Speedo Adjustment

[rocket rod]

I know this has been discussed many times but I thought I would give it one more shot to get it right. According to the GAP website
For 2010+ vehicles
Only one of the following settings will work and it depends on the car itself. Menu–Car Config–Instrument Pack–
This setting, Tyre Size MSB – IPC
Or this one, Tyre Rolling Radius (mm) – ABS
So my question is, what numbers have people used to get a speedo that reads 4% too low to read correctly?

[dirvine]

Run car on265/65/18 tries. Speedometer accurate but odometer is now inacurate

[John_D4]

Is there an issue with having a speedo that’s 4% too low? The way I see it is if I drive as per the speedo I’m less likely to get a speeding fine. The 4% less value will add only a small amount of time to your trip to the next town or when you’re going to work. How often do you pass a car driving 5k under the limit only to have them pass you when you stop for a pee?

[Tombie]

Is there an issue with having a speedo that’s 4% too low? The way I see it is if I drive as per the speedo I’m less likely to get a speeding fine. The 4% less value will add only a small amount of time to your trip to the next town or when you’re going to work. How often do you pass a car driving 5k under the limit only to have them pass you when you stop for a pee?

ADRs state speedo can be -10% of real speed up to Real speed but not indicate over. So you can’t be sitting on 100km/h indicated and be doing 101 km/h.

Hence all Speedometers tend to err on the side of very cautious to allow for tyre diameter variations between brands and also wear.

[DiscoMick]

Here's an explanation of the speedo rules.

Speedometer Accuracy - Speedo Regulations - RACQ (https://www.racq.com.au/cars-and-driving/safety-on-the-road/driving-safely/speedo-accuracy)

The speedo must not indicate a speed less than the vehicle's true speed.
It must not indicate a speed greater than the vehicle's true speed plus 10% plus 4 kmh. So, if the vehicle is doing 100 kmh the speedo must not read higher than 114 kmh.

[John_D4]

ADRs state speedo can be -10% of real speed up to Real speed but not indicate over. So you can’t be sitting on 100km/h indicated and be doing 101 km/h.

Hence all Speedometers tend to err on the side of very cautious to allow for tyre diameter variations between brands and also wear.

I am aware of that. I was just trying to ask what the big deal is? Not sure if it’s just me or not, but I know my D4 is doing about 95-96kmh when the speedo reads 100kmh. I just can’t work out why it’s a bad thing. I’m not going to get a speeding ticket, and 4-5kmh over most trips really doesn’t add anything to your trip. Even on a trip from Adelaide to Melbourne, we keep passing the same cars many times and get passed many times by cars wanting to drive <5kmh more than us.

[DiscoMick]

I am aware of that. I was just trying to ask what the big deal is? Not sure if it’s just me or not, but I know my D4 is doing about 95-96kmh when the speedo reads 100kmh. I just can’t work out why it’s a bad thing. I’m not going to get a speeding ticket, and 4-5kmh over most trips really doesn’t add anything to your trip. Even on a trip from Adelaide to Melbourne, we keep passing the same cars many times and get passed many times by cars wanting to drive <5kmh more than us.It sounds normal to me. Our Defender's speedo reads 100 when the GPS says 96. Our Mazda is similar.

[rocket rod]

Thanks for the replies but I've heard all this before. I just like things to be accurate. If the speedo is acceptable to be out, then what other items can be as well? Where does it stop?
Anyway back to the question, what numbers have people used to change the Rolling radius or Tyre size to get the speedo accurate? Note that I have 18" wheels.

[DieselLSE]

It's a simple CCF change. I've posted the settings on here before. I'll look them up again and advise.

[Tombie]

Thanks for the replies but I've heard all this before. I just like things to be accurate. If the speedo is acceptable to be out, then what other items can be as well? Where does it stop?
Anyway back to the question, what numbers have people used to change the Rolling radius or Tyre size to get the speedo accurate? Note that I have 18" wheels.

Understand Rod, for reference 18” wheels makes no difference - rolling diameter does.

Where does it stop - in vehicles it stops with the speedo - odometer is accurate to within 1%.

What you won’t be able to do is set your speedometer at 100% and maintain it - ever. Tyre size varies between brands and models of tyre. Tyre size varies between new and a tyre that is wearing down.

You can aim for close, and at least these vehicles allow that - none you may have had previously would likely have been correct unless you manually intervened..

[DieselLSE]

The usual "as built" speedo setting is set at 3.5% below actual and at +2kph (Market:0).

Although each of these parameters has a number of settings, it appears as if there are only two main market settings. In Genesis, this is displayed under Jaguar Speedometer Calibration Market which can be found under the Warnings and Messages tab. The choices are:
Market 0: 3.5% + 2kph
Market 1: 2.0% + 0kph

Accordingly, if the above is set to Market 0, then the following parameters are also set:
Jaguar KPH Offset: +2kph
Jaguar Percentage Scaling Factor: 3.5%

To effect a change, all three setting must be changed.
I changed all three settings as follows:
Market 1: 2.0% + 0kph
Jaguar KPH Offset: 0kph
Jaguar Percentage Scaling Factor: 2%
This gives a spot on speedo on 19” 255/55/19 and 18” 265/60/18

[rocket rod]

Understand Rod, for reference 18” wheels makes no difference - rolling diameter does. So what value do you use for rolling diameter given that it's probably not the actual rolling diameter of the tyre fitted? Do increase or decrease it by a certain amount to fudge the correct speedo reading?

[Tombie]

So what value do you use for rolling diameter given that it's probably not the actual rolling diameter of the tyre fitted? Do increase or decrease it by a certain amount to fudge the correct speedo reading?

I adjusted by the % error as a baseline. And went from there...

Take the number in there, convert to diameter. Work out ratio of that size to real, then go back in and change by that percentage.