Photo courtesy of Negaro UK at Flickr.com.
For years, auto designers have been using wind tunnels to improve car designs. Wind tunnels make it easy to see how different features affect aerodynamics. Hoods, spoilers, and even mirrors have been engineered and re-engineered due to wind tunnel testing. This is because friction consumes roughly 80% of all gasoline that’s used while driving. By reducing friction, wind tunnel tests improve gas mileage and boost performance.
There’s one thing that wind tunnels miss though – the friction between a car’s tires and the road. This overlooked detail has gained new attention recently. Due to tightened CAFE standards, many cars now come standard with tires that improve gas mileage.
Expected improvements are in the 1-2mpg range in highway driving, depending on the vehicle and the previously specified factory tire. The gains aren’t enormous, but as Scott Miller, GM’s vehicle performance manager for full-size hybrid trucks said, “Every bit helps.”
Unfortunately, these factory issue tires are often replaced with gas hogging aftermarket tires. What makes some tires get better mileage than others? It’s all about friction, or “rolling resistance”. Rolling resistance is a measurement of how much friction a tire produces. Tires with low rolling resistance (LRR) convert less energy into heat and noise.
So, what’s the trade off?
In the automotive world, there’s never a free lunch, and low rolling resistance tires are no exception. There are certain trade-offs that come with reduced rolling resistance. In order to minimize rolling resistance, LRR tires are designed with less surface area in contact with the road. That saves gas, but it also reduces traction and increases stopping distances.
Tires with low rolling resistance are stiffer and flex less. This means LRR tires can feel uncomfortable because they provide less cushion on rough roads. Some LRR tires are also less durable and wear out after 30,000-40,000 miles. They are also slightly more expensive than other tires, but they can save money over the life of the tire (the savings can be substantial on cars with low MPG ratings).
How do you actually find tires with low rolling resistance?
This is where things get tricky. Tire companies have been slow to report the rolling resistance ratings on their tires. Rolling resistance values vary based on the testing situations (different cars produce different rolling resistance values), so a raw number isn’t meaningful to all customers. Also, the testing process can be time consuming. Here’s how Bridgestone responded when we inquired about why rolling resistance is not listed on their website (emphasis added):
Rolling resistance has traditionally been measured thru SAE (Society of Automotive Engineers) test procedure called J1269. It measures the force required to roll a tire against a dynamometer at a fixed speed of 50 mph. Within Bridgestone/Firestone, we have over 1,300 passenger and light truck products in the Bridgestone line alone and, conceivably, each one could have a different rolling resistance. The tread compound is a major factor, but construction, size, and even tread pattern can have an influence. At least 3 tires must be run in each configuration to get a good average. At approximately 1 hour per rolling resistance test, this amounts to 3,900 hours or over 6 months just to run the Bridgestone brand.
This explains why these values are estimated. We have some data, however it frequently does not line up with those sizes or patterns requested. Therefore, estimation is required.
The weight of the tire will have some affect on gas mileage. What is more of a factor, though, is the tire “footprint”. This term refers to the actual area where the “rubber meets the road”. The same size tires may have different contact areas and therefore different gas mileage implications. More rubber coming in contact with the road can create increased rolling resistance. Generally, taller, narrower tires are better for fuel economy, if you retain your current wheels. Increasing the tire aspect ratio, for instance from 70 to 75, will provide additional load carrying capacity.
Your local mechanic may be slightly more helpful, but don’t count on it. Right now, the best way to find a tire with low rolling resistance is to find a chat board dedicated to your car and surf the wisdom of the crowds. There are also several non-comprehensive lists of LRR tires, but they may not be available in your area and the lists quickly become outdated as new models are introduced.
Hopefully, this situation will change soon. A California law went into effect in 2008 that requires all companies to list RR ratings for replacement tires sold in the state. As more people become aware of green tires, there will be rising demand. This demand will drive innovation and may also bring prices down. In the near future, we may even see a Green Seal on tires with Low Rolling Resistance, just like the Energy Star label on appliances.
The spike in gas prices in 2008 has focused attention on several ways to improve mileage without adopting radical technologies, and low rolling resistance tires are only one of several inexpensive ways to get significant improvements.
Before you go out and buy new tires, there are several ways to reduce the rolling resistance of your current set. Start by removing any excess weight from your car – all that junk in the trunk is pressing the tires down against the road and increasing the contact area. Also, check the air pressure on your current wheels:
The easiest way to reduce rolling resistance… is to make certain that the tires are properly inflated. A vehicle that requires its tires to be inflated to 35 psi (based on the vehicle’s tire placard) will have an increase in rolling resistance of approximately 12.5% if the tires are allowed to become underinflated to just 28 psi.
Photo courtesy of Tamaki at Flickr.com.