The speedometer works part of the time to read right speed and then it goes crazyreading what ever it wants either high or too low some times it gets stuck on 15 mph in park or reads 10 to 15 when driving on thruway doing 55 to 70 and stay with traffic not saying they are doing the speed limit thats posted but im sure they are doing something close to it is it stil cable driven or eletrical?its a 2005 chevy monte carlo

car starts engine runs ok and drives ok and all gears work?>
check all fuses first
the scan the pCM for error ,for sure speed or comm errors
if the speed sensor fails so will transmission does it?
yes car is sold with manual transaxle, and are very different..
so this is automatic transmission.(guess1) A/T BOX.
NO junk $13 code reades, real scan tools see the Bnnnnn codes.
for VSS errors of any kind.or P07nn tranny codes and more.

no ABS errors or lamp, or ESP lamp glows?
no BCM errors, for sure driving and scanning or wheels up spin tires.(on stands:) safely.
the service manual covers all, read it yet?
car is not AWD is it?
many cars now use ABS to speed. (gee it's freed data why not ) ?
The PCM sends speed to the cluster.
the big problem here is assuming we know how this works.
and not reading the manual first.
the schematic show (on my screen now) the VSS1 on tranny is a 3pin hall sensor.
power 12v to sensor is fuSE B/UP in bay box.
speed out pin3 goes to vast things.

1. cluster pin a17 (obviously car must be moving to get output from pin3 above)
2. tire pressure contr
3. BCM
4. atm key control
5. DLC pin 6 can_bus high ( obd2,P6, WE CAN SCOPE VSS !
6. AC (hvac) controller

ALL 6 MODULES WILL DO DTC ERRORS FOR SPEED. IF FAILS.
EVEN STEERING CONTROLS TOO.

SEE DLC PIN6 ABOVE HOW EASY IT IS TO SEE VSS1?
A POWERFUL HALL OUTPUT SQUARE WAVE.
THE USA CAR USES A 16 PIN OBD2 DLC CONNECTOR
OUTSIDE USA THE DLC IS NOT 6 AT ALL.
IT IS 20 PINS 3ROWS OF PINS.

YOU DID NOT POST YOUR VIN# SO IT IS IMPOSSIBLE FOR ME TO KNOW WHAT CAR THIS IS. AND BUILT FOR WHAT COUNTRY EITHER. NOR IF A/T OR M/T BOX.
ABS LAMP IS NOT FAILING.
SORRY. ABOVE IS ALL PURE USA SERVICE MANUAL FACTS.
ONLY



I HAVE SCOPE AND CAN SEE PIN 6 ANY TIME i WANT AND EASY
OR GOOD OLD SKOOL, ANALOG VOLTMETER WITH NEEDLE
SCALE, AND SEE IT WAG HIGH LOW AS FRONT WHEELS SPIN.

MOSTLY NO VIN NO JOY HEAR SAD AND BIGTIME PAR.
THE TRANSMISSION NOT TOLD AT ALL .


COMMON DTC ERRORS VSS ( ROAD SPEED)
P0721 Output Speed Sensor Circuit Range/Performance
OR 722 OR 501
P2159? NOTE A P2 IS OEM FACTORY CODE !
A;/T BOX HAS 3 SPEED SENSORS, TURBON, MID AND TAIL
U0100 COMM ERRORS?

MOST MODERN CARS
WE SCAN IT DRIVING, AND LOG DATA
AND SEE IF THE PCM SEES SPEED ERRORS
AND SCAN ABS TOO, FOR SURE STICK SHIFT M/T BOX.
WITH PCM KNOWS CAR IS IN DRIVE, AND UP SHIFTING
AND VSS IS DEAD.
NEVER SEEN ANY CAR NOT SHOW THAT DATA IF VSS IS DEAD
REGARDLESS OF VSS SOURCE.
UNLESS TOY CODE READERS USED. (DON'T?)
GOOD LUCK TELL MORE GET MORE , THIS POST IS TOO LONG DUE NO VIN TOLD OR WHAT TRANSMISSION IS THERE.

Speed sensor is probably faulty.

Are the fans running on low or high ?
Circuit/System Description
The engine control module (ECM) commands the fans ON in either high speed or low speed, depending on cooling requirements. In low speed, both fans are turned ON at a reduced speed. High speed has both fans turned ON at full speed.
In low speed operation, the ECM applies ground to the coil side of the fan low relay. This energizes the coil and applies voltage directly to the left cooling fan through the switch side of the fan low relay, which is fed by the fan 1 fuse. The right fan is connected in series to the left fan through the de-energized fan control relay so that both operate at low speed.
In high speed operation, the ECM applies a ground to the coil side of the fan low relay, the fan control relay , and the fan high relay. On the fan high relay, the energized coil closes the switch side of the relay and applies voltage directly to the right cooling fan through the switch side of the relay, which is fed by the fan 2 fuse. At the same time, the ECM energizes fan control relay pulling the switch side over, providing a direct path to ground for the left cooling fan, which has voltage applied through the energized fan low relay. In high speed mode, the fans are operated as a parallel circuit with full voltage applied to each.
You should have it hooked up to a scan tool that can read engine sensor data parameters . Check for DTC'S - diagnostic trouble codes etc...
Circuit/System Verification

1. If DTCs P0480 or P0481 are set, perform those diagnostics first.
2. Ignition ON, verify with a scan tool that the control module is not commanding fan activation.
3. Ignition ON, observe that the fans are not activated.

Your best bet is to take it to a ASE certified repair shop !

Sounds like the speedometer is faulty. If the speed sensor were the problem, the odometer would not work correctly, the transmission would not shift correctly, and the check engine light would be on.

GM had problems with a part inside the instrument cluster ,called a stepper motor .2003 to 2006 This stepper motor controls the gauge needle .
How to repair GM Instrument Cluster Speedometer Gauge Stepper

so you know where the cable is thats good.Next take some oil and pour it down the cable ,because that all is wrong .adrian,,,,

It is probable that the cable for the speedometer is streached and your MPH reading is incorrect all or most of the time, get a friend to drive behind you at say 55 MPH so you will know the odometer reading ( may prevent speeding ticket )
All metal expands when heated and contracts as it cools. Your cable becomes slack when hot. Stay Safe!

Possible speed sensor. I know the speed sensors are a problem with hondas. A new speed sensor is about $200 and can be easily replaced. It bolts to the top of the transmission on the firewall side. Do NOT buy an e-bay one. They are only about $30 but are junk

replace the cable that goes from the back of the speedometer to the tranny

It is the cable that connects to the trany..