Speed

Francis H Murphy

Travelling on the M7 last week I set my speed to cruise control at 120 km per hour but noticed that I was not doing 2 km per minute. The new road signs now located every half km gave me a benchmark.

I had to increase my speed close to 130 km/hour to travel 2 km per minute

Is this normal, that the actual speed of the car is less that that shown on the speedometer .
The car has not been modified with standard wheel size & Tyre size as purchased

mickdw

Yes your gauge will over read to keep you safely under the speed limit.

tomtucker81

You'll find that most speedometers are not 100% accurate. As from your example, you were 10kph off the actual speed. Over time most speedometers go like this, and would need to be annually calibrated to maintain their accuracy.

I asked a friend of mine about this, she's a traffic garda, and she said their lasers etc have to be calibrated every year. She said for example, a patrol car driving at 100kph on the clock, approached a speed checkpoint and was only actually doing 91kph.

the higher the speed the more inaccurate the speedometer will be. If you were driving at 160kph, you might really be somewhere in the 140's. Meanwhile if you're at 50kph you might be at 47ish kph.

could be to do with wear and tear on wheels and tyres over the years, resulting in inaccurate speedometer readings. Open to correction on that though!!

Chris_Heilong

use a GPS speedometer app on your smart phone to get more accurate speed readings.