Car speed V Sat nav speed

[Sado]

Having recently been given a satnav as a present, I have noticed that the speed given by the satnav differs from the speed indicated by mine and my wife's car speedos.
Which is correct?
(please, no jokes about the cars wearing speedos)

[squarebear]

We had this thread before. Sorry I can't find a link but the general opinion was that the sat nav speed is more accurate.

[Sado]

Thanks for the speedy answer. I tend to agree, but I'd rather not leave it up to a court to decide.

[Disco Fever]

The speed on the sat nav is generally 10% lower than your actual speed due to the fact it's getting it's signal 24,000 miles away, yes 24,000 that's 12,000 to the satellite and then 12,000 back to your sat nav....

[Disco Fever]

If you follow a learner driver they will never go over 30 mph in a 30 zone so you will know what speed your doing to the speed on your sat nav, there's one way to find out....

[Ethel]

Satnav speed is usually quicker than than the speedo speed, not slower.

[ChuckFickens]

I disagree with both above points!

The time for the signal to get from (yes just from!! not there and back, sat navs don't send signals to satellites they just receive them) is taken into account in the calculations for working your speed and position out.

Also if your following a learner driver they won't go over 30mph as shown on their speedo, which is probably no more accurate than your own speedo!

Car speedos normally read over by anything upto 10% and I bet your satnav always shows a lower speed than your speedo.

The satnav will be more accurate if your holding a steady speed for a little time but it may not be so accurate if you are speeding up or slowing down.

[rov1200]

I have found that a satnav is not very accurate for distance when counting down to a junction or roundabout say. As velocity is distance/time then if the distance is wrong so will the real time speed at any instance.

[kira000]

When driving, my sat nav (tom tom one), shows my speed as slightly slower than my speedometer. Interestingly i have a new car, and in the new car, the difference between them is slightly less than in my new car.
I did ask the dealership about this, and they told me that the new the cars, the more accurate the speedo is. This is one reason why the police allow speed limt +approx 10%; the speedo's werent consistent at showing speed.
As support for this, there are several of those roadside speed indicators on my route too and from work that tell you how fast you are going and give a smiley or frowny face accordingly. The speeds that show on these signs s pretty much in line with my Sat nav, but is slower than my speedo. I.e Car says 31MPH, Sat nav and sign says 28MPH.

[kira000]

Sorry, noticed an error. i meant to say that in my new car the difference wbetween sat nav and car is smaller than between old car and satnav. 34/30 then compared to 32/30 now.