May 23, 2014
If you’re like most drivers of a car that came with run-flat tires from the factory, you didn’t buy the car because of the tires. You chose the car, but it was the vehicle designers who specified it roll off the assembly line riding on run-flat tires.
Run-flat tires do bring several advantages to the vehicle design equation. Their ability to keep going even when flat (typically up to 50 miles at up to 50 mph) makes them a superhero when a simple puncture would render a conventional tire helpless and flat. Since run-flat tires greatly reduce the probability of drivers ever needing to install the spare on the side of the road, the vehicle designers eliminate the spare, jack and tools, which increases chassis design flexibility. The end result is more space for car designers to work with at the beginning of a car’s life, along with less material being scrapped at the salvage yard at the end.
Contrary to urban legend, run-flat tires aren’t a "man of steel" like the fictional comic hero. They can get punctures just like any other tire. The difference is run-flat tires rely on careful engineering to carry the load when there isn’t enough air pressure to do the job. We’ve all seen what happens to a conventional tire without air - squished sidewall. Run-flat tires have special rubber reinforcement in the sidewalls that can temporarily support the weight of the vehicle in the event of low or no air pressure. But that additional sidewall reinforcement often brings a trade-off in the form of increased ride harshness versus a conventional tire. The extra weight and sidewall stiffness may also cause a modest increase in fuel consumption, as it can take some extra energy to roll a run-flat tire down the road.
If your car came with run-flat tires from the factory, odds are you don’t have a spare or tools onboard to change it out in the event of a flat, so switching to conventional non-run-flat tires may not be desirable. Having a flat conventional tire and no way to change it guarantees a flat tire will strand you on the side of the road.
So how do you improve the ride of your car while still retaining the run-flat capability? Bridgestone thinks they have an answer. As a leading supplier of run-flat tires for Original Equipment fitments since 2002, they have invested heavily in R&D on how to improve the ride quality while still retaining extended mobility. Their latest run-flat technology is called 3G RFT (third generation run-flat technology) and brings together a combination of advances, ranging from clever aerodynamics around the tire sidewall to a special formulation of the rubber sidewall reinforcement.
Bridgestone’s newest 3G RFT tire design is simply called DriveGuard, a run-flat tire designed to equal or better the ride of any OEM run-flat, and even be comfortable enough to install on cars that didn’t come with run-flat tires as Original Equipment. To find out how DriveGuard performs, the Tire Rack team conducted a Real World Road Ride and Performance Track Drive with our BMW 3 Series test cars that come from the factory with run-flat tires. For comparison, we also drove on the Pirelli Cinturato P7 All Season Run Flat, one of several Original Equipment run-flat tires fitted to the BMW 3 series. Our evaluation used a pair of identical 2014 BMW F30 328i sedans riding on new, full tread depth 225/50R17 tires mounted on 17x8.0 wheels.
Our 6.0-mile loop of expressway, state highway and county roads provides a great variety of road conditions that include city and highway speeds, smooth and coarse concrete, as well as new and patched asphalt. This route allows our team to experience noise comfort, ride quality and everyday handling, just as you would during your drive to school or work.
BMW was one of the first vehicle manufacturers to adopt run-flat tires and is the only one that offers them on almost all of their car models today. By design, the BMW 328I has a taut suspension that communicates well without feeling overly harsh or stiff.
Original Equipment tires often have an advantage in complementing a vehicle’s road manners, because the tires were developed alongside the vehicle and tailored to enhance noise comfort and ride quality. Out on the road, the Pirelli Cinturato P7 All Season Run Flat tires showed they matched the chassis very well. The OEM Pirelli rode well for a run-flat tire and generated little tread pattern noise which confirmed how much Original Equipment run-flat tires have improved with years of fine-tuning.
The Bridgestone DriveGuard also felt like it was tailor-made for the BMW 3 Series’ suspension and offered a ride quality that did a better job of softening how the car drove over the sharp edges of patched potholes and expansion joints. Impact noise from the small- to medium-sized bumps was moderately lower with the DriveGuard, too. Both tires offered similar handling, feeling direct and responsive, and very appropriate for Grand Touring All-Season tires.
Our 1/3-mile per lap test track course includes 90-degree street corners, a five-cone slalom and simulated expressway ramps. Run in both dry and wet conditions, the test track allows our team to experience the traction, responsiveness, handling and drivability normally only encountered during abrupt emergency avoidance maneuvers or competition events.
In both dry and wet conditions the overall handling and traction levels were very similar for both tires. Both performed at a level appropriate for tires in the Grand Touring All-Season category, and felt predictable, responsive and sure-footed during cornering and braking.
