How do you get the mph on the display when recording, i am doing rockingham friday and want to record some laps. Would be nice to have mph on the display. Thank. Kev

You mean HUD graphics on the video?

I, as do many others, use a phone app that uses the built in GPS. It logs the data which can then be exported to a software that maps the video and speed.

I have done it, see below. I used Trackmaster for my Android (Harrys lap timer for my iphone) and Racerender to mesh the lot together.

Using regular phone gps for speed is pretty inaccurate, mainly for reaction time as generally they only work at 1 hz.
Heres me using a racechrono app to log and race render to present he video. Notice how slow the speedo is to react.
https://www.youtube.com/watch?v=DnogK6sYy3E

Then this is my latest, using same software but running a 10htz qstarz blue tooth gps receiver.
The revs and throttle position were logged on racechrono via a bluetooth obd reader, which obviously needs a car with a plug.
https://www.youtube.com/watch?v=5ekhdPecSek

If you have a camera that records the GPS or some other log such as ECU log you can use Dashware to put telemetry on the video.

http://www.dashware.net/

Cheers
Stot