Speedometer - vs - in car GPS

[Inmykombi (Geoff)]

Hi,
Please excuse me if this topic has been on a previous thread, however I have not seen one on this subject.

I am wondering what the general feeling is about either using the car speedometer, or the in car GPS navigator to provide a more accurate speed reading while driving.

In my work Van, there is a difference of about 5 kilometers per hour between them.
ie: When my car speedo says I am traveling at 110 klm/h, my GPS says I am traveling at 105 klm/h.

What would be the most accurate ?
My feeling is that the GPS would be more accurate.

Whats your opinion ?

Geoffro.

[Bart]

It is a known fact that vehicle speedometers are deliberately faster than actual road speed if the vehicle is post 2006. Pre 2006 the law is less stringent.

Basic interpretation of ADR (Australian Design Rule) 18 (2006 Revision) indicates that a motor vehicle can never be going faster than shown on the speedo. So, if the speedo indicates 80kph, the vehicle must be doing 80kph or less.

Manufacturers always build an error into the speedo and we guess that some of these reasons would include protection for the consumer so they never get booked for speeding and also to reduce the impact speed in an accident. There is also some unproven speculation that it helps them fudge the fuel economy figures as those advertised are generally never achievable in the real world. This has an effect on the actual odometer reading which is also covered by an ADR. Tyre and rim modifications have also been legislated around this to provide better protection to consumers and states that you can fit any tyre and rim package to a vehicle you like as long as the new package is within + or - 15mm of the original rim package, which again limits the amount of error tyres and rims can induce into a speedo.

Yes, your GPS is more accurate, but why bother running the risk of its inaccuracies when your vehicle is designed to protect you if you are obeying the road laws.

[Inmykombi (Geoff)]

Thanks Bart, that makes perfect sense the way you explain it.

[gary]

Hi Geoff,

The current Australian Design Rule standard for speedometers is based on
United Nations agreement on uniform speedometer installation.

It states for purposes of calibration -

* That the tyres shall be of the ones normally fitted to the vehicle and
at the correct pressures.

* That the test be carried out with the vehicle at its unladen weight.

* That the test be carried out at 23 +/- 5C.

* That the speed indicated shall not be less than the true speed of the vehicle.
At the various test speed specified, there shall be the following relationship
between the speed displayed (V1 ) and the true speed (V2).
0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h

So in a nutshell current speedometers must not under estimate. However, they
are allowed to over estimate by 4k/h plus the 10%.

In practice, most do intentionally over estimate.

GPS in itself only inherently provides a positional fix with an associated error
radius. SatNav systems take two position fixes and calculate an average speed.
The number of available satellites that are being received, GPS canyoning
in city streets, etc. can give rise to a larger radius of error in the position and
hence a larger error bar on the velocity estimate.

In a nutshell, it is normal for your GPS to display a speed that is slower than
the speed of your speedometer because of the standard by which speedometers
are calibrated.

However, given that all modern vehicles pretty much around the entire planet
have their speedometers calibrated in an identical way (by UN Agreement), then
it is worth keeping in mind that if you are keeping up with a car in front and your
speedometer indicates that you are both at the gazetted speed limit, then the driver
in front is very likely seeing the same readout on their speedometer.

If your speedometer is at the gazetted limit but you then glance on your GPS and
see that it indicates below the limit, then I'd suggest it is good driving practice to
drive by the speed on the speedometer with respect the traffic around you.
They will be driving by the instruments fitted to their vehicles and that is the
only information they have available to them.

Best regards

Gary

[desler]

While the ADR's do their thing, Australian standards require a car tested must show a speed reading within + or - 10 km/h at 100 kmh as depicted on the vehicle speedo at the point of new car compliance requirements.

In 15 years in the auto industry, I've never come across a car that when it says 100 on the speedo is actually doing 100. most are around the 92 to 94 mark and I cant remember one being over 100.

The gps might be accurate, but without calibration and evidence to support it, who'd know?

Darren

[Kunama]

Unless you drive a 1999 Toyota Echo, which with the correct tyres fitted shows 100 when you are doing 100. (Of the 41 cars I have owned it is the only one where this occurs - bought it for my kids to learn in)

[rally]

Correct tyre size is a key requirement

I got given 5 almost new offroad tyres and subsequently paid the price - three speeding fines in the same month due to the tyres being oversized from the OEM spec !
Luckily I was only a few kms over the limit so I only lost 1 demerit for each offence. but it cost me about half the value of the tyres
. . . and I had to pay to get the speedo recalibrated

So check your rotating tyre diameter

[LewisM]

VW Passat manual states that the actual speed is 4 km/h LESS than that indicated by the speedometer. You can check this by accessing the onboard computer through the climatronic (climate control interface). I have checked it many times especially after changing tyres, and ALWAYS 4 km under speedometer. So, I drive speedometer, I don't get a ticket.

