What Speed am I doing?

Freakbro

Freakbro

Freakbro

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I've noticed people regularly talking about how your (analog) speedo registers higher than your actual speed - why is this anyway?

The 350z also has a digital speedo as one of the options on the display which registers a few mph less than the analog one, so I thought that was correct. But yesterday, driving back from Devon I had my Tomtom Sat Nav on and it was different again! :confused:

For an example, the analog speedo would say 75, the digital 72 and the Sat Nav 70

So....which is the most accurate....and what speed am I doing? :p
 
The speedo on the car works by looking at how fast your wheels (or axle) are turning and knowing the circumference of the wheels, that lets it work out your speed.
Because the circumference changes slightly depending on how much tread is left on the tyres and how well inflated they are the speedo will give a reading that is out, but they're only allowed to over estimate the speed so the reading will tend to be slightly high.
The digital speedo might be averaging the readings out slightly to avoid it flickering from say 50 to 51, to 50 repeatedly (in a very distracting fashion), whilst the analogue speedo will be constantly updating.
I think there may also be slight differences depending on if it's a mechanical speedo, or a fully electrical one.

The sat nav's speed will be worked out as the average over the last X seconds (I've seen a sat nav say the car is moving at 5mph when the handbrake has just been applied), whilst the in car speedo will be almost instantaneous.

At least this is my understanding of the basics, the actual methods vary depending on age of car, model, and any extras.
 
I think it's generally accepted that the GPS speed is the most accurate one (assuming you are on a relatively flat plane). I too have a digital and analog speedo, and whilst they appear to marry up, they over read by perhaps 1-2MPH when compared to a sat nav.
 
Speedos are likely calibrated to the largest wheel/tyre combination fitted to the car (e.g. including any optional wheel sizes), which could be a couple of % different to the smallest options.

Digital speedos may also apply some smoothing/filtering which may also affect the reading vs an analog speedo.
 
Raymond Lin said:
I got clocked doing 73mph on my Satnav and 70mph on my dash cam GPS last weekend...

I am hoping the 10% rule is not a old wife's tale.
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In a 70? I'm pretty sure you'll be fine. I'd be more inclined to trust the satnav's GPS over the dashcam one but even so I'm sure it's fine. I've done a satnav indicated 77-78 past a police van before and I've been fine so in Hampshire at least the 10% +2 guidelines do seem to be adhered to :)

My speedo indicates 81 or so when the satnav indicates 77 in my car so it's a little bit out.
 
Phemo said:
In a 70? I'm pretty sure you'll be fine. I'd be more inclined to trust the satnav's GPS over the dashcam one but even so I'm sure it's fine. I've done a satnav indicated 77-78 past a police van before and I've been fine so in Hampshire at least the 10% +2 guidelines do seem to be adhered to :)

My speedo indicates 81 or so when the satnav indicates 77 in my car so it's a little bit out.
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In a 70, yes.
 
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Also aren't speedos calibrated for 30mph meaning further away you are from that speed, the less accurate they are. Always taken gps as most accurate.
 
Exactly same on my Golf.
Analogue : 70mph
Digital : 67mph
GPS :65mph

As mentioned above, the digital is probably a two second average to save it being annoying and changing constantly.

GPS is always more accurate
 
SDK^ said:
GPS is always more accurate
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Actually, no it isn't. Not always.

GPS can be very inaccurate under certain situations, some of which happen more often than you might think. The simplest scenario which can give you a false speed reading is changing elevation - the more rapidly your elevation is changing, the worse it gets. This could be magnified by either only having 'visible' satellites close to the horizon or only having visible satellites clustered close together, which are the GPS scenarios that gives the largest margin of error.

Curved roads and rapid changes in speed also tend to screw up GPS-based speed measurement.

I've stood at a precisely known elevation and had a GPS elevation reading that was significantly out. I've also been at sea and been a few hundred metres above sea level according to GPS.

If you were to calibrate your speedo properly, that will be far more consistently accurate than GPS.
 
My 911 is pretty much 10% over reading the whole way compared to my GPS, quite funny watching a digital speedo in a performance car when you accelerate hard becomes a bit of a blur.
 
savagesam117 said:
My car says I'm doing 80 when i'm actually do 70. It goes wrong at 65 then back to the correct speed once you hit 100. Very strange!
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I'm sure my speedo reads 95 when I'm doing 70 ;-)

I believe a cars speedometer has to be accurate to within 10% but must not under read. So for example it must not show 70 when you are doing more.

Not sure when this is actually tested though, probably only at type approval, once for every car model?

Andi.
 
Evo8 said:
Even if you did get a ticket through, I'd contest it and it'd get thrown out.
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I was in cruise control so I know there is little chance of me having a heavy foot for a split second for speed to fluctuate too much. There was a 4x4 who overtook me though...I think he got caught, as I am doing 70 already.
 
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