Winter weather is often unpredictable; and snow-covered roads change with every passing vehicle as they churn snow into slush or pack it down to polished ice. A constantly changing test surface makes side-by-side comparisons difficult, so we use a dedicated winter testing facility in Northern Sweden with acres of groomed snow that provides the consistency we need to get reliable acceleration and braking comparisons. This facility also has a prepared snow-handling course where we evaluate the stability and control of each tire during abrupt maneuvers. To simulate the icy conditions found at intersections or the black ice experienced out on the highway, we use ice at a local hockey rink and measure acceleration and braking traction.
The Pirelli Cinturato P7 All Season Run Flat fitted as Original Equipment on a BMW 3 Series provided adequate overall snow traction during our testing. While the Bridgestone DriveGuard matched it on ice, it fell well short of the Pirelli in snow traction, taking significantly longer to start and stop than the Pirelli along with delivering lower overall scores in our snow-handling test.
Our Real World Road Ride features a relatively flat 6.0-mile loop of 65 mph expressway, 55 mph state highway and 40 mph county roads along with two stop signs and one traffic light every lap. Our team drove each tire approximately 400 miles over the course of several days. Since we wanted to compare fuel consumption results that typical drivers would experience, our drivers were instructed to maintain the flow of traffic by running at the posted speed limits and sustain the vehicle’s speed using cruise control whenever possible. They did not use hypermiling techniques to influence vehicle fuel economy.
| Tire | Test MPG* | Gallons/Year @ 15,000 Miles |
% vs. Most Efficient |
|---|---|---|---|
| Bridgestone DriveGuard | 30.6 | 490.2 | -1.0% |
| Pirelli Cinturato P7 All Season Run Flat | 30.9 | 485.4 | -- |
| *Our evaluation used Linear Logic ScanGauge II automotive computers to record fuel consumption, and Race Technology DL1 data loggers to record true distance travelled. | |||
With ever-increasing emphasis on better vehicle fuel economy, the vehicle engineers have worked with OEM tire suppliers to continue to reduce rolling resistance as one way to help improve the vehicle’s fuel economy numbers. That means most Original Equipment tires are among the best in rolling efficiency. Our observed vehicle fuel economy showed a negligible difference between the two tires. Based on our results, the small 3/10ths of a mile per gallon difference between the Original Equipment Pirelli and non-O.E. Bridgestone tires would result in an annual difference of just over 5 gallons of premium gasoline. At the current cost of $4.00/gallon, it would amount to just about $20 for drivers driving 15,000 miles per year.
It’s important to note our test’s fuel consumption measurements follow consistent procedures designed to minimize variables that could influence the results, however they do not represent an exhaustive long-range fuel consumption study. While our procedures require the test vehicles in each convoy to run under the same prevailing conditions, the week-to-week differences in ambient temperatures, barometric pressures and wind speeds that we experience over a season of testing can influence vehicle fuel consumption and prevent the absolute mpg values of this test from being compared directly against those of others.
Larger differences in consumption between tires may indicate a difference that might be experienced on the road, while smaller differences should be considered equivalent. As they say, your mileage may vary.
Bridgestone’s 3G run-flat technology in the DriveGuard appears to have taken a measurable step forward in improving the common shortcoming of run-flat tires — ride quality — while other areas such as handling, wet/dry traction and fuel economy essentially match that of the tuned OEM Pirelli Cinturato P7 Run- Flat tire. One area the DriveGuard falls somewhat short in is snow traction, where its performance was considerably lower than the OEM Pirelli.
Bridgestone DriveGuard (Grand Touring All-Season Run-Flat): Choose to keep moving. The DriveGuard is Bridgestone’s Grand Touring All-Season self-supporting replacement run-flat tire designed to satisfy drivers looking for a longer wearing, better riding option to replace Original Equipment run-flat tires, as well as safety-minded drivers whose cars were not originally available with run-flat tires. Read more.
Pirelli Cinturato P7 All Season Run Flat (Grand Touring All-Season Run-Flat): The Cinturato P7 All Season Run Flat is Pirelli’s Grand Touring All-Season run-flat tire developed for sports coupes, sedans and small crossover vehicles. Cinturato P7 All Season Run Flat tires feature Pirelli Self-Supporting Run-Flat Technology that enhances safety and convenience by providing temporary extended mobility in the event a puncture allows complete loss of air pressure. Pirelli Self-Supporting Run-Flat Technology allows tires to literally run-flat for up to 50 miles at 50 mph (80 km at 80 km/h) unless otherwise specified by the vehicle manufacturer for Original Equipment tires. The Cinturato P7 All Season Run Flat is designed to deliver all-season traction and handling for drivers who operate their vehicles in America’s various weather conditions, including in light snow. Read more.
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