Have you ever wondered why your monthly GPS reports show a different top speed to the one you saw on your speedo that day? Or why your speedo and your engine management display show different speeds?

Speeding was listed as the cause of 25% of truck crashes in 2012, and enforcement agencies and fleet managers are increasingly using speed management as a basis for underpinning safety programmes and, in some cases, driver bonus schemes.

With more and more technology around, there are more ways than ever to check if you are driving at the right speed. But which one is the right one? Which one can you trust? And will your boss and / or the police be using the same speed record as you?

Speed is a measure of how fast you are moving across the ground. It doesn’t sound complicated, but trying to measure it accurately is. And it’s partly for the same reasons that there is a difference in the distance recorded between your mechanical hubo and your speedo.

The speedo in your truck is most likely reading from a mechanical source (like the gear box) and calculating the speed that the truck is travelling. This is subject to variances such as:

  • The size of your wheels being the same as the manufacturer expected
  • How worn your tyres are
  • How worn your drive train is
  • Whether the sensor or cable carrying the speed information is worn

As an example, after a puncture up the East cape, I had to put a tyre on my motorbike that was 5mm higher profile than I normally use. This affected the accuracy of my speedo by 5%. Essentially, the bike thought it was travelling at one speed, but it had no way of knowing that the wheel had suddenly become bigger, so it couldn’t account for the difference.

As you drive down some stretches of road there is a helpful “your speed” indicator to save you the trouble of glancing down at your dashboard. Have you ever glanced down anyway, to find that your speedo is reading higher than the speed on the sign? These signs use radar (think Police speed radar) and calculate the time it takes the radar signal to bounce back from your truck. Vehicle manufacturers appear to “err on the side of caution” when displaying the speed to you in their vehicle. It doesn’t take too much to imagine the court cases in the USA if your speedo accuracy earned you a speeding ticket when you thought you were travelling at the right speed. We have seen speedos that show your speed as being up to 10% faster than your true speed, to keep you safe.

Your GPS uses satellites to work out its own position relative to those satellites using triangulation. Then it works out where it was, where it is now, and works out how long it has taken to travel between the two points. So GPS is telling you how fast you are travelling over the ground, but it is still calculated, not an instant speed.  Because sometimes the GPS signal can get distorted by physical things blocking the GPS signal, the speed is not perfect, and many GPS systems check that you have maintained a speed for a number of seconds before it will add it to the GPS records that the manager can see. Things that can block the GPS include:

  • Metal walls (think warehouses),
  • Tall buildings (think Auckland CBD), or
  • Natural rock walls (think the Lyttelton access road before the Tunnel)

This can mean that if you have GPS speeds listed on the wall in your smoko room, they could be lower than you were expecting for two reasons:

  1. Your speedo is reading high to keep you safe
  2. Your GPS system dismissed a high speed because you only hit it briefly

On the other hand, if the GPS shows a speed that is well above what you have ever seen on your truck speedo, check where it happened, to see if there is anything there that could have confused the GPS.

Oh, and a word to the wise, if you are looking at speeds in urban speed zones, dig out my article from May for the things that can catch you out when GPS is used to check speeds anywhere other than the open road.

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