Yes, it's OK.
Typically, the GPS speedometers are more accurate than those in the vehicle. The former tell the speed based on purely mathematical calculations (how much time it takes you to move from one opsition to another), the latter rely on the measurements of the wheels or driveshaft rotation, and then apply a bit of math.
That's why in case with car navs, things like wheel diameter and tyre pressure are factored in. Plus, it's a normal practice with car manufacturers to calibrate the speedometers to show the speed being higher than it actually is.
Here are a few good sources to learn more about why these differeces arise:
- How accurate is your car speedometer
- What is a GPS speedometer and is it better than my regular speedometer?
- A Quora thread on the subject