Legal Question in Traffic Law in Texas

It could be. This method of speed detection by police is referred to as "pacing". Basically the officer drives the speed you are driving and makes a note of the speed on the speedometer in the officer's vehicle. The speed on the officer's speedometer will be an accurate speed of your driving speed IF the officer's speedometer has been certified as being accurate. (Just like a Radar-gun must be certified as being accurate.)

Note: Most traffic officers are "trained"/"certified" to be able to guess a driver's speed just by sight within 5miles. So if your officer is trained to guess speed and you were going more than 5 mph over the speed limit then the officer will be able to testify that you were speeding.

You should consult with a traffic defense attorney in your area for a more in-depth analysis of your situation.

Good Luck!