Tires and Fuel Efficiency: A Car Match Made in Mileage Apr 20, 2017 by Rick (Driver Weekly)
There’s a lot of noise these days about the difference between traditional tires and ‘new and improved’ variants with low rolling resistance; but when it comes right down to it, does the hype stand up to reality? To get to the heart of that question, one first has to understand how tires affect fuel usage, and how the construction of particular tires deliver more or less efficiency when considering real-world driving, operated by most drivers.
First, bear in mind that this discussion is largely based on stock tires fitted-out with specific cars. Next, the construction of these ‘standard’ tires, their sizes, their shapes, their tread patterns, the chemical structure of their compounds, and how well they respond to dynamic forces like friction, consequent heat and wear apply. Now, before your start to roll your eyes at a jumble of technical terms, in truth, each element is critical to understanding the bottom line—which, in our case, it how easily a tire rolls across the pavement and the corresponding decrease in fuel consumption needed to move the vehicle.
Back in the day, tires were largely seen to be round rubber tubes that just prevented the metal wheels from grinding on the pavement. However, tire manufacturers leveraged chemical and physical science to improve their technology until, today, tires now offer a range of practical advantages depending on the given car and its driving requirements. Consequently, let's take a look at the tires in question and see where the ‘rubber hits the road’ as it were.
Traditional Radial Tires
Today’s consumer sedans and SUVs typically fit standard radial tires for a good reason—they roll more easily. This reduces the power needed to get the vehicle moving and keep it moving on the road and thus increases fuel economy. These characteristics are particularly important due to the current consumer demand for low-weight, small-displacement internal-combustion engines (ICE) to reduce costs. The easier it is to move the car, the better.
Radial tires have a solid list of advantages including:
1. Fuel consumption is reduced with lower dynamic resistance, i.e. friction.
2. A ‘softer’ and more confident ride because the fire can flex at the sidewalls while keeping its ‘tire patch’ firmly mated to the road.
3. Reduced gyro-oscillation (vibrations) at higher speeds. Again, this largely relates to the tire’s ability to sway and flex, therefore dampening most physical movements while cushioning the ride.
4. Extended service lives since their chemical construction, which reduces the damaging impact of the tire’s heat cycle.
Low Rolling Resistance Tires
From a construction perspective, low resistance tires are still based on radial tires, but their chemical compounds are further optimized to reduce heat driven by friction. As a practical matter, then, the real point of the hubbub is about; 1) enhanced chemistry for alternatively-powered vehicles; 2) the potential impact of additional federal regulations; and 3) environmental marketing, primarily targeting the EV and Hybrid vehicle segments.
Nevertheless, proponents offer what they see as a similarly central list of advantages including:
1. Decreased universal energy requirements (the tires are more eco-friendly and have longer lives) primarily driven by reductions in the effects of heat generation.
2. As suggested earlier, reduced fuel consumption due to reduced friction.
3. Reduced emissions, especially for vehicles that combine the tires with EV or hybrid powertrains.
Now, what does all this mean to the ‘average’ car and driver? Well, if you own an ICE vehicle, don’t run down to the local tire store and plunk down an additional $100 on a set of low-resistance radials. Frankly, it isn’t worth the time, effort, or money; since in this case, National Highway Transportation Safety Agency (NHTSA) metrics offer only a small increase in fuel efficiency over regular radials. On the other hand, if you’ve purchased an EV or hybrid car, and low resistance tires are fitted as standard, stay with that choice going forward. There are a host of reasons for this, ranging from factory optimization to negative impacts associated with downstream fuel mileage, but in short, if the tires came with the car, stay with those tires.
Also, bear in mind that I previously mentioned NHTSA’s tire efficiencies. But those numbers are really just educated guesses that are accurate only under strict testing conditions. The real efficiency differences usually end up in the range of 1% - 2% when all is said and done. The fact is that at least 70% of all tires on the highway are under-inflated to one degree or the other; and low tire pressures create more friction, which requires the driver to pump more gas to get the car moving. So, on balance, whether the car is an ICE, an EV, a hybrid, it’s more important to check and maintain your tire pressure to increase fuel economy and decrease rolling resistance. Don’t buy into the hype around low rolling resistance tires—just inflate your standard tires.