I also verify the speed by GPS, and it concurs.

[Baddad (Marty)]

I have a friend, Col, who has been a transport driver for a great many years. He informs me that the speedos in large vehicles are very accurate and calibrated to show actual road speed.

I told you that story to tell you this one.

Col has a GPS. When he drives sedans or small vans the GPS always indicates 3 to 6 klms/hr less than the speedo. However in the large transports the GPS agrees with the speedo.

Sometimes there are devices which indicate vehicle speeds on the roads. In huge lights so the driver can see the read out on approach. The GPS always corresponds with the speed check device.

I hope this places a little more light on the subject

Cheers

[Octane (Humayun)]

My 2005 Echo is bang on, too; 100 km/h is 100 km/h.

H

[Lee]

Our Subaru Outback is way off.... From memory when speedo shows 100, we are actually under 90 according to GPS and timing the speedo check signs....

[mithrandir (Andrew)]

My 2004 Prado with OEM rims and tyres has always at 100kph indicated read 2-3kph less than what each of my three GPS say. That may be because I run the tyre manufacturers recommended pressures and not what Toyota suggests. This increases the rolling diameter.

I only ever compare the speedo and GPS when it has a good fix and I've been traveling on a straightish section for several seconds. You need the GPS track points to fall on the road otherwise the GPS will read slow.

I hadn't heard of the method of getting the GPS speed for SWMBO's Passat. The Satnav system does not display on screen what it thinks the ground speed is. I could use the Garmin which does.

[ZeroID (Brent)]

The VR4 has 17" rims with 225/17/45 Eagle F1 D3 tyres, not factory spec but they are the same diameter. My GPS shows a difference on 3km /hr at 100 kmh, speedo is low by 3%.
It's all about having the right diameter tyres on.

[Kunama]

Originally Posted by Lee

Our Subaru Outback is way off.... From memory when speedo shows 100, we are actually under 90 according to GPS and timing the speedo check signs....

Our Liberty wagon is also quite far off, in fact I will be stepping up to 205/55R16 instead of the 205/50R16s on it now, that should put the car speed at 98-99 when the speedo reads 100.

Getting back to the OPs point, I find that on long straight runs the GPS gives a very steady reading, around twisty roads not so much. I have run 2 GPS units side by side (Garmin and Navman) and both indicated almost identical readings. In the Subaru 100 on the speedo was 93-94 on both of the GPS units.

[DavidU (Dave)]

My BMW speedo shows 100 when the GPS is 94.

[AdrianF (Adrian)]

Wondered why following drivers were impatient until I used two GPS units to check speed of the Pajero. 100 on the speedo shows 89 on the GPS's and that's with factory spec tyres.

Adrian

[Inmykombi (Geoff)]

Thanks everyone for all your input.
I think I will stick to the speedometer as I have in the past, after reading all the comments.

Cheers
Geoffro.

[sam22 (Sam)]

I used various GPS apps on my motorcycle, but they are like 4 to 7 kmph slower than the speedometer. A lot of test studies claim that on test bikes an error of up to 10% is found in the speedometer. Also, there is an ECE regulation that specifies a speedometer can read high by as much as 10 percent plus 4 km/h at a specified test speed, but under no circumstances can the speedometer read low. That may be one of the reasons why motorcycle speedometers show higher speeds than the GPS. According to a scootyblog research article, new-age motorcycles use sensors that are triggered by gear teeth or slots in ABS wheel ring. The ECU of the motorcycle calculates the speed from the frequency of the square wave signal received from these sensors.

[JA]

If you are interested you can check the calibration "in a fashion" over a known distance with some careful technique.

Best
JA

[mswhin63 (Malcolm)]

I personally use OBDII as that bypasses the tolerance set by manufacturers. This is usually set to +0% to -10%. The problem with using GPS or OBDII at least here in SA is that you can be fined for doing 1 km over the limit which happened to my wife only recently. Police know the law is based on tolerance and that there is no excuse.
So drivers beware if you use GPS as the speed is done using averaging otherwise variation in GPS accuracy can give multiple fast fluctuations.