Why does my sat nav speedo say I'm travelling 10kph slower than my car speedo? Which is more accurate?

Answer 1

Harry M is right. Car speedos are 10% inaccurate.

If you do 70 mph per your speedo, you are doing roughly 63 mph. If you get caught speeding above 70 mph then you should be quite aware that you were speeding because your needle was 10% higher than that in front of you.

GPS speed is more accurate so go by that.

Answer 2

Hi there in the Uk we have Construction and Use regulations which state that up to 30mph the accuracy of the speedometer of any vehicle has to be within 3% and cannot read lower than the speed yor travelling. After 30 mph the speedometer can be a maximum of 10% out but again not lower than the actual speed. I have a sat nav as well and have matched my speed by some of the automated speed warning signs many times and the sat nav has always been more accurate than the speedo of the vehicle and the same as the sat nav, the signs i know are calibrated on a monthly basis (my friend does that job) so i know they are accurate as some have cameras in which have been used in legal cases

Answer 3

In Europe, by law, the speedometer can never show slower than actual speed.
EEC directive 75/443 (1975) and the follow-up EC directive 97/34, which you can find here:
http://europa.eu.int/comm/enterprise/automotive/directives/vehicles/dir75_443_cee.html
from paragraph 4.4: "The speed indicated must never be less than the true speed."

Although it is not in the English version, I am told that the German version of the document has further formulas in it. The translation I was given is:
"At the speeds specified for the test in 4.3.5 above and between these speeds, there shall be the following relationship between the speed indicated on the dial of the speedometer (V1) :
O * V1 - V2 * V2/10 + 4 km/h"
So therefore, the speedometer in an EC compliant vehicle will show at least 10% over the true vehicle speed.
http://autos.groups.yahoo.com/group/Prius-Europe/message/595

Also, as others have mentioned, if you change tyres on your vehicle, or worn vs. new, will have a different diameter or revolutions per mile figure, and that can also cause your speedometer to be off a few %.

Your GPS will usually be more accurate.

Answer 4

I own 3 gps. I once compares 2 GPS vs. my car speedo. The GPS were almost in complete agreement of each other.

The was as slow by about 5 MPH (at 70 MPH).
===
It makes sense. If tons of people got tickets because they were just over the speed limit, you can imagine all the law suit to the car manufacturers.

Good Luck

P.S. Jake Dey above is completely full of BS. It is the delay that the GPS use to figure out the position. The delay is accurate to better than 1 part in million (not 5 or 10 percent).

Answer 5

yes most car speedos are out by 10%, i have gone into my settings on climate control ( vw passat ) and lcd readout reads different from speedo and is spot on with sat nav

Answer 6

your sat nav will get you a speeding ticket..most cars speedo is only off 2 or 3 miles at the most.

Answer 7

It depends.

Car speedometers are not allowed to under-read, but are allowed to over-read by 10%, according to EC regulations. Therefore they are designed and calibrated to over-read about 5%, to keep them within the regulations.
Also, as a car speedometer actually records the speed at the gearbox not at the road, as your tyres wear (or if they are under-inflated) the reduction in tyre rolling radius means it will over-read more.

GPS systems measure speed relative to satellites, and are not bound by regulations to prevent under-reading.
On a flat road, with good satellite reception, they are very accurate. However any gradient will make them under-read (as the actual distance travelled on the road is slightly greater than the flat distance measured via the satellites), and poor satellite reception will reduce their accuracy.

Answer 8

Car speedos usually over-read by anything up to 10%, your satnav's the one I'd believe.

Answer 9

Your car may be the most accurate. Keep in mind that there is a delay between the sat and your nav system.

Answer 10

Usually the GPS is much more accurate actually. But It depends on how many satelite signals it has picked up. The more signals it can pick up the more accurate the reading will be