Sat Nav speed accuracy

Larkspeed

Larkspeed

Larkspeed

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Joined
11 Oct 2005
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766
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Emmen, Netherlands
I put this in the motors section as it is basically a driving thing but if this is in the wrong place please move :)

I currently drive a 1994 escort and I have noticed when using satnav that the current speed reading is always lower than what my speedo says I am doing.

I have used various different sat nav units in different cars and there is always a difference to some degree or other.

Here is a small chart I figured out yesterday with my current car and Navigon 2.6.1 running on a HTC Desire.

30 kmh on clock = 26 kmh on satnav, difference 4 kmh
50 kmh on clock = 44 kmh on satnav, difference 6 kmh
80 kmh on clock = 73 kmh on satnav, difference 7 kmh
120 kmh on clock = 110 on satnav, difference 10 kmh

My wifes HTC Desire is running Co-Pilot live 8 and she has a difference close to the same range.

I'm wondering if anyone can tell me if I should trust my speedo in my car or the current speed reading on my sat nav.
 
I think the only time that Sat Nav can be anything but 100% accurate is when the altitude is changing or going round corners. But even then its error margin for the speedo calculation/algorithm is going to be far better than the car's speedo.
 
As said, the satnav will be pretty much bang-on once you're up to a stable speed, and most car speedometers will read a little bit over, so if you're doing an indicated 33-35mph you don't have to suddenly panic and jab the brakes down to an indicated 25mph once you see a speed camera like most people seem to ;)
 
I wish cars just had speedometers that read true, tbh. I hate being lied to. I have to re-process the speedo reading in the 530 and reduce it for over-read every time I look at it.

It's annoying as the car is easily able to tell the correct speed within about 2% (so less than 2 mph out for most of the time - that's from an accurate measurement at 7mm tread depth to over-read at 1mm tread depth, assuming it's perfectly solid). I could accept a needle width's difference from reality as fact, rather than the ~5 mph over-read fiction.

If I use the digital speed readout (via the annoying to enter diagnostic menu or by re-setting the average), it returns the exact real speed, so it annoys me even more than the car knows but isn't telling me.

My 330 had a perfectly accurate speedo for some reason, and it was nice.
 
PMKeates said:
If I use the digital speed readout (via the annoying to enter diagnostic menu or by re-setting the average), it returns the exact real speed, so it annoys me even more than the car knows but isn't telling me.
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I was about to post this - mine's the same, if I enter the diagnostic mode and get the speed/rpm etc, the speed is spot on. It's silly :(
 
ok thanks guys I thought it would be the sat nav that would win.

My wife just looks at me funny when i and doing 130 down the highway according to the clock but now I can prove to her that I am not actually speeding :)
 
PMKeates said:
I'm going to find out if there's a way to get the analogue speedometer over-read disabled.
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OK, let me know
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Larkspeed said:
well there are companies that calibrate speedometers so surely if you talk nice to a tech you can get him to recalibrate it true instead of the 10% low
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That just makes it seem overly complex. There must be a value somewhere that is saying take the actual speed and multiply by X or add Y (or both)!
Lucero said:
to be fair, my cars speedo is almost bang on with the sat nav speed. Which is annoying.
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Why is it annoying?! Your speedometer is telling you how fast you are going. This is its intention! :D
 
PMKeates said:
That just makes it seem overly complex. There must be a value somewhere that is saying take the actual speed and multiply by X or add Y (or both).!
Click to expand...

I think it's more simple mechanical inaccuracy.
My old Nissan had a speedo with solder links on the back you could add or remove to calibrate it. After I did that it was just about spot on at all speeds.
 
Speedy Cables do speedo calibration. Certainly on old stuff, not sure about new fangled BMWs etc.

TBH I like having a slight over read on my speedo, saves that panic as you spot a camera and you doing slightly over the limit.
 
I know on my car the ECU speed and the indicated speed are exactly the same (and within 1mph accuracy up to about 90 which is nice)

but i think the odometer has to be pretty accurate so that'll be why the ECU's speeds are spot on?
 
burns said:
TBH I like having a slight over read on my speedo, saves that panic as you spot a camera and you doing slightly over the limit.
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For me the speedo inaccuracy makes that situation worse.

I'm either:

1) Not consciously choosing a speed at all

2) Consciously choosing a speed, and that speed will be appropriately higher to factor in speedo inaccuracy

In scenario 1 when I suddenly see a camera, I have to reduce to the speed limit indicated. If I am doing a true 50/indicated 55 in a 40 limit, I have to go from my unconsciously chosen true 50 to true ~36. If I had an accurate speedo, I would drop from true 50 to true 40, making it less of a panic.

In scenario 2 when I suddenly see a camera, I have to reduce to the speed limit indicated. If I had a true speedo I would have to drop from 50 to 40. If I have an inaccurate speedo I have to drop from 55 to an indicated 40.

In both cases the speed reduction is worse with an inaccurate speedo than it is with an accurate one, as I drop to an indicated 40 in both cases with an accurate speedo or not. In reality I probably drop to a slither above 40 because I know it's inaccurate, but you can't measure it out so I'm still dropping below a true 40.
 
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