If you plan on using this app to give you accurate readings that you need to rely on for some reason then calibrating the device or comparing it with a calibrated device is essential.
DigiHUD app doesn’t know how accurate the GPS data is that it receives and doesn’t make any allowances for inaccuracies in the data. All the app does is display what the (usually cheap, low-cost, low precision) GPS sensor is telling it to. So if the sensor is inaccurate then the app will be too.
I use the app every day (well, every car journey actually) and because I use a high precision GPS receiver, and also because DigiHUD matches exactly several local roadside speed warning signs, I’m very comfortable that the speed I’m travelling is accurate enough for me. If the accuracy was unknown then I’d be uneasy travelling on the speed limit.
So please remember the following, taken from the app’s Help text:
Although we strive to make all readings as accurate as possible they are only as accurate as your device’s sensors and should only be regarded as approximations.