My speedo is fifteen mph. Over actual speed 1995- 2500 GMC 2 wheel

Hi @viper2964 ,

It may be a faulty Vehicle Speed Sensor. (VSS)

Here’s a link that shows how to recalibrate the VSSB (vehicle speed sensor buffer) that may help if the sensor itself is OK

Just wondering though if the error margin is within allowable tolerances.

There are regulations in various countries that mandate the accuracy of car speedos, (see below), but as best as I can find out this is not the case in the USA. There most of the manufacturers adhere to SAE J1226_201108. It isn’t cheap to get a copy of the specification however so I don’t know what it says.

In Australia and EU countries it is up to a maximum of 10% of reading + 4km/h but not less than the speed, e.g. speed of 100km/h could read up to 114km/h but the speed is not less than 100km/h.

In the U.K. it is up to a maximum of 10% of reading + 6km/h but not less than the speed e.g. speed of 100km/h could read up to 116km/h but the speed is not less than 100km/h.

You should never get caught for speeding if the speedo registers the same as the posted speed sign ;-)