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Subpar EV driving range? Tire pressure may be to blame
2020-11-06   |  Editor:et_editor  |  135 Numbers

If you find your electric car’s total driving range diminishing with each charge cycle, you might want to hold off on a trip to get your battery checked out. Instead, tire pressure may be the culprit.

Lower tire pressures result in higher friction and therefore worse energy efficiency; that is certainly nothing new. But as EVs become more and more popular, mileage-sensitive car owners tend to misdiagnose their EVs’ reduced driving range as a sign of faulty battery. In response to this, Geely, ExxonMobil, and China-based Linglong Tire undertook an in-depth study into the relationship between tire pressure, tire manufacturing process, and EV driving range.

Let’s cut to the chase: using worse tires can result in a 5% reduction in yearly driving range on average. That is, approximately 36 kWh of electricity is wasted given a yearly mileage of 12,000 km (assuming the EV’s tires are not pumped for the entire year).

Prior to a deep dive into the report, it should be pointed out that the automaker, tire manufacturer, and oil company all share the same common goal of finding any possible reason under the sun to get the consumer to either replace their tires or upgrade to higher-end ones. Nevertheless, they have provided a plethora of data worth investigating in their report, but the main conclusion would lead one to further conclude that buying new tires is relatively unnecessary so long as one pumps one’s tires every month.

The report repeatedly made references to IPLR, which stands for inflation pressure loss rate. This concept, in plain English, refers to the rate at which a tire loses its internal air (pressure). Under normal circumstances (that is, without structural damage to the tire itself), the lower the IPLR, the slower the “attrition rate” of the tire.

In the report, “good tires” are defined as having an IPLR lower than 1.8%, while this percentage rises to 3.1% for “bad tires”. These definitions exclude other factors such as tire tread or stability/agility.

The research team discovered that, for an EV with a theoretical maximum driving range of 360 km, using tires with an IPLR lower than 1.8% would result in an average driving range of 353 km after one year, while using tires with IPLR higher than 3.1% resulted in an average driving range of only 344 km after the same period.

In terms of energy efficiency, EVs with low IPLR tires consume about 11.6 kWh of electricity per 100 km, while EVs with high IPLR tires see an increase to 11.9 kWh. Assuming that the yearly mileage for the average driver is about 12,000 km, the difference between tires with high vs. low IPLRs works out to be about 36 kWh.

▲Lower IPLRs mean higher average driving ranges (Source: SAE).

Hypothetically, equipping the 5 million EVs that are currently in operation globally with low IPLR tires would result in a yearly power saving of 18 million kWh – all this, from a mere change of tires. Alternatively, and as has been previously mentioned, being diligent in pumping one’s tires to ensure optimal air pressure at all times works too.

Especially relevant for those of us who happen to be early adopters with a case of gear acquisition syndrome is that fact that many tire manufacturers are now releasing airless tires specifically for the EV market. Airless tires forego the conventional tire’s two-part structure, which features an outer tire and an inner tube. Instead, they have a honeycomb structure that is highly shock-absorbent and highly gripping without running into potential risks of leaky or even blown tires. As of now, however, airless tires remain in the product testing stage, since manufacturing them is prohibitively expensive. Michelin, for instance, is expected to release its own airless tires to market in 2024 at the earliest.

Aside from this type of disruptive technology, tier-one tire manufacturers have all been investigating possible ways to slow the loss of tire pressure, whether through reinforcing tire structures, using better materials, or using a thicker inner tube lining. A good candidate for improving air sealing in tires is BIMSM, a material patented by ExxonMobil that strengthens the adhesiveness of rubbers.

Although high tire pressure means lower energy consumption, it also comes at the price of driver comfort. Take the Tesla Model 3 and BMW 320i, cars with similar curb weights, for instance; Tesla’s recommended tire pressure for the Model 3 is 42-45 psi, whereas the same for the 320i is 32-35 psi. In spite of the two models’ similar curb weight, structural and spec differences mean that Model 3s become difficult to handle at tire pressures below 39 psi. Conversely, conventional fuel vehicles with tire pressures above 40 psi are relatively uncomfortable to drive, since vibrations become more noticeable at such high tire pressure.

From fuel-based vehicles to EVs, from human-driven cars to self-driving cars, tires serve as the first point of contact between the vehicle and the ground. As EVs see a gradual increase in torque and curb weight, tire manufacturers have in response designed tires specifically for EV use, touting benefits of increased durability and tire grip. As it stands, increased driving range will likely be the next major selling point for these manufacturers.