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I have a facelift 156 JTD which has trip computer etc. Problem is my speedo, odometer and all the mileage trip functions (range, mpg etc) are all about 10% - 15% optimistic. (checked them with satnav, kilo posts on Mway etc.)

It's as though the entire speed measuring system is calibrated for bigger dia tyres (I have standard 205/55-16 tyres).

The problem is not a showstopper, but it's annoying when the speedo says 85 and I'm only doing 75.

Question - is the speedo software driven? Can tweaking the software change speed calibration? If so, who do I go to to get this done?
 

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i have noticed with sat navs there always a few mile per hour out so i wouldnt be so sure your car is that far off i own 3 cars all of which are on standard tyres an the speedo never matches what my tomtom says usualy out by 5 mile per hour thats on 3 different cars and with multiple sat navs tomtom an garmin
 

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It can be done on Examiner, but the JTDwas designed to have 17" wheels, so you thoughts are correct, is was meant to have larger diameter wheels.
 

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I have a facelift 156 JTD which has trip computer etc. Problem is my speedo, odometer and all the mileage trip functions (range, mpg etc) are all about 10% - 15% optimistic. (checked them with satnav, kilo posts on Mway etc.)

It's as though the entire speed measuring system is calibrated for bigger dia tyres (I have standard 205/55-16 tyres).

The problem is not a showstopper, but it's annoying when the speedo says 85 and I'm only doing 75.

Question - is the speedo software driven? Can tweaking the software change speed calibration? If so, who do I go to to get this done?
It can be done on Examiner, but the JTDwas designed to have 17" wheels, so you thoughts are correct, is was meant to have larger diameter wheels.
The rolling radius of the tire is neglible in these calculations as they are pretty much the same. Maybe 1-2mm difference..
 

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... but the JTDwas designed to have 17" wheels, so you thoughts are correct, is was meant to have larger diameter wheels.
The rolling radius of the tire is neglible in these calculations as they are pretty much the same. Maybe 1-2mm difference..
And the std 17" size (215/45R17) is smaller in diameter than 205/55R16.

So that doesn't add up either.

When I've checked mine against the satnav it has been spot-on. I'm on 205/55R16.
 

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I had 15's and now i've got 17's.. as the others have pointed out the rolling diameter is negligble.

My JTD has always read over by about 10% - tomtom indicated. Most cars in my exerience do this. If it can be DIY recalibrated then i'd like to know how.. it annoys me.
 

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my last car (seat leon) and my current 156 both show 5mph below what the tom tom is showing accross the range, as the sat nav is working it out using distance and time it should be correct.

from what i've read legally dials can show 10% above actual speed and nothing lower. so there all withing the rules and tollerances.

But yes it'd be nicer to have the dials showing a bit more accuratly.
 

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All car speedos over read, the manufacturers stick on the side of safety, after all they wouldn't want drivers getting done for speeding becasue the speedo was under reading. If you know how far out it is after comparing it with your sat nav then there is no need to try and re calibrate it.
 

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So my JTD really doesn't do 145 :confused:

Only Joe King, recently I got flashed by one of the speed signs at 28 mph and the speedo was reading 34. But passing cameras in 70 mph limits I seem to be at the same speed as others.

Confused? You bet, I'm seriously considering getting a Road Angle Pro or Pogo Alert - any comments?

Rob
 
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