How do I know if my speedo is accurate?
QUESTION
Hi John,
I can’t say too much but we have a proceeding at the moment where an unfortunate person has accumulated 12 speeding traffic infringements over about four weeks captured by our fixed speed cameras. This person has purchased a used car, not sure if the safety certificate included reference to the size of the tyres.
After getting the infringements he went to a mechanic, they put the vehicle on a dyno and is travelling about 1.2% higher than the speed read out. Unfortunately, even with this discrepancy each of the infringements are exceeding the speed limit (cut throat). This is something people buying used cars should be mindful of and check the tyre specifications on the inside of the driver door.
This got me thinking, I bought a Nissan Navara brand new in 2007, I set the cruise control at 100 on my daily commute on the Warrego Highway towards Brisbane. Quite often I was getting passed by semi-trailers so I increased the cruise control levels. I then did some testing and we found that on average the vehicle was travelling at 6km under the speed on the speedometer, apparently this complies with ADR and is an extra safety feature. Nissan did nothing to assist my position, my view was they hid behind the ADR.
What I have also found is that some dual cab utilities are sold with different tyre sizes, so the base model may have 17-inch, the top model could have 19-inch, so I dare say the speeds are set off the base models, but are more accurate in the top models. I don’t have any real data to prove this but I find this an interesting fact.
Josh
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ANSWER
Hey Josh,
By law, new cars cannot report a speed less than the speed they are actually moving at. (Can’t report, say, 95 when moving at 100.)
Although there are different tyre sizes per model grade, they are also usually lower profile with the parger diameters. Thus the actual rolling diameter is very similar.
Carmakers don’t release cars that are not compliant with ADRs - all new cars are within the spec on speedo accuracy. Generally, driving at an indicated 100km/h means actually moving (in the ballpark of) about 95.
JC
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