Tyre tips for fuel efficient driving

Fuel prices might be on the wane again, but fuel economy is always paramount in the thought of the average haulier. John Loughran looks how tyres can have a positive or negative impact on fuel efficiency.

A vehicle’s aerodynamics and its travelling speed have a dramatic effect on how much fuel is consumed. The force created by the aerodynamic drag of a vehicle goes up exponentially with the speed of the vehicle. Tyre rolling resistance increases with speed, but tyres represent a proportionally smaller percentage of the total drag on a vehicle as the speed increases.
A heavily loaded truck will use more fuel than a lightly loaded truck. For a truck, a good rule-of-thumb is that for each five tonne increase in load, fuel economy will drop by about 5%.

As a vehicle travels down a road, there are a number of factors that contribute to the amount of fuel it will use in getting from Point A to Point B. Tyres are just one of many factors.
As each tyre on a vehicle travels down the road, it creates a drag force. It is composed of the energy loss created by the deflection of the tyre sidewall and the compression and deformation of the tyre tread in the footprint at the road surface. This drag force is commonly called rolling resistance and can be measured in a laboratory.
The contribution of tyres to the total energy required to move a vehicle down the road is dependent upon the effects of many outside factors, which include: aerodynamics and speed, load, wheel alignment and inflation pressure, and driving style.
If any of the wheels on a typical articulated truck and trailer are not properly aligned, the total drag on the vehicle increases. There is greater “scrub” of the tyres against the road surface and, potentially, greater aerodynamic drag when the tractor and trailer are not tracking parallel to the direction of travel.

The driving habits or “style” of the operator of a vehicle can have a very large influence on the amount of fuel consumed. Aggressive drivers can negate many of the gains obtained from investments in fuel-efficient tyres and engines, aerodynamic devices or synthetic lubricants. With today’s technology, it is possible to accurately measure the amount of fuel an engine uses over a period of time so programmes can be set up to reward drivers for fuel efficiency.
Ambient air temperature, weather conditions, road surfaces (gravel, asphalt, concrete) and terrain (flat, hilly or mountainous) are “environmental” factors that are impossible to control, but have a direct effect on fuel consumption.
Each of the wheel positions contributes a portion to the total tyre rolling resistance. Since the drive and trailer tyres account for over 85% of total rolling resistance, tyre companies concentrate their fuel efficiency efforts on these axle positions.
In general, fuel-efficient tyres will wear faster than standard tyres. But the tread wear gap between these two types of tyres is less today than a few years ago.

Most of the gains in fuel efficiency can be obtained from the tread of the tyre, from the tread compound, tread design and/or the tread depth. Research shows that the tread contributes to over half of the rolling resistance.
It is important to keep tyres properly inflated so that the strength of the tyre is not compromised. Fuel economy falls off sharply when tyres are under inflated.
If a rolling resistance improvement is made with the tread only, the improvement diminishes as the tread is worn down to 0 tread depth. If a rolling resistance improvement is made in the casing components of the tyre, that improvement will remain throughout the tread life of the tyre.

Tyre manufacturers often market their tyres on the premise that they reduce rolling resistance and therefore contribute to fuel efficiency. However, the following example shows that some manufacturer’s claim can be over sated.
Consider two trucks - one has “standard” drive axle tyres and the other has “fuel-efficient” drive axle tyres. Assume that the wear rates to be equal for each tyre.
The fuel-efficient drive tyre may have 20% lower rolling resistance and can deliver a 4 per cent improvement in fuel economy when all the tyres are new.
But as the tyres wear, the fuel economy improvement of the low rolling resistance tyres diminishes when the drive tyres need to be replaced on both vehicles. Each vehicle is at the same point of fuel economy improvement (both are getting the same miles per gallon).

In this case, the 4 % advantage of the fuel-efficient drive tyres decreases to 0% advantage when the tyres are worn out. So, the overall average advantage is really only half 2%.
If the standard drive axle tires get better tread wear than fuel-efficient tyre here is actually a point where the truck with the standard tyres gets better fuel economy than the truck with low rolling resistance tyres
There are steps that can be taken to increase average fleet miles per gallon. The miles per gallon achieved by a given truck depends on many factors, the major ones being:
* Vehicle, engine and accessory design and maintenance
* Driving style
* Tyre selection and maintenance
* Environmental conditions

Fuel-efficient tyres with a low rolling resistance are offered by all major tyre companies. Although such tyres can improve vehicle fuel efficiency, the gains will not be as much as demonstrated in standardised tests designed to eliminate or reduce the many other environmental factors that affect fuel usage.
The fuel economy advantage of low rolling resistance tyres tends to diminish as the tyres wear down since most of the gains come from changes made to the tread of the tyre. So, the advantages gained when tyres are at full tread depth can be cut in half over the life cycle of a tyre’s tread life.
Many Fleet Managers feel that the investment in low rolling resistance tyres, which tend to cost more, is money well spent. Depending on the type of fleet operation, vehicles used, driver situation and the fluctuating cost of fuel, a Fleet Manager must analyse all factors affecting fuel efficiency to make an informed decision regarding fuel-efficient tyres for his fleet.