Improving fuel economy is high on any fleet manager's agenda. Poor economy is literally burning money and profit. There are many factors that contribute to good or bad fuel economy. Tyre choice, maintenance and condition can affect up to 1/3 of the vehicle's total fuel consumption.
Tyres – rolling resistance
Each tyre creates a type of drag called rolling resistance. A low rolling resistance results in better fuel economy. A great deal of this rolling resistance is the direct result of energy loss due to tyre deformation as it travels over the road.
The use of fuel efficient tyres on all axles can make a significant difference in fuel consumption, a reduction of 10% rolling resistance on a complete vehicle results in approximately 3% reduced fuel consumption (approx 0.9 litres/100 kilometres on a vehicle which consumes 30 litres/100 kilometres).
Rib type tyres are more efficient due to their lower rolling resistance when compared to block design tyres. This is mainly due to less movement of the tread in the contact patch area. Additionally, low aspect ratio tyres are stiffer, allowing for less flexing under load, thus they typically have lower rolling resistance compared to high aspect ratio tyres.
To help buyers make a more informed decision, most tyres sold in the European Union now include a standard EU Tyre Label. As well as classifications for wet grip and exterior noise, the label includes a fuel economy classification – giving the tyre a classification between 'A' (most fuel efficient) and 'G' (least fuel efficient). The difference between a complete set of new 'A' class and 'F' class tyres could equate to a fuel consumption difference of up to 15%, which is equivalent to an annual difference of more than £6,000.
Rolling resistance is heavily dependent on inflation pressure. An inflation pressure 1 bar below the recommendation for the axle load can lead to a 5% increase in rolling resistance, resulting in a significant fuel cost. In addition, under inflation can have a negative effect on tyre durability – resulting in premature tyre failure, and a carcass unsuitable for retreading.
1 bar under inflation in every tyre can cost £800 of fuel per year.
Rolling resistance increases linearly with speed. However, a vehicle's aerodynamic resistance goes up exponentially with speed. As a result, tyres are a proportionally smaller percentage of the total drag on a vehicle as the speed increases. Reducing speed will reduce aerodynamic resistance and, to a lesser degree, rolling resistance too.
Fuel economy - other considerations
Some causes of fuel economy variation cannot be controlled - these include ambient air temperature, weather conditions, road surfaces (sand, gravel, asphalt, concrete) and terrain (flat, hilly or mountainous). However, there are other factors that can be improved.
Incorrect axle alignment drastically influences rolling resistance, decreasing fuel economy and causing accelerated tyre wear.
The driver has a huge influence on the amount of fuel the vehicle consumes. An aggressive driving style can negate the gains obtained from investments in fuel efficient tyres and engines, aerodynamic devices or synthetic lubricants. It's now possible to accurately measure the amount of fuel a vehicle uses, allowing for drivers who employ a good driving style to be rewarded for their good fuel efficiency.
Using fuel efficient tyres in place of standard tyres in combination with good vehicle and tyre maintenance with an economic driving style minimises fuel consumption.
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