Should I trust Google Maps Speedometer?

Just download a MPH app on to your phone. That’s what I use.
That only works on smart phones LOL

Also, "it might be" that those aps do NOT work on GPS, but rather the tower information / location in your phone. If you are out somewhere rural, and you only have access to 1 tower, they are not accurate, and not much better if you only have access to 2 towers