Incorrect Speedometer Reading

[Rognmoe]

Hello, all!

I have an issue here and I want to see what your take on it is: my speedometer is off between 2 and 7 MPH, depending on how fast I'm going. I'm running the stock tire sizes, so I'm not sure why it would be off. Anyone know of any quick and/or easy fixes for something like this?

[Shazam]

I've found Saab speedometers to be a bit optimistic. Mine always seems to be off by about 7%. So when I'm going 72, the speedometer reads about 76 or 77. I remember Hirsch saying his 9-3 reads a bit high as well.

[nordwulf]

It's normal for speedometers to show a slower speed than your actual speed. 5% difference seems normal.

From the WIS

The ESP control module reads 29 pulses per wheel revolution. Depending on the dimensions of the tyre and rim (physical circumference), one kilometre will correspond to a different number of pulses when the main instrument unit receives the value from ESP. It is therefore possible to change how the main instrument unit interprets the total number of pulses per kilometre so they correspond to the tyre dimension fitted on the car.

Programming must be carried out so that the speedometer and odometer can give the driver the correct information. On certain markets, it is a legal requirement that the speed the speedometer shows is between 0 and 10 km/h too high.

It can be adjusted with Tech2

The current tyre dimension is programmed with Tech 2. Select the correct tyre from the list in Tech2 or choose to program the dynamic circumference of the tyre manually.

If the speedometer is programmed for a certain tyre and new wheels with larger circumference are fitted, the speed indicated by the speedometer will be too low.

The speedometer in my 9-4X is showing the exact same speed as my GPS unit so it differs by vehicle.

[Burnsside42]

Other way around Frank - the Speedometers in Saab's often show FASTER then you are really going Average by about 5mph off once you are over 45MPH. This is something they always seem to never correct (in US spec cars anyway). The NEW 9-5 and the 9-4X I think are dead on - as Sport Sedans were very close too.

Part of owning an older Saab - thinking you are doing 100 - but really just 93 or so

[Rognmoe]

So, Frank, what is Tech 2? Some kind of OBD plug-in?

[Shazam]

Tech 2 is basically a way more complex OBD tool. A OBD reader can only read error codes, and clear them. A Tech 2 can do that, and change many settings in the car that are more directly related to the ECU, and more in depth features. Dealers, and very well equipped repair shops have Tech 2's, while pretty much every shop has a OBD reader. A Tech 2 is a couple thousand dollars, while a OBD reader is $50-200+.

[Rognmoe]

Oh, OK. Thanks for that. I'll have to ask my repair shop about that when I take it in to get the fuel tank checked out.