Which is more accurate - GPS or Speedo ?

[KiwiTT]

I thought my speedo was pretty accurate. I would drive along those odometer test counters and look down at my odometer and it would be dead-on. So I naturally assumed that the speed indicated on the dash would also be accurate.

Well as a retirement present, the company gave me a Tomtom 250 XL GPS. I placed this the window. I noticed it would also read the speed that I was traveling at and it would go red if I was driving too fast. It was then that I glanced at the actual speed and noticed that my indicated speed was even faster.

So while my speedo was telling me I was going 110kph, the GPS was telling me I was going only 105kph. I checked this about a dozen times and the difference would always be 5kph regardless of speed.

[elnevio]

For steady speeds, GPS will be accurate. The speedo will vary with tyre size and wear, as well as with any factory overreading built in to the speedo.

[bradc]

An 8g speedo does overread a bit, while
the odo is perfectly accurate. Considering your 225/40/18 tyres I would have expected you speed at 110 indicated to be about 108 actual

[AndyD]

As the GPS is coming from a satellite.. they typically have a little bit of lag to them as far as I know. As Nev said, steady speeds, GPS will be great, but during accelerating, it will be slightly behind what you're actually doing I believe.

[horndog]

My speedo always runs lower than GPS, at steady 40mph Speedo I usually get 43/44mpg on satnav (I checked on two different types)
using 225 45 17

[elnevio]

I'm surprised it's that far off Marc, but the effect gets a bit more noticeable as the tyres wear down. Pretty much every car speedo will overread slightly. Don't suppose a manufacturer wants someone to try and claim that because their speedo was underreading, they get clobbered for speeding and it is partially the manufacturer's fault!