Speedometer calibration?

J

JVT

Member
 Czech Republic
Hello all, new owner of a 2008 3.0si. While I know all speedometers on virtually all cars read high, mine is reading about +15% compared to GPS, which seems way too much. Car has 18" wheels with OEM tire sizes (225/40 fronts)

Would a faulty wheel speed sensor throw off the speedo like this? Or is there a way to calibrate the speedometer?

Tried searching, couldn't find much, but apologies if this has been discussed and would appreciate any help.
 
Welcome to the forum by the way. :thumbsup:

I've never checked mine with GPS, but when I am in Sutton they have loads of those signs that flash your speed up and when I'm doing an indicated 30 they always flash up 27!

That is with OEM sizes, fronts the same as yours, rears 255/35 x 18. Which wheel(s) does the speedo reading come from?
 
Thanks!

Rears are also stock - 255/35. Wheel speed sensor is driver's side front I believe.

Used a speedo app on my phone to compare. Indicated 100kmh, actual is 85kmh. All of my other cars are approx +5%, so this is noticeable to the point where I can feel the speed more accurately than the speedometer itself
 
Sorry, I didn't read where you were, I'm in the UK so my crude test was in mph but it's still only 10% regardless.

But the results were the same for both my 3.0Sis, an E46, an E91 and my current E90.

15% does seem excessive though - might cleaning the contacts on the sensor connector help perhaps?

At least you'll avoid speeding tickets, if you go by the speedo.!
 
If you’re running standard size tyres, you should also be able to check actual speed by the RPM, though you’ll have to use google
 
Each vehicle manufacturer has to obtain type test approval of their speedometers, the EN standard states that the maximum allowed error is 10% plus 4 km/hr. So at a true speed of 30mph the speedometer can read a maximum of 35.5 mph. If it’s above that it’s faulty.
 
Welcome to the forum. 15% over is unlikely. Don't assume the GPS is correct, have you double checked it using another device. :)
 
Used the same app with in my other car and it was accurate (+5%), which is nearly the same difference as to what the roadside radars show.

I guess I could replace the sensor just to rule that out as it's not expensive, but I don't think that will be the cause...
 
JVT said:
Used the same app with in my other car and it was accurate (+5%), which is nearly the same difference as to what the roadside radars show.

I guess I could replace the sensor just to rule that out as it's not expensive, but I don't think that will be the cause...
Click to expand...
So you haven't double checked and are assuming the app is correct. Fair enough. It's your money. :)
 
It won't be anything to do with the wheel speed sensors. You do find that different tyre manufacturers sizes vary for the same width/profile so probably just that.
 
Mister T said:
It won't be anything to do with the wheel speed sensors. You do find that different tyre manufacturers sizes vary for the same width/profile so probably just that.
Click to expand...

At first that's what I thought, but doing the math, the size difference would have to be huge to even make a 5% difference.
 
buzyg said:
JVT said:
Used the same app with in my other car and it was accurate (+5%), which is nearly the same difference as to what the roadside radars show.

I guess I could replace the sensor just to rule that out as it's not expensive, but I don't think that will be the cause...
Click to expand...
So you haven't double checked and are assuming the app is correct. Fair enough. It's your money. :)
Click to expand...

Checked on a different vehicle, using a navigation app, using a speedo app as well as roadside radars. It's +15%, no doubt about it.
 
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