Speed readout ~ Satellite calculated v Speedo?

#1
I have a digital speed readout on a satellite “device” in my car and I’ve noticed that the speed I’m travelling at on the digital readout is different to the speed shown by my speedometer.
When my “satellite” calculated speed is showing as 70 mph my speedometer in the car reads 66 mph?
I know 4 mph doesn’t seem much but is this common with other vehicles and are car speedometers manufactured to show a speed slightly lower than the speed that the vehicle is actually travelling?

Has anyone else noticed this in their car?
 
#2
Very - interestingly a volvo is different by 10mph (part of thier sfatey stratergy)

the GPS readout is accurate, the Speedo reading is dependant on how much air is in the tyres, how much tread is left (rolling radius changes) and they arnt that accurate to start with.

Go by the GPS reading, i always do and havnt had a ticket in 3 years (touch wood)
 
#3
My car's speedo's the same after about 35mph your 5mph out ie 40mph on the speedo 35mph on the satnav I would go with the satnav's reading's if you have a really good signal lock. my car's a SEAT so I'd imagine all VAG group cars are the same
 
#4
Unless you're going up or down a very, very steep hill, then the SatNav is giving the correct reading when you're driving at a steady speed.

But if you value your licence, then drive according to the speed shown on your speedo. :)

edited to add: case in point is if you're decelerating from, say, 70 to 40 and there's a speed camera just after the speed limit sign. The SatNav needs a bit of distance to calculate your speed. The time lag involved may be just enough to get you a ticket. Your speedo, on the other hand, gives an instant reading of speed and should be between 90% and 100% of your true speed.

There are fuller answers on the FAQ pages of many SatNav sites.
 
#5
My GPS unit (handheld, but very sensitive) calculates 68.4mph when my speedo reads exactly 70mph. Most cars have a small error - my last car was out my 0.9mph at 70mph. There are many different reasons, most listed above, but also the mechanical accuracy of the speedo mechanism, has it been properly calibrated etc
 
#7
I've noticed this too, using one of those road side radar guns, you know the type that has a smiley face when you are within the speed limit and an unhappy one when you are not. I get the same speed on my satnav and the Radar sign, but the speedo reads about 8-10% faster.
 
#8
Part of the Swiss MoT is to test the speedo reading. The official printout showed 120km indicated was 106km actual. This differential is exactly matched by the speedo/satnav reading.
 
#9
The-Goose said:
I believe most police forces allow a 10% error before proceeding to stop you.
Do they really or is this an urban myth? Do speed cameras also allow for 10% error?
 
#10
TheBigUn said:
The-Goose said:
I believe most police forces allow a 10% error before proceeding to stop you.
Do they really or is this an urban myth? Do speed cameras also allow for 10% error?
I went through a speed camera at 78mph indicated (about 76 real) in a 70 recently and I did not get flashed. It is believed that speed cameras will normally give 10% +2mph, but I'd forget the +2mph and just assume 10% for errors with the speedo.
 

Legs

ADC
Book Reviewer
#11
I've had this with a couple of different SatNav systems in a couple of different cars. Personally I think it's a conspiracy to keep traffic speeds down...
 
#12
I recall a letter to a national newspaper a couple of years ago from a big chief from one of the major suppliers of GPS systems. Sorry, I don't remember the full details and I haven't even attempted to find a link.

From what I recall, he said that the GPS speed was more accurate than the speed shown by the speedo because of deviations introduced by a number of mechanical factors such as tyre pressure but his advice was to play on the safe side and stick to the (generally lower) speed shown by the mileometer.
 
#13
Legs said:
I've had this with a couple of different SatNav systems in a couple of different cars. Personally I think it's a conspiracy to keep traffic speeds down...
hence my coment on volvos...

could never understand why when the M1 roadworks near Liecester Forest Services and the speed limit at the time was 40 mph with specs.. i would set the cruise control to 40mph and lorries would flash me and honk horns... i got the satnav and found out why...was driving home at 30mph... no wonder it took forever.

but from a safety point of view... you think you are doing a ton - showing off but are actually doing 90...

or a more realistic senario.... a kid steps out in a 40mph zone and you hit him/her at 30 - good chance they will live.
 
#14
crabby said:
TheBigUn said:
The-Goose said:
I believe most police forces allow a 10% error before proceeding to stop you.
Do they really or is this an urban myth? Do speed cameras also allow for 10% error?
I went through a speed camera at 78mph indicated (about 76 real) in a 70 recently and I did not get flashed. It is believed that speed cameras will normally give 10% +2mph, but I'd forget the +2mph and just assume 10% for errors with the speedo.
This 10% thing is an urban myth, they give you 10% on the speed the camera gets you at, for the 10% your speedo is out.
 
#15
Does anyone have a link to a clip of the top gear where they tried to explode the "you can drive so fast that a speed camera wont catch you" myth.
 
#16
Modern car manufacturers will tell you openly that your speedo is designed to show an error of 5-10% in your favour ie: 70mph indicated is likely to be 63-67 actual. The reason they do this is simple, it is to avoid you blaming their technology when contesting a speeding fine.

In Audis and most VAG group vehicles it tends to get larger with speed. My car indicating 30mph is actually doing 29mph but indicating 100mph it is doing 92mph.
 
#17
Pocoyo said:
but indicating 100mph it is doing 92mph.


I'm assuming you were driving on the continent on a motorway where there is no speed limit? :wink:
 
#18
TheBigUn said:
The-Goose said:
I believe most police forces allow a 10% error before proceeding to stop you.
Do they really or is this an urban myth? Do speed cameras also allow for 10% error?
As I understand it, the ACPO guidelines are that cameras should trigger at or above "speed limit + 10% + 2mph". That removes any "honest guv, my speedo must be reading low" excuses.

I also understand that manufacturers will try and create an accurate speedo, but accept a tolerance of -10%/+0% accuracy. In other words, they probably design them to display 95% +/-5%.

But you also have to accept that the speedo depends on how fast the wheels are going round; a smaller wheel will have to rotate faster to cover the same difference. You effectively get a smaller wheel if your tyres are under-inflated (a large proportion of the cars on the road) or if the car is heavily loaded.

So, people shouldn't be surprised that their car speedo reads high when they haven't checked their tyre pressures for two months, and they've got four adults in the motor...
 
#20
Wiki has the answer.

United Kingdom

A speedometer showing mph and km/h along with an odometer and a separate 'trip' odometer (both showing distance traveled in miles).The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39. [12]

The Motor Vehicles (Approval) Regulations 2001[13] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower that this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph. There is also the added problem of cars not complying with the United Nations standards, being imported and allowed to be registered, making the situation even more complicated. This needs further investigation.
 

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