Speedo calibration

[hangman3]

I have just fitted some larger wheels to the D3 & was wandering if anyone knows how to re-calibrate the speedo as this will obviously be affected?

[PillowSmuggler]

Just for interest, which size tyres are you now running, and do they have the important 111 load index rating?

As far as I'm aware you cannot change the speedo and further should not change the tyres diameter for this car.

All the cars dynamics systems (not just the speedo) are calibrated to the rolling diameter of the stock tyres, which remains close to the same for all the alloy wheel sizes from 17" to 21".

For instance the SatNav system switches to dead reckoning mode when GPS vanishes, enabling surprisingly accurate turn and location information even when in tunnels or underground car parks and so relies on a known rolling diameter of the tyres (to the point where there is a user setting called calibration which allows the SatNav to re-calibrate when new tyres are fitted, to address the minor rolling diameter difference just 5mm of tread can make).

The dynamic stability and ABS systems also work together with the steering angle sensor during a slide/skid on a corner to maintain your nose direction to be where you pointed the wheel even if the tyres are hardly gripping - again I believe that the tyre diameter would be an important factor in the cars calculated braking solutions.

I don't think you can even change the 4 cog wheels that the ABS sensors read at each wheel to change the number of teeth per revolution (from these the car determines the average speed that gets reported to the instrument panel) as then the engine/gearbox would be turning slower than expected for a particular wheel speed and the car could think it was perpetually spinning the wheels potentially leading to fault codes due to "plausibility errors" from the data it has available to it.

In short, if this was a series or early defender it really wouldn't be much of an issue, but in a complex and heavy car such as the Disco I really wouldn't recommend running different sized tyres at all.

[anglefire]

And to add though, what sort of difference are you talking?

People fit MUD's that are larger than standard - by a reasonable amount too - 17" BFG 275/75 I **Think** are a common fitment and I've never heard of any issues. Basically they will still be ok if the tyre has to be deflated to get one in the spare wheel well. But I would suggest that is the limit.

[jamespope]

I've got 4 BF Goodrich All Terrain T/A KO2 265/70 R17 S (121) on mine, no issues at all so far and the dash speedo is closer to the GPS speed than it was with the factory tires on (which were quite a bit smaller).

James

[hangman3]

The tyres that came with the car are 255/60/18 112v. & the tyres that I have put on are 255/55/20 110w. Apart from it feeling ever so very slightly taller geared, there has been no other affect as yet that I know of.

jamespope,
How have you measured this with the GPS please? It was suggested to me by a chap at ATS to go on a distant drive at steady speed to compare the satnav with the speedo & odometer reading but I cant seem to find the appropriate bit on the satnav. not yet fully familiar with satnav as I never had it before.

[jamespope]

My D3 doesn't have the built in satnav, but I use an app called Waze on my phone instead. Your speed is just displayed at the bottom of your phone screen when you're driving. I think there is another screen that gives journey information like mileage, avg. speed etc so you could compare that to the D3's odo or trip computer, but it wouldn't take much of difference in speed for the two odos to drift quite a way apart on a long journey, so i'm not sure how useful a comparison distance traveled would be to you.

James

[anglefire]

The tyres that came with the car are 255/60/18 112v. & the tyres that I have put on are 255/55/20 110w. Apart from it feeling ever so very slightly taller geared, there has been no other affect as yet that I know of.

jamespope,
How have you measured this with the GPS please? It was suggested to me by a chap at ATS to go on a distant drive at steady speed to compare the satnav with the speedo & odometer reading but I cant seem to find the appropriate bit on the satnav. not yet fully familiar with satnav as I never had it before.

You've increased the diameter by 3.3% (About an inch)

So at an indicated speed of 70mph will actually be 72.3mph.

I wouldn't worry about it in truth.

https://tiresize.com/tyre-size-calculator/" onclick="window.open(this.href);return false;

[hangman3]

Cheers Guys,
Such a small speed difference is not to be worried about cheers.
I have installed Waze before but it wont work with a Windows phone. Bit naff really that most companies don't make apps for Windows.

[hangman3]

I have now found that is possible to re-calibrate for the different wheel size so have done so. Although as informed previously in this thread that the difference is minimal, it made sense to have it correct where possible.

[anglefire]

Is this for a D2 or D3? Only your profile says D2?

The D3 has CCF settings for wheel size, but they don't appear to do anything - and as far as I know the suggestion is to leave well alone.