Speedometer Vs Sat Nav

[Circuitmarshal]

My speedometer shows 6 mph MORE than sat nav which one would be right?

[Adam43]

Both will probably be wrong (depending on how accurate you want to be!). However it is usual for speedos to show a little higher, so it is probably your Sat. Nav. that is nearer.

My Sat Nav is a manufacturer unit so doesn't show speed. However I have a performance box GPS logger (similar workings to a Sat. Nav.), which is apparently accurate to <0.1mph. It shows all speedos as running high. Some only a couple of mph (at 30-100mph), others 5-10mph high. For the cars I've tried it in the newer ones are closer, but all speedos have read high.

[terence]

Manufacturers built in error,makes the cars seem faster than they are and helps save license's.

[Adam43]

I think that they did build in the error, but maybe not quite for the reasons you state. It is a bit like the baker's dozen; they just want err on the safe side. My current car is only 2mph out at 100mph, only just over 1mph at 30mph. Last car only 4mph out at 140mph(!). They're more confdent it is right nowaays.

[Cougar]

It also depends on the diameter of your wheels.. Years ago my dad put smaller wheels on his car and where before we could only do 140km/h down the motorway we were all of a sudden doing nearly 160... It was a Nissan 4x4

[Adam43]

Good point and a good reason to build in a margin of error. It certainly can make a diference.

Thinking about this there is no reason why a modern car with its own GPS can't recalibrate its own speedo. Does anyone know if any manufacturer does this?

[terence]

I have often wondered that Adam,chance of a little cost cutting for the manufacturers to be able to use the speed readings from sat nav,it would save on the rest of the sensors being installed.As far as changing the wheels goes Cougar,that only alters the final drive,not the speedo.

[Hepatic]

think of the court actions if they were slightly lower than real speed and you were caught by the cops, overstating slightly is definitely the right way to go. mines around 3/4 mph out according to the *****in' betty

[Dave Brand]

Originally Posted by Hepatic

think of the court actions if they were slightly lower than real speed and you were caught by the cops, overstating slightly is definitely the right way to go. mines around 3/4 mph out according to the *****in' betty

I believe Construction & Use regulations stipulate that speedos may over-read by up to a specified figure (10%?) but may not under-read.

[zac510]

I have a gps and have tested it against several cars' speedometers. The car speedos almost always overrate the speed. The only limitation I see is that the sample rate of my unit is only about 1hz so sometimes it can be behind when the car is accelerating or decelerating but at a steady speed it can be trusted.

[JimW]

Your GPS will always be more accurate except by chance.

That statement depends upon:

Access to adequate number of satellites.
Constant speed (see below).
Neither being 'broken', i.e. outside calibration.

Car speedos are indeed allowed +10%, -nothing (in the UK). Note that the error will vary with the wear on the tyres. Now the difference between new and worn tyres in RPM per km/hour is 2%. (Assumes depth of new tread is 8mm and worn is 2mm.) So your speedo reading will have a difference of 2% between new tyres and ones you are just about to ditch. For a given true road speed, the reading will be 2% lower on new tyres (and so worn tyres make you go faster - at least according to your speedo!)

The quality and complexity of your GPS will deliver different accuracies. For a relevant race tool one RaceLogic product (here) claims a velocity accuracy (they probably mean speed since they are quoting it separately from heading) of 0.1 Km/hour. (Measured over four samples which is five accurate readings per second.)

Your speedo will lag wildly behind that sample rate and might take (I'm guessing) a second to settle to an indication within its accuracy following a speed change.

'Domestic' GPS systems will be less accurate than the RaceLogic one. A typical one (Garmin eTrex Vista ) claims 0.05 meters/sec but only quotes for "steady state".

So even a cheap handheld will be way more accurate than any speedo, even 'calibrated' police ones (which are, I think, calibrated to 1%.).

(Costs? Garmin - £280, VBox don't know. As their website does not quote a cost one can assume lots and lots of £.)

