Tachometer and Volt Meter in addition to the Speedometer.

Hope this helps,

Fastest Robe

if nothing on the guage cluster is working, chances are you lost power to the whole cluster. try checking your fuses. betcha one of em is blown.