MyGeotab: Why is there a difference between GPS speed readings and engine road speed readings for the same time period

• The difference is due to the GPS speed beingmore accurate than engine road speed.
• Engine road speed is affected by a variety of factors including difference in wheel size due to wear, pressure, and temperature.
• Industry officials have said speedometers are set "optimistically" to give drivers an inflated sense of speed (and help them avoid tickets),

Please refer to the following article that talks about how speed is calculated differently between GPS and Engine Measurements (speedometer)

• MyGeotab: What is the difference between Trips History Speed and the Engine Road Speed measurement

An article from the Geotab's blog

• GPS Speed vs. Vehicle Speedometer