South Wales Police – Patrol car
digital speedometer calibration

The Challenge

The South Wales Police roads policing team were delighted with their new Seat unmarked pursuit vehicle.

Its performance and appearance fitted the bill. Unfortunately, when the vehicle was supplied by the Volkswagen Group to the fleet it was found that the speedometer within the new vehicle was a digital speedometer that could not be adjusted.

This type of speedometer display means that there is no separate instrument that can be removed from the vehicle and adjusted to cause the speedometer dial to display the true speed of the car.

Several enquiries were made to see if there was a provider of some form of adjustment and calibration of this type of speedometer, but no practical solution was found that could be provided to all police fleets.

It has long been standard practice for car manufacturers to make vehicles with speedometers that read a speed that is faster than the true speed of the vehicle.

This is because UK and European regulations state that the speedometer of a vehicle can either read the true speed of the vehicle or a speed that is a percentage higher than the true speed.

The regulations say that the speedometer must never read a speed that is below the true speed of the car. Manufacturers use a simple method to allow the speedometer to comply with the regulation for the life of the vehicle.

They introduce a small positive error so any wear in the vehicle never puts the speedo outside of the regulations. It means that speedometers do not read the exact vehicle speed.

This positive error is not useful if a police officer uses the speedometer to pace a subject vehicle and make a measurement of the vehicle’s speed.

For approximately 50 years, police vehicles have been having their mechanical or electronic speedometer gauge removed and an adjustment made to them or a new scale applied, so that the speedometer in a police patrol car can be made to read an accurate and close to true speed.

The adjusted speedometers were given the term “calibrated speedos” but the method of calibration would perhaps not pass an accreditation test to make this a truly traceable and validated calibration of the speedometer.

That said, the adjustment method produced reasonably true speed readings and was accepted in courts when presented in evidence. This has created a cryptic puzzle for all fleet managers and their suppliers, who are planning to make all vehicles with multi-media integrated display systems in the future.

How could a calibrated speedo be supplied so target cars and drivers could be prosecuted with reliable and accurate evidence from a speedo that cannot be adjusted?

The Solution

The issue posed little problem for the Road Safety Support (RSS) calibration laboratory, that came up with the perfect solution.

The laboratory, which gained UKAS Accreditation as a BS EN ISO17025 calibration lab, is led by Steve Callaghan, a qualified electronics and forensic engineer.

Steve listened to the problem and, as an expert in the calibration of speed enforcement systems, explained that RSS had come up with a quick and reliable digital calibration method.

The method involves measuring the performance of the speedometer gauge, establishing its inaccuracy and providing correction figures to the readings of that gauge.

This process can be carried out at the RSS ISO accredited calibration laboratory or at any other chosen location.

The records of the test and calibration must be traceable and verifiable to allow an ISO accredited calibration to be applied to the speedo.

The method is a practical, efficient and affordable solution for the police, their fleet managers and fleet suppliers. 

Because the speedometer or the speedometer multi-media display does not need to be removed from the car, a huge saving in mechanics’ costs and time is made. No disassembly and reassembly of the vehicle is required.

The Result

The South Wales Police roads policing team have been using their calibrated vehicle successfully since February 2019.

The Seat, which is currently the only force vehicle with a digital speedometer, has been added to their fleet of vehicles.

The force can now consider the purchase of any vehicle that is suitable to add to their fleet, irrespective of whether the speedometer is an analogue or digital type.

Inspector Gareth Morgan, of the South Wales Police Western Roads Policing Unit, said: “In my opinion, the work carried out by RSS has transformed a vehicle that, although ideally suited to the RPU role, was not fit for purpose, due to the lack of a calibrated speedometer.

“Due to the new digital type displays, our traditional solution was not an option, and we had to consider other options.

“RSS were, to my knowledge, the only company to provide a solution, and we were at the point of decommissioning an otherwise excellent vehicle.

“The RSS method is suitable for vehicles with either digital or analogue displays, it is far quicker than the previous solution and it does not require the display to be removed from the vehicle.”