Speedometer Error

[Felt Stirling]

My 2000 Z Roadster's speedometer read just about 5 mph higher than the actual speed at 70 mph. Since I do not know the vehicles history, I don't know if a previous owner changed the rear-end ratio??? It does keep me below the speed limit.

Does your read higher that actual?

 

[t-tony]

In a local village they have recently installed some speed measuring devices which tell you your speed and whether you need to slow down ( or speed up !) and I have noticed that my VW Touareg shows 42-43 mph when the display only shows 38-39 mph when I pass it. So there is some discrepancy there too. I suspect car speedometers usually read on the high side.

Tony.

 

[Dickymint]

My Z its about right but my Alfa reads about 4mph to fast though for most trips I just use the sat nav anyway as the speed turns red when I go over, its very hard on these modern cars as you never feel like your doing the speed you are

 

[SteveZ3]

Found this info:

How a car speedometer works

The Car Expert explains why your car speedometer may not be accurateSpeed is the measurement of distance over time. But a car speedometer doesn’t actually measure how fast you travel from Point A to Point B. Car speedos usually work by measuring rotation of the car’s driveshaft, axle or wheel. They then use some basic maths to extrapolate that rotation and determine how fast you are travelling. It’s a very similar concept to a bicycle speedometer.

However, if the diameter of the wheel/tyre alters, the extrapolation calculation will be incorrect. For example, the diameter will increase if you put new tyres on the car (more tread, which wears down over thousands of miles) or increase the tyre pressure. This means that, for each revolution of the wheel, the car is travelling further, meaning your speed is greater.

If the diameter decreases (eg – worn tyres, less air in the tyres, a different brand of tyre with slightly different dimensions, more load in the car weighing it down and compressing the tyres), then the car will be travelling a shorter distance for each revolution of the wheel, therefore you will be going slower.

Margin of error in a car speedometer

The differences in wheel diameter resulting from the above circumstances could be tiny (maybe a few millimetres), but at 30mph your car wheels are rotating 6-7 times every second, so it can quickly make a difference of a few miles per hour. This margin for error is taken into account in how the law is applied, and how manufacturers calibrate their car speedos.

 

[Jack Ratt]

All our works vehicles have GPS trackers and also monitor journey times, maximum, minimum and average speeds. My vehicle, a Ford Fiesta Van displays higher speeds than my GPS tracker report.

 

[Grant]

The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.
To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount, this would cover issues such as tyre wear which will affect the size of the tyre rotation and the reading on the speedo.

 

[jonco]

@Grant 's reply covers most of the points. There was a lengthy `debate' on another forum awhile ago and the eventual consensus was that the truest measurement available for the general motorist is GPS/sat nav.
From personal experience I have used same TomTom on both of our cars and at 70mph on either speedo - satnav indicated 67mph. Being a sad b****** , I even re-checked it when I replaced worn for new tyres and there was no noticeable difference.

 

[Felt Stirling]

Thank you all for your input.