Ask Phoebe: Beware of GPS speed readings

Photo / Steven McNicholl

We recently bought a GPS device for use when we travel overseas and need rental cars. While we were trialling it here, we noticed quite a discrepancy between the car speed as shown on the GPS and what was showing on the car's speedo. For example, at one point the speedo was reading 100km/h, while the GPS measured 90km/h. And then we thought: Which is likely to be correct, or more accurate? Which one should we pay attention to? And how do they both compare with what a speed camera would register? Neil Davidson, Mt Eden.

The AA's motoring advice manager, Jack Biddle, says vehicle speedometers are certified by manufacturers to follow either the United States SAE (Society of Automotive Engineers) J1226 or the European version, ECR-39 regulation. The European standard (km/h) is the most common for cars on New Zealand roads.

The European standard does not allow underestimation of speed, but does allow overestimation of up to 10 per cent plus 4km/h.

In comparison, the SAE rule allows readings to be plus or minus 2 per cent of true speed, or on a sliding scale from -1 per cent to +3 per cent at low speeds, or zero to +4 per cent above 55mph.

Testing the AA has carried out using a GPS on a range of different vehicles (makes/models/age and mileage) has confirmed that inaccuracies of up to 10 per cent are not uncommon. In other words, indicated speed (on your car speedo) is faster than the GPS speed. This means when vehicles are travelling at an indicated speed of 100km/h, their true speed could be up to 10 per cent less.

Tyre wear and pressure are reasons it is difficult for a speedo to remain 100 per cent accurate.

For those using a GPS unit as a way of driving to a posted speed limit, the risks of tipping over that speed are very high as there is no room for tolerance.

For example, if you were to drive downhill at the posted speed limit using GPS, it would be so easy to go over the speed limit without realising it. The end result could be an increase in speeding fines.

Using the indicator speed creates a safety cushion for motorists.

Why do petrol prices always end in 0.9? Norman Milligan, Remuera.

I'm not sure that they do, Mr Milligan, but here is one explanation.

The price of fuel in New Zealand is dependent on the international price of crude oil, the currency exchange rate, and taxes levied on fuel. The decimal points allow the fuel companies to reflect these variables more accurately than rounding the price up or down to the nearest 10c might allow. Because fuel is a volume purchase (how often would you buy one litre?) the decimal points become absorbed, and can in fact save you money.

For instance, if 91 octane was priced at $1.78c a litre at the pump, 50 litres would cost you $89. If the price per litre was rounded up to $1.80, as the nearest 10c mark, 50 litres would cost you $90.