Discrepancy Between Speedometer & Satnav - Which is Most Accurate?

Question

At 100 km/hr my Satnav shows a sped about 6 km slower than my speedometer. I suspect the Satnav is more accurate as it is an external measure of time and location, but I also can see an argument that its rate of sampling and curving roads may affct accuracy. Does anyone know?

Answer 1

In most cases, trust your speedometer. There are always times when it could be wrong, but for satnav, it is never 100% correct because there is ALWAYS about a second of lag.

Answer 2

SATNAV, because your speedometer may be inaccurate for mechanical reasons (i.e your tire pressure) while SATNAV software calculates your displacement to a very accurate clock. The software found on most SATNAV systems know about compensating for curving roads.

Answer 3

Your speedometer is allowed to overread by up to 10 percent.Thats the law.It is not allowed to underread at all.Your satnav gets a new measurement roughly every 30 feet.Curving roads etc should be negligable on both instruments.I drive wagons,vans and cars .The wagons are fitted with a tachograpgh.The anologue tachos i have compared the satnavs against have been spot on.The cars and vans have been around 3 or 4 miles overread.I havent bothered to check the satnav against a digital tachograpgh.Mainly because the digital is an expensively underdeveloped piece of shite.

My advice?

Trust your satnav for a true road speed.

Go with your speedometer to avoid speeding tickets.

Answer 4

No speedometer is 100% accurate.

That is why police have to allow you a certain amount of leway before they will book you for speeding.

If you change tires on your car to a higher/lower profile, your speedometer will change. To a lesser extent, if you put more or less air in your tires, the speedometer will change.

If you change your diff ratio, the speedometer will change.

Now, the gps is typically more accurate than the cars speedometer, however, once again, depending on the gps, there is often lag, by the time the signal leaves the device, hits the satellite and returns to the gps, and also the gps brand, and the software the gps is using.

Answer 5

Neither is absolutely, incontrovertibly accurate, even a slope can affect a satnav due to the change in angle between satellite and antenna.  However, the satnav is far and away the closest. Speedos are never allowed to under-read, they may over-read by up to 10%.

As the greyfox says, rely on your speedo to keep safe from speeding tickets, if it reads 70 you're probably doing somewhere in the late 60's.  And the satnav for the most accurate reading. And again, don't bother bickering with a policeman about it if you're caught . . .

Answer 6

The sat nav is the MOST accurate.

I own 3 gps.  I compare then against each other (and against the car).  The GPS are all in agreement (down to +/- 1 mph or even less).  But I find that most cars speedo are often wrong.

I think (my guess) that the car companies makes the speedo read a little higher so that

(1) they don't get sued for accidents and tickets

(2) you don't get  a ticket driving at the max limit

It seems like all cars are about 5 MPH too fast at 60 mph (100 KM/hr)

Good Luck...