Regards

Jim

[garcon]

Originally Posted by Dave Brand

I believe Construction & Use regulations stipulate that speedos may over-read by up to a specified figure (10%?) but may not under-read.

That's my understanding too. Not sure about the 10%, but I do know they MUST NOT under-read.

[JimW]

Originally Posted by JimW

Car speedos are indeed allowed +10%, -nothing (in the UK).

"The Road Vehicles (Construction and Use) Regulations 1986 (SI 1986/1078)"

Jim

[strider]

I had an old Audi ur quattro for quite a while. It probably started life on Pirellis or something similar, but by the time it reached me it was on Dunlops and, compared with my Road Angel Classic, the speedo read 9% fast! The Road Angel (and anything similar) relies on a GPS signal, so has to be more accurate than the speedo.

[Woolley]

Mine reads about 5mph over at all speeds. Tom-Tom exactly matched one of those roadside speed detectors that tell you your speed to make you slow down. Except that now I'm confident I was going slower than the speedo said, I can go faster!

[Pro Racer]

i have never bothered with GPS but i always (correction most of the time) sit abit below the speed limit on my speedo even itf it over rates it.

[icemachine]

Depends on the vehicle, but most speedo's now use electromagnetic sensors that calculate speed based on output shaft speed of the transmission, and then multiply by differential ratio and factory supplied tire/rim sizes.

[Cougar]

Originally Posted by terence bower

I have often wondered that Adam,chance of a little cost cutting for the manufacturers to be able to use the speed readings from sat nav,it would save on the rest of the sensors being installed.

They would still need a backup for instances where there is no GPS signal.

Originally Posted by terence bower

As far as changing the wheels goes Cougar,that only alters the final drive,not the speedo.

What icemaachine said in the previous post...

[Circuitmarshal]

My speedometer is on a Peugeot 406, ten years old and 200 thousand miles, the sat nav is a Tom Tom One.
The speeds I was looking at were 50, 60 & 70 mph.
And the difference was the same on all.

[Mr V]

How true it is i don't know, but i've been told that on the flat, the sat-nav is more accurate, on a hill the speedo is.

All i know is, up the road from me there is one of these gismo's that flashes your speed at you and in both the Audi and the Astra, the speedo says just under 40 and the gismo says 31/32 mph

[Cynic]

Mine seems to vary depending on how fast I'm going. Anywhere below 30 the difference between the speedo and the shat nav is 1-2mph. It gets progressively bigger at faster speeds. at 70mph (speedo), Mr Garmin claims I'm actually doing 62. Unfortunately this means I have started speeding up, assuming that the speedo is wrong.

[JimW]

Originally Posted by Cynic

Mine seems to vary depending on how fast I'm going. Anywhere below 30 the difference between the speedo and the shat nav is 1-2mph. It gets progressively bigger at faster speeds. at 70mph (speedo), Mr Garmin claims I'm actually doing 62. Unfortunately this means I have started speeding up, assuming that the speedo is wrong.

Yes, that ties in with the (UK) permissible error being quoted as a percentage. So +3mph at a true 30 (speedo will read 33), up to +7mph at a true 70 (so the speedo will read 77).

The size (rolling radius) of the driven wheels is crucial to all speedo systems.

Regards

Jim

[Mr V]

Originally Posted by JimW

Yes, that ties in with the (UK) permissible error being quoted as a percentage. So +3mph at a true 30 (speedo will read 33), up to +7mph at a true 70 (so the speedo will read 77).

What it also means of course is the fact that we're all getting worse fuel economy than we thought we were

[garcon]

Thought this was more appropriate to the road car forum.

[blackx]

Originally Posted by terence bower

I have often wondered that Adam,chance of a little cost cutting for the manufacturers to be able to use the speed readings from sat nav,it would save on the rest of the sensors being installed.As far as changing the wheels goes Cougar,that only alters the final drive,not the speedo.

Different diameter wheels will alter the reading the speedo gives which is why when increasing the diameter a reduction in profile is required to reduce the rolling radius..