Over-inflated tires are a bad idea

Q: If I were to increase the inflation pressure of my tires to improve fuel economy, what considerations should I be aware of? Can I run at the maximum pressure marked on the sidewall?

A: There has been much bunk posted on the Internet about how easy it is to save fuel by over-inflating the tires on a vehicle. The people never mention the downside: driver safety.

I have canvassed all the major tire companies and not one was willing to recommend deviating from what the vehicle manufacturer's placard recommends.

Michelin, in particular, noted, "Michelin's recommendation is, and always has been, that the correct and only inflation pressure for your vehicle is that which is specified by the OE vehicle manufacturer, either on the placard (inside the driver's door jamb) or owner's manual."

Are the tire companies part of the cabal that killed the 100-mpg carburetor and just want you to wear out your tires faster?

In short, no. The thing that kills fuel economy and increases tire wear is underinflation. Driving with tire pressures 10 per cent too low will increase fuel consumption by 4 per cent. And the tires will give 30 per cent less mileage before wearing out.

The tire pressure numbers on the placard are not just a random whim by the car company. A lot of research went into choosing the tire size, type, speed rating and pressures for your car. The pressures have to take into account passenger comfort, vehicle grip, vehicle load, tire life, rolling resistance and fuel economy.

All car manufacturers would like to increase the fuel efficiency of their cars. But in dealing with tires, for everything you gain, you lose something elsewhere on the product map.

That's why tire performance charts are spider graphs. The perfect tire would have a perfect circle graph to show its performance, but that never happens. The charts end up looking like spider webs.

The area where the tires touch the ground is the contact patch. By increasing tire pressure, the contact patch shrinks.

According to Michelin, "The first thing to understand is the tire's interaction with the pavement is through contact-footprint forces, which are significantly affected by inflation pressure. Altering these contact-footprint forces from the intended design with an inflation increase will vary by vehicle and inflation pressure, but generally results in less than optimal performance for the tire application on the vehicle."

Jon Bellissimo, Goodyear's director of consumer tire technology, states, "On wet surfaces, lower inflation pressure generally produces slightly higher braking traction – until the onset of hydroplaning."

On the lateral-grip (how well the car holds in the corner) aspect of overinflation, Bellissimo concludes,

"If pressures are increased without the corresponding increase in load (adding weight), lateral grip will generally decrease. For vehicles in the market place, the placard inflation generally provides optimum handling."

The Michelin engineering team ran a computer simulation on a family car with pressures increased to 44 psi from the proper 35 psi.

They report, "Increasing inflation pressure can negatively affect performances such as handling, wear life, worn appearance and braking traction. For instance, our modeling simulations predict, with an increase from 35 to 44 psi, up to 6-per-cent degradation in stopping distance can occur on wet/dry surfaces."

To put that in context, a Chevy Cobalt travelling at 110 km/h takes about 49.8 metres to stop. An increase of 6 per cent adds another 3 metres to the distance – three-quarters of a car length.

The Michelin report concludes, "The key learning is that inflation pressure affects the tire-vehicle interaction, and the only correct inflation pressure for proper balanced performance is what is specified by the vehicle manufacturer on the placard. Never exceed the maximum inflation pressure, as branded on the sidewall of the tire."

Email tire questions to John Mahler at thetireguy_1@hotmail.com.

Please include vehicle's make, model and year, tire brand and size, as well as your name, address and phone number.

Volume of mail prevents us from answering all queries or providing personal replies.

Follow Wheels.ca on
Facebook
Instagram #wheelsca