Our Experts on: Tyre pressure, part 2
First, consider the tyre pressure for a VX/VT series Commodore. According to the Tyre and Rim Association load tables, 4 tyres under a Holden Commodore inflated to 250 kPa (36 p.s.i.) can carry a load of 2.6 tonne. That’s a pretty overloaded Commodore. With two passengers and luggage, you’re weighing in at about 70% of that. According to the Tyre Standards, that means you only need about 23 psi in the tyres of your average Commodore.
But if you look at the actual tyre placard on a Commodore, it says to inflate the tyres to 26-29 psi. Why the difference? First off, because engineers generally don’t specify pressures that are barely adequate – that way lies trouble. Secondly, a Commodore with tyres inflated to 23 psi would handle like mush. As we’ve noted before, the higher the pressure, the better the handling.
Here’s where our detective work comes in. What if we look at the tyre placard for a Calais? For those not acquainted with it, the Calais used to be thought of as half of a limousine ride – a luxury car. However, if you look at the tyre tables, the tyre pressure specified is 36 psi, significantly higher than the Commodore. That gives you some quite twitchy steering, and means you feel a lot more bumps – not exactly what’s expected when you buy a luxury vehicle.
So what caused the engineers who designed the VE Series Holden Calais to specify such high pressures? GMH is very coy about the weight of the car- it cannot be found in the owner’s manual, only the maximum weight permitted over each axle. The answer might be found in the delays that accompanied the release of the new VE model Holden. Loaded with extras, the VE Calais did not meet the intended design parameters for fuel consumption. Reportedly, the engineers were chasing a figure 0.1 litres per 100 km lower. Their solution apparently was to increase the tyre pressures. This lowers the rolling resistance of the tyres, and improves fuel consumption. There is a lesson here for all of us, if the fillings in your teeth can stand the ride – increase the tyre pressures, and your fuel consumption will reduce. Even maintaining the pressure at that intended will pay off in lower fuel consumption.
So have a look for your tyre pressure placard somewhere on the car, or in your car handbook. Mostly, it’s on the driver’s door or door pillar. It specifies the pressures that the car engineers are happy with to make the car ‘handle’ the way they want it to. If the fillings in your teeth can’t cope with it, accept that there is a penalty involved, paid for at the fuel bowser.
The writer drives a VX SS Commodore, and would just love to get his hands on a VE 6 litre “SS”, because “it handles”.