I use a set of GoPros but the GPS signal on them can sometimes be poor. Even when they have been outdoors doing laps every now and then they will lose the GPS signal and that then means you lose data about speed and track position. You can buy a separate GPS receiver to go in your car to accurately record your GPS data. Mobile phones are generally restricted to a GPS update rate of 1hz but a proper GPS receiver will give you an update rate of 25hz
My set up is:
1. Record video footage on a front facing GoPro and a read facing GoPro.
2. Use a RaceBox Mini S
www.racebox.pro to capture the GPS data and lap times.
3. Once I have all the video and GPS data import it in to a RaceRender template - then sync up the two lots of video footage and the GPS footage.
RaceRender will then be able to display information like speed, track position and lap times depending on the display elements you are using. RaceRender will allow you to sync all of the your video and data and create a video that uses those separate sources of information. In the video I posted above the speed is recorded by the GPS receiver and does not take the speed measurement from the car. The timing is based on the GPS data. The bits of video footage have been synced so even though they started recording at different times. The rear footage reflects what is going on at the back of the car at the same time as what it going on at the front of the car. RaceRender also allows to you manipulate video so the rear footage is flipped horizontally to show what you would actually see in a rear view mirror. If wasn't flipped cars that approached from behind on the passenger side would appear in the video to be on the drivers side.
I started using Harrys Lap Timer but I found that it was bit cumbersome on an iPhone and the files it was generating were difficult to transfer due to their size.
I run RaceRender on a Mac and it works really well.
My only criticism of RaceRender is that the software is now getting old and I have not seen it updated for about 2 years now. I doubt it will ever be updated again.