Inconsistent speedodometer reading

[sifan]

The car is 1995 Toyota Tercel with 293K miles. Its speedodometer sometimes gives higher reading than actual like 3 to 5 mph depending on the speed. I usually up shift to 3rd gear when the speeddometer reading reaches 35 mph. Sometimes I felt I was lugging the engine. I discovered the discrepancy after hooking up my ScanGauge to the OBDII port. Now, I always look to the ScanGauge for speed reading when shifting. Under the bright sun it is very hard to read the display from the ScanGauge. Then I have to rely on engine sound to shift which is very frustrating.

The repair shop told me that the speedodometer does not use a cable but an electronic sensor instead. I have several questions:

1) Does the speedodometer inaccuracy affect the engine timing?

2) Is the sensor going bad? Or there are other causes.

3) If the sensor were bad, how easy would it be to replace it? What is the name of this particular sensor? Is it expensive?

Thanks.

 

[chrome]

You need to verify that your speedometer is really reading incorrectly. The ScanGuage takes OBD2 data from the ECU, but it actually doesn't measure real time speed, plus you can actually manually trim the speed reading on the SG itself. The best way is to check your speed on a GPS unit, then calibrate the SG to match the GPS readout.

 

[StevieC]

Originally Posted By: chrome
You need to verify that your speedometer is really reading incorrectly. The ScanGuage takes OBD2 data from the ECU, but it actually doesn't measure real time speed, plus you can actually manually trim the speed reading on the SG itself. The best way is to check your speed on a GPS unit, then calibrate the SG to match the GPS readout.


+1

 

[Gary Allan]

Sometimes the indicated mph is not directly related to the output from the VSS. What input is generated to get the needle to point where it is ..is related to the PCM's output as in integrates the pulses out of the VSS ..but doesn't mean that the needle is pointing where it is supposed to be.

While I have a gear that I can swap on my jeep t-case (it drives a VSS output sensor) I can have the dealer make the speedo read anything I want using their scan tool (DRBII)

 

[SHOZ]

You can go out on a straight stretch of Interstate and check you speedometer with a clock. Just drive 60 mph and you will go one mile in a minute.

 

[sifan]

In fact I discovered this problem at first hand while testing the Microsot GPS map software on my laptop. I noticed my speedodometer reading is 3 mph higher than what the GPS reported. At that time I was not sure if this was GPS error or not. Later I confirmed it with the help of ScanGauge.

 

[Quattro Pete]

Most speedos are designed to read high by a few mph. No need for concern. If you're lugging the engine, then just wait until you reach a higher speed before shifting.