Thread: Speedo Adjustment

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

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?

Originally Posted by John_D2

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.

Here's an explanation of the speedo rules.

Speedometer Accuracy - Speedo Regulations - RACQ

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.

Originally Posted by Tombie

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.

Originally Posted by John_D2

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.

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.

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