Speedo Accuracy

[TonyH]

Checked the speedo in my S2 against a sat nav. Accoding to the sat nav, my speedo is 10% high, is that normal ?

Can they be adjusted ? At least it's inaccurate the right way but it didn't stop me getting nicked a couple of years back.

[Paul.P]

Checked the speedo in my S2 against a sat nav. Accoding to the sat nav, my speedo is 10% high, is that normal ?

Can they be adjusted ? At least it's inaccurate the right way but it didn't stop me getting nicked a couple of years back.

Tony, they have to be inaccurate the right way, or a lot of manufacturers would be getting sued over speeding tickets, its perfectly normal.

[Foo]

10% high seems to be common. My current car says 70 while my sat nav reads ~63

I think the sat nav is correct. Since it's charting accurate distances over time from maps. Not sure I'd risk a ticket on pushing to 10 + 2 + Satnav reading though.

[markv]

Mine shows 10% high as well. But I don't think this is that uncommon - I'd always been told that speedos generally read 10% higher, and it's certainly true for our Honda CR-V as well. My Honda Accord is more accurate - showing 73mph when the satnav says 70mph.

[tweetdriver63]

I started a thread on this exact thing a few months ago. Apparently, 10% is pretty normal, but not for most cars. For example, driving my Mercury Mariner Hybrid on the highway with the speedo indicating about 78 mph, the GPS was reading 77.

[BFG in the BRG]

Mine also reads 10% low as does my other car, partly no doubt because of manufacturer worries over getting sued by owners, but also because possibly, as the tyres wear down the circumference gets smaller so the tyre rotates faster, so the speedo will get more inaccurate with time, until you replace the front tyres., I know it is only a matter of mm in tread depth, but over the circumference of the tyre it might make a difference - no doubt someone will work it out - I might also do it myself......just to prove myself wrong

now was it pi R squared or 2 pi r h........

[Dave Eds]

My sat-nav says -

"Slow down, do you have to drive so fast? It's not a race ,you know! Who do you think you are , Lewis Hamilton?"

"Do I look like I've parked in the back of a Ferrari?"

[BFG in the BRG]

so does mine, especially when approaching corners - I have tried explaining that is why I bought the car, but there is no arguing with navigational aids, mine also tells me when the car in front is slowing down.....and traffic lights are on red......and cars coming.......

[BFG in the BRG]

By the way, I've worked it out, (dunno why, felt the urge to justify my existance on this planet) 6mm loss of tread equates to just over 2% loss of circumference - hence 2% on the speedo possibly...........it's been a while since I did algrbra, or is this geometary.....
although I did remember pi = 3.1415926536 or roughly 22/7 which was quite scarry

That Grammar School education and degree from the University of Life with honors from the School of Hard Knocks was not wasted

[Dave Eds]

How do you know the speedo isn't calibrated for bald tyres?

Is it algebra or geometry?

I know it ain't telemetry - ask Lewis!

[lotusgirlmarie]

...and people wonder why I changed over to a programable Speedo..... other than the good looks

[DeeAitch]

If you can be bothered have a read of EU directive 75/443/EEC & 97/39/EC here

If not:

"For all true speeds of between 25 mph and 70
mph (or the design speed if lower), the difference
between the indicated speed and the true
speed shall not exceed -
V/10+6.25 mph
where V= the true speed of the vehicle in mph"

[dapinky]

......no doubt someone will work it out........

I can't be ar$ed to search for it now, but i did all the maths a year or so ago (and posted it) to see what effects tyre pressures, wheel/tyre sizes, new vs old tyres tread depth etc etc would have.......

The conclusion was NOT A LOT

IIRC there was only about a 4-5% diifference between the 'smallest' circumference with bald tyres and the 'largest' circumference with new (8mm depth) tread.