speedo vs GPS - deviation?

[daniel575]

I've noticed that at low speeds (30-50 kph) the speedo gives speed as ~3 kph slower than GPS.

This rises to a deviation of 7 kph at 130 - when the speedo shows 137 kph, GPS shows 130 kph.

Measured with my phone GPS, on cruise control on flat roads, many times.

Who do we believe? IMHO 7 kph is a significant deviation. I usually keep my speed at +4-5 which is what most people do but on the motorways here (Netherlands) nobody gives a damn about speed. Everyone is always speeding and I just don't want to get fines, so I'm slow. So I'm just wondering whether I'm being too slow and could actually add a few kph still without risking fines.

[edit] I'm planning to do a test some time soon. I'll drive at exactly 100 kph for exactly 10 minutes (stopwatch), using motorway markers, and then see just how far I came. Should be able to calculate the actual speed from that. Has anyone else tried something like this before? By the way, I'm using the original size tyres only (205/55 R16).

 

[Dt-spd]

5% down at 130kph is pretty good to be honest.

Sounds like you could be doing an indicated ( on speedo ) 140kph without gaining any attention from the police, especially if everyone else is going quicker.

No need to do 100kph for 10 mins, just time one set of motorway markers and work it out from there.

 

[MattW35]

Mine is typically 3-5 mph out. As pretty much every car I've had has been.

 

[niggle]

The OP's issue seems to be in the 30 - 50 kph range where the speedo reads too low. This is unusual and leaves the driver prone to getting caught in traffic calming areas if the speedo is taken at face value.

 

[daniel575]

No, it's always showing lower speed than GPS. At 50 on cruise control, GPS will show 46-47.

I assume all our cars would have more or less the same deviation (if you're using original size wheels).

 

[chrisRibiza]

This always seems to be the case, I noticed it in my Ibiza when using a gps that the speedo read about 3mph lower than the speedo.

 

[philhoward]

By law, a car speedo can't under-read, but can over-read by up to 8% (I think). This allows for difference in tyre sizes (despite the wording, not all tyres of a specific size have exactly the same rolling radius), plus a little bit of a safety factor.

 

[Brady93]

Measured this myself the other day, when my speedo reads 80mph, my gps reads 77mph. All on private property you understand.

Sent from my SM-G935F using Tapatalk

 

[Cupra Belfast]

Speed not directly related

GPS and speedo velocity shouldn't really be expected to match. Unless a military spec GPS device they do not give an accurate reading of speed. They aren't accurate enough in positioning to do so. They work on triangulating the position of the vehicle and reading it's speed in a linear fashion, the sum of vectors essentially. Don't kid yourself because the GPS says you aren't speeding that you won't get a ticket.

 

[Seastormer]

Try watching the road a bit more than the speedo, if you don't want tickets.

 

[dw911]

GPS and speedo velocity shouldn't really be expected to match. Unless a military spec GPS device they do not give an accurate reading of speed. They aren't accurate enough in positioning to do so. They work on triangulating the position of the vehicle and reading it's speed in a linear fashion, the sum of vectors essentially. Don't kid yourself because the GPS says you aren't speeding that you won't get a ticket.

Wouldn't really say that's the case now, many years back maybe, and most certainly when SA was switched on, then they were so inaccurate you had to have land based stations to get a vaguely accurate position. But now if you have a decent lock on several satellite's even a cheap unit should be very accurate

 

[Cupra Belfast]

Accuracy

Wouldn't really say that's the case now, many years back maybe, and most certainly when SA was switched on, then they were so inaccurate you had to have land based stations to get a vaguely accurate position. But now if you have a decent lock on several satellite's even a cheap unit should be very accurate

They still haven't introduced sufficient accuracy yet to give a true speed reading. Only the military satellites and hardware are sufficient to do that. Even these days a land based station is required, calibrated to a benchmark, for good accuracy. Speaking from experience taking levels for drainage networks (not glamorous, but essential of building hydraulic models of the network).

 

[dw911]

They still haven't introduced sufficient accuracy yet to give a true speed reading. Only the military satellites and hardware are sufficient to do that. Even these days a land based station is required, calibrated to a benchmark, for good accuracy. Speaking from experience taking levels for drainage networks (not glamorous, but essential of building hydraulic models of the network).

I'd still tend to disagree with that regarding the accuracy were talking about here, which seems to relate to how lightly are you to be nicked for speeding if you rely on a gps unit to tell you how fast your going.
There have already been cases thrown out or over turned on account of the accused producing logged dater from a gps - I'm guessing from the sort of units companies use to track their drivers or vehicles.
Also most large GPS makers like garmin give a Speed accuracy of +or - 0.1 mph

Like you I've had many years of using gps based systems, although for different applications to you, so like you I too do agree they have their flaws.
But for the application were talking about here I,.e how fast is my car travelling.
Then in that case I'd take comfort in the fact their speed accuracy has stood up in a court of law and the fact the manufacturers are happy their products have a high degree of accuracy.
So I'd be happy to use to take what a gps is telling me to be my speed, unless of course I was driving through a 10 km swiss tunnel, in which case I'd revert back to the old fashioned speedo

 

[Havsgaard]

I would in general state that a GPS is the most accurate when reading out speed - and especially in recent years with improved hardware. However, there might be some circumstances such as going through a tunnel, atmospheric disturbances, weather conditions etc. - which results in a lower accuracy.

A car is over-reading, always a bit higher than the actual speed, which is fine. The GPS however is calculating how fast your are travelling between point A->B, and with a civil GPS accuracy down to ~3-5 meters, that gives by far the best reading compared to how a car calculates speed.

A personal experience in this regard - I set my car cruise control on 97 km/h (same as shown on GPS), but my car was stating 100 km/h in the display. On top of that, I was 'laser beamed' by the police and got a speeding ticket. I was measured to a speed at 97 km/h by the police laser.
To my luck, there's a 3% deviation, and so the fine was set on a basis of 95 km/h - which is max speed in the lowest fine category when speeding in a 80 km/h zone here in Denmark.

 

[srytter]

I can't Seem to find my speed on the navigation system. Actually I don't Think it's there. I have the "normal" nav, not the one with a slightly bigger screen.

Am I not looking in the right places?