Friday, April 12, 2013

Car's speedometer - does it tell you the true speed?

Naked truth: Quite often speedometer in your car doesn't show you the actual speed.




If we exclude a speedometer malfunction, there are still two main factors:

Factor 1. Legal matters

E.g., in European Union, there are legal acts that force car manufacturers to calibrate speedometers to show up to 10% higher speed. Let me cite from here: http://discussions.tomtom.com/t5/GO-800-1000-2000-Via-Start-20/Speed-difference-between-my-tomtom-and-my-car/td-p/135981

"Lord Whitty: The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed."

Aren't those guys awesome at keeping us safe from wrongly curved bananas/cucumbers and reckless speeding?!  
Are countries outside EU affected by this? Surely! If there is a chance your car was assembled in EU or for EU, there is a high chance its speedometer will be lying out straight on the stock tires. Which brings us closer to the tyres question itself...



Factor 2. Tire size


So lets say your car has nothing to do with EU or 
any other country where law forces manufacturers to "lie" about the speed. Can you be sure your car's speedometer is right? NO!

You never actually know what is the right size of the "stock" tyres for your car. In reality, you can pick from few options for it. It might be good to know what tire size manufacturer calibrated the speedometer for, but this is rarely possible.


Does car tyre size lead to a significant difference in speedometer and odometer readings? Let's see... Here's a great page to tell: http://www.performanceprobe.com/misc/index.php?do=calc&page=tire


We can see that speedometer showing 60mph with R15 tyres will show 57.7 mph with R16 tyres. And was it right showing 60mph with R15? You don't know, unless you TEST it!

How to test my speedometer and know how much OFF it is?

The easiest option of all is to use another, GPS way of measurement. Almost everyone nowadays has a GPS navigation or there are iPhone/iPad or Android apps showing you the speed based on GPS.

For iPhones and cellular iPads you can grab our FREE app. Just beware that non-cellular iPads don't have a GPS module, same is true for iPods.
For better results be sure to achieve some constant cruising speed before you compare. Also try on few speeds, lets say those 30 and 60 mph at least to see if difference is proportionally the same on different speeds or not. Mark the GPS speedometer readings against your car's speedometer.

 We believe that knowing that difference helps you make more educated decisions while you drive, making your driving safer.

This is almost it, let us give you few more helpful external links:
How to on tire size for your car
Formula to calculate the speedometer error for different tire size
How to adjust a speedometer in your car