GOODYEAR

 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 rolls 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 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.

A vehicle’s aerodynamics and its traveling speed have an extremely large 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 are 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. Think of the extra fuel you might use when taking the entyre family (and luggage) on vacation compared to the fuel you might use in the same vehicle with you as the driver and no passengers. For a truck, a good rule-of-thumb is that for each 10,000 pound increase in load, fuel economy will drop 5%.

If any of the wheels on an 18-wheel tractor 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 programs 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 treadwear 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 underinflated.

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.

• Shallow original tread tyres have lower rolling resistance than deep original tread tyres
• Rib tyres have lower rolling resistance than lug tyres
• Old tyres have lower rolling resistance than brand new tyres
• Worn tyres have lower rolling resistance than brand new tyres

Consider two tractor/trailers. 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 type (standard and fuel-efficient) for the steer, drive and trail position tyres.

Chart 9 illustrates the effect of tyre wear on fuel economy improvement as each of the vehicle wheel position tyres are worn out and replaced over 350,000 miles.

Even though the fuel-efficient drive tyres have 20% lower rolling resistance and can deliver a 3 1/2% improvement in fuel economy when all the tyres are new (full tread depth), as the tyres wear, the fuel economyimprovement 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 3-1/2% 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 of the 3-1/2% or 1-3/4%.

If the standard drive axle tyres get better treadwear than fuel-efficient tyres (a more realistic solution), there is actually a point where the truck with the standard tyres gets better fuel economy than the truck with low rolling resistance tyres.

The average fuel costs of a given trucking fleet are related to two factors:

• Average fleet miles per gallon
• Average fuel cost per gallon

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
• Tyreselection and maintenance
• Environmental conditions

Fuel-efficient tyres with a low rolling resistance are offered by all major tyre companies. Although such tyres can enhance vehicle fuel efficiency, the gains will not be as much as demonstrated in standardized 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 will 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 analyze all factors affecting fuel efficiency to make an informed decision regarding fuel-efficient tyres for his fleet.