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Sat Nav v Speedo

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Sat Nav v Speedo

I started a topic similar to this a couple of years ago but,  a) I've got a new Sat Nav and b) the technology has moved on, and I also have a different car now.
Whilst accepting the speed readout on a sat nav can be affected by hills and valleys etc. We were on the M40 yesterday the section between Stokenchurch (Bucks) after is passes through a deep cutting and the A34 turn off (Birmingham bound) which is pretty flat for at least 20 miles or more.
I noticed a marked discrepancy between the car speedo reading against that on the sat nav. The speedo read 70 the sat nav 65. The sat nav was much harder to keep at a steady speed the reading fluctuating 1 or 2 mph either side of 65, the speedo needle was rock steady.
So do I trust the sat nav and speed up to a speedo indicated 75mph, or stay at the speedo 70mph and switch off the sat nav reading (possible)? and later explain my thinking in court when pulled for speeding.
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Re: Sat Nav v Speedo

Speedo every time for me.
Sat nav is working out your speed by the distance travelled and then using the time to work it out. It's also using previous values to work out an average to smooth the jerky speed calculations.
Speedos do apparently become less accurate the faster you go. They're allowed to over read your speed but not under read. They're usually set to read slightly faster than you're actually going - usuall 2 - 3 mph faster than your actual speed.
I very much doubt that sat navs are setup to do the same thing because mine will calculate my speed down to as low as 1mph indicating that its using real measurements without any modification of the numbers.
Also as you accelerate, a sat nav will take a few seconds to recalculate your speed. A speedo will be much quicker.
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Re: Sat Nav v Speedo

My sat nav is always 2mph under my speedo well the digital reading on my HUD at any rate.
maybe a way to judge the accuracy is to find a 50 limit and a convoy of lorries get behind them and see which is sat on 50
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Re: Sat Nav v Speedo

Try to find a "reasonablly flat/level" section of road, with the old milestones set on the verge.... do a couple of "measured mile" timings using the tried and tested formula.... ( forgot it, but someone will chime in) and see how accurate your speedo is compared to your timings
CX
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Re: Sat Nav v Speedo

The company car I had at my old job was a Citroen C3 (with a digital speedo readout). It appeared to be very accurate - it pretty much matched the value given my my sat-nav, those temporary speed activated signs in town, and frequently read 56mph when sat behind lorries on the motorway.
This thread reminded me of this:

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Re: Sat Nav v Speedo

Don't forget it can never be that accurate, tyre wear and pressure can vary it by at least 1%.
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Re: Sat Nav v Speedo

The sat nav of the type I use for walking (and I have no reason to believe the car sat navs work on a different principle) calculate the speed directly from the doppler effect of the signals from the satellites
http://gpsinformation.net/main/gpsspeed.htm
It is therefore not very accurate when under tree cover which mucks up the signal but in a car on an open road it should be fairly accurate. How accurate depends on the sensitivity of the receiver and the strength of the signal
There is a smoothing algorithm as the GPS typically calculates the speed once per second
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Re: Sat Nav v Speedo

For a long time there has been a lengthy contra-flow on the M1 near Luton with 'average speed cameras' (I don't know why they don't use 'good speed cameras'  Crazy), my satnav knows about these and displays the average speed through the section.  The satnav also beeps gently when I reach the average speed limit (making it easier to concentrate on traffic IMO), the satnav indicates 50mph in this section at about 53 on my speedo, I stick to 50 on the satnav and haven't got a ticket yet.  I will continue to trust my satnav as long as the discrepency from the speedo reading remains about the same.  Smiley
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Re: Sat Nav v Speedo

Quote from: CX
The company car I had at my old job was a Citroen C3 (with a digital speedo readout). It appeared to be very accurate

It should have been! It was still a speedo connected to the gearbox not a GPS working it out by distance, time and averages like a GPS. Car speedos wouldn't be allowed to work like that.
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Re: Sat Nav v Speedo

@sprite
how do sat navs weork? When you answered that you should see that they are accurate compared to a mechanical speedo.
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Re: Sat Nav v Speedo

Yes but remember sat navs aren't accurate GPS wise. I think its something down to 2 - 3m. With that in mind, how could they ever possibly be 100% accurate?
Gears on a gearbox will always turn at a countable frequency from which you can work out your speed.
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Re: Sat Nav v Speedo

True Sprite but tyres, even if correctly inflated, loose about half an inch in diameter between new and legal minimum (based on 8mm tread depth on an new tyre (I believe this is about the minimum for a new tyre on most vehicles) and legal minimum of 1.6mm, difference = 6.4mm, double as the diameter includes both treads = 12.8mm = nearly 1/2"), allow for under-inflation and speed is bound to be (at least a bit) over-measured at times.
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Re: Sat Nav v Speedo

Quote from: Sprite
Yes but remember sat navs aren't accurate GPS wise. I think its something down to 2 - 3m. With that in mind, how could they ever possibly be 100% accurate?

3 in 1000 so that a 0.3% error compared with what seem to be the average error of speedo at 70mph which is 5%
Quote
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.[11]
The Motor Vehicles (Approval) Regulations 2001[12] 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 than 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.
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Re: Sat Nav v Speedo

Yes but also don't forget that at the same time the sat nav is refreshing the screen and doing all sorts of other things that could cause a delay in the works.
It doesn't matter how much you think a processor can multitask, it's still running a single block of code very fast while comparing what time it needs to execute certain bits of code at. If it suddenly has a screen refresh to do that will naturally take more time than reading a clock signal.
Again, add into the mix the fact that GPS's also use an average of previously measured speeds to keep the readings smooth and it's a recipe for inaccuracy imo. Plus you accelerate or slow down and the sat nav won't pick it up for a couple of seconds. That should prove just how inaccurate it is. It's only an estimate of your speed.
I do of course also understand about the weat on tyres however as they wear down your speedo will simply tell you that you're going faster than you really are. Again, that indirectly protects you from yourself.
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Re: Sat Nav v Speedo

Also, it's fair to point out that EU legislation on speedometer calibration states:
Quote
The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

Also:
Quote
The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.

source
So, factory fitted speedometers will show a range of "correct" to "correct +5% + 10km/h"
So, a car travelling at actual 70MPH means the speedo can read anything from 70MPH to ~79MPH
In addition, as people have intimated, factors such as tyre inflation levels and wear levels can have an affect.   Average tread depth for a new tyre is around ~8.6mm, dropping to a legal minimum of 1.6mm.  This is a variance of 14mm (diameter) which can be anything up to 3% of a wheel's entire diameter, which conveys a linear reduction in circumference of 3%, thus your speedometer will read that you are travelling 3% faster than you actually are, when running on a set of tyres near the legal minimum.
The tl;dr is that Speedometers in cars will always read consistently faster than the car is actually travelling, from the day that the car rolls out of the factory and with nearly all variable factors, this will only ever INCREASE.  Thus, a satnav is actually likely to be much more accurate than your speedo!
B.