Which is more accurate - my car's speedometer or my sat nav?

What is the most accurate speed indicator - analogue speedometer, digital speedometer or GPS on sat navs?

Why has my VW satnav stopped giving traffic updates?

My satnav no longer picks up traffic, roadworks etc. VW tells me it is no longer supporting its own satnav. Is that genuinely true?

My daughter bought a car with sat nav but it's missing the SD card, can I ask the dealer to pay for this?

My daughter has bought a secondhand Mercedes A-Class and subsequently discovered there is no SD card for the sat nav. Does she have any rights for the garage seller to pick this cost up? What kind of...