Bro I request you to kindly use some common sense. If u have read some science, then u should know that GPS satellites orbit the earth at a height of 20200 km. It is not a small height. So never expect accurate readings from GPS. GPS in smartphones have accuracy of upto±10kmph.
Only military grade GPS devices show accurate measurements.
And moreover, all the goods loco (G5/G7/G9) can never exceed 100 kmph because of their gear ratio. THEY ARE NOT DESIGNED TO RUN AT SPEEDS HIGHER THAN 100kmph.