When it comes to tire inflation pressure maintenance, there are three categories of consumers.
The first group includes the ones you see driving down the road with underinflated tires. They don’t regularly check their tires or even think about what’s keeping their cars rolling down the road. Unfortun-ately, they account for a large portion of the driving population.
The second category includes the safety-conscious people armed with a reliable tire pressure gauge who check pressures regularly and abide by the vehicle placard. They do not deviate from the vehicle manufacturer’s recommended tire pressure.
The third category is a derivative of the second in that they, too, check pressures regularly, but have become self-taught tire engineers and have determined that both the vehicle and tire manufacturers are keeping something from them. They take it upon themselves to determine what tire pressures are best, especially when it comes to their fuel economy, hence their category term of “hypermiler.”
While the debate continues on Internet forums as to what tire pressures are best for maximum fuel economy, the overall idea that higher pressures translate to higher miles per gallon is widely accepted. So, what are the real numbers? In order to understand the relationship between tire inflation pressures and fuel economy, we must first take a look at rolling resistance.
As a tire rolls down the road, deformation takes place at the tire footprint. The sidewalls deflect and the tread rubber squirms due to the vehicle load. This deflection creates a certain amount of resistance, preventing the vehicle from rolling freely. Approximately 5% of a vehicle’s energy loss is attributed to rolling resistance. Anyone who has ridden a bicycle knows that low tire pressures require more energy input in order to keep moving down the road. Eliminating tire deflection, therefore, will decrease the amount of rolling resistance.
An extreme example of low rolling resistance can be seen at a railroad crossing: Trains ride directly on their steel wheels on steel rails, which maximizes load carrying capacity and minimizes rolling resistance.
So, it is obvious that increasing inflation pressure gives the tire a higher spring rate or stiffness, effectively reducing the rolling resistance.
Truth in Numbers
An extensive study was conducted by the National Academy of Science to determine the effects of tire rolling resistance and inflation pressures on vehicle fuel economy. The 2007 study found that for passenger tires inflated between 24 psi and 36 psi, a 1 psi drop resulted in a 1.4% increase in rolling resistance. Pressure changes below 24 psi resulted in even greater rolling resistance changes.
A 1.4% change in rolling resistance equates to a fuel consumption change between 0.15 and 0.3 miles per gallon. This range in fuel economy drop-off is due to the fact that vehicles respond differently to changes in rolling resistance. A vehicle with a recommended inflation pressure of 35 psi running 20%, or 7 psi, below recommended pressure will result in a 10% increase in rolling resistance – which translates into a 1.5% decrease in fuel mileage. For a vehicle that averages 35 mpg, that is a decrease of 0.5 mpg. For a vehicle that averages 20 mpg, that is a change of 0.3 mpg.
Imagine the amount of fuel that could be saved if all of the 220 million passenger and light trucks traveling a combined 2.9 trillion miles annually on our nation’s roads had proper tire pressures.
Now taking a look at the other end of the spectrum, hypermilers take this data and go to the extreme. Trolling through various Internet forums shows that hypermilers are exceeding vehicle manufacturers’ pressure recommendations by 10 psi and higher, often exceeding tire manufacturers’ maximum inflation pressures.
Tire pressures as high as 70 psi are reportedly being used because the thought is that tire burst pressures are above 100 psi; therefore, operating pressures below that should be fine. Little do these drivers know that wheel failure will occur well before a tire fails.
As with most tire performance parameters, there are tradeoffs. There is a point of diminishing return with regard to rolling resistance vs. inflation pressures and its effect on vehicle handling. Increases in inflation pressure reduce a tire’s contact patch area, which decreases the available traction. From a vehicle dynamics point of view, increased inflation pressure reduces lateral grip, not only from a reduced contact patch, but also from a high vehicle stiffness or spring rate. That, however, is another topic of discussion.
When a hypermiler comes across an “uh-oh” moment on the road and needs to perform an emergency maneuver, chances are they will fail because they have given up so much grip for their increased tire pressures. The tradeoff between vehicle handling and inflation pressures exists, and it is the task of vehicle engineers to find the best balance of performance, economy and safety. Much testing goes into finding the proper inflation pressure for a specific vehicle, and consumers should recognize this and follow the manufacturer’s recommended inflation.
Tire manufacturers continue to advance technology by formulating lower rolling resistance tread compounds while maintaining adequate grip levels for varying weather and load conditions. This trend will continue as vehicle manufacturers design and build higher efficiency vehicles, government regulations become tighter, and consumers demand more fuel-efficient replacement tires.
What things other than maintaining correct tire pressures can consumers do to improve their vehicle’s fuel economy? Since much of the energy loss (about 65%) occurs in the vehicle’s engine, proper vehicle maintenance is critical. Consumers should make certain that their engine is properly tuned, change their air filter as prescribed by the vehicle manufacturer, and use a quality full-synthetic oil to reduce engine friction.
Another area for improvement in fuel economy is to remove excess cargo. Items such as golf clubs should be removed if a tee time isn’t on the schedule that day. Same with other items – tools, toys, other athletic gear – that aren’t needed in the vehicle. By reducing the overall weight of a vehicle, there is less mass to get moving from a stop, thereby improving fuel economy.
A final tip pertains to driving style. Avoiding quick acceleration (jack rabbit starts) and driving the speed limit are certainly ways to avoid frequent stops at the gas pump.
Proper tire inflation (as recommended by the vehicle manufacturer) – with these other easy-to-do strategies – is by far the safest way to getting the best balance of fuel economy and vehicle handling.