How accurate is the speedo...

Here's an interesting item. I have a Garmin Nuvi 250 that I use in my truck. At lower speeds, the speed numbers match perfectly. If I'm driving through town at around 50 or 60 km/h.....the GPS say the same speed. However, at highway speeds I see a sure difference. And it makes me wonder which is more correct. For example; if I'm driving at about 120 km/h (speedo).....the GPS will show 117 km/h. It seems pretty consistant that once over about 105 to 110, the speed difference from GPS to speedo is about 3 or so km/h.

Which would be more correct?

(p.s. Yes, I'm Canadian....so all my numbers are metric)

 

There is a tolerence on a speedometer, they are not 100% accurate, my GPS and speedometer does similar to yours. I tend to put more faith in my GPS. Either way it's a very small difference.

 

According to my Scanguage my speedo is only about 2 MPH off from what the SC says. I heard that once you get a little bit bigger tires, that goes away

 

It can- but by the same token it can go the other way depending on how big you go. Your speedo will register too slow (but you'd need some large tires for that)

 

x2

 

It's the same with me. If I'm goin 65 the GPS says I'm doin 67. I trust the GPS more than the Speedo.