September 30, 2011
Your performance car, sporty coupe or sedan came with performance run-flat tires. It's time to replace them, and you've decided to stick with a run-flat design to maintain the extended mobility capability they offer. Or your lease term is almost up, and like most, your agreement requires that run-flat tires be on the vehicle when you turn it in. Not long ago your selection process would have been simple – install more of what came from the factory, as there weren't many (or any) replacement run-flat tire designs to choose from. But with the number of run-flat equipped BMW, MINI, Chevrolet Corvette and other vehicles on the road today, growing demand is leading many of the tire companies to develop additional replacement run-flat tire options.
To evaluate the differences between some of the latest run-flat tire designs, the Tire Rack team conducted a Real World Road Ride and Performance Track Drive with a group of Ultra High Performance and Max Performance Summer run-flat tires. We included the Bridgestone Potenza RE050A I RFT as a reference (Original Equipment on BMW 3 Series), comparing it with the Kumho Ecsta SPT XRP, Michelin Pilot Sport PS2 ZP and Pirelli Cinturato P7 Run Flat replacement tires. Our test used 2011 BMW E92 328i coupes with full tread depth 225/45R17 tires mounted on 17x8.0" wheels.
Our 6.6-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.
All four of the tires in our test felt nimble and responsive. Of note was the direct steering feel of the Michelin Pilot Sport PS2 ZP, which responded immediately to driver inputs. This tire feels eager even when driving at the pace of surrounding traffic. The Bridgestone Potenza RE050A I RFT also felt rather responsive, but didn't quite match the urgency of the Michelin tire. The Pirelli Cinturato P7 Run Flat doesn't quite possess the crispness of the Michelin or Bridgestone tires, while the Kumho Ecsta SPT XRP felt a little slower to respond to driver commands through the steering wheel.
When it comes to run-flat tires, ride quality is an important component in the decision process for most drivers. The Pilot Sport PS2 ZP received the highest scores for ride quality from our drivers, feeling a little firm but well-controlled and not too harsh. The Cinturato P7 Run Flat also did a good job softening the bumps, riding somewhat better than the BMW O.E. Potenza RE050A I RFT. A little further back was the Ecsta SPT XRP, which didn't soften the bumps quite as well as the others.
The Pilot Sport PS2 ZP also lead the group in terms of lowest overall noise level, producing only modest tread noise at it rolled over the various asphalt and concrete surfaces along our route. The Cinturato P7 followed, and was also well liked by our team. Again the O.E. Bridgestone tire slotted in just behind the Michelin and Pirelli tires, producing a modest but discernable tone as it rolled along. The Ecsta SPT XRP rounded out the group generating a little more volume than the other three tires.
Our 1/3-mile per lap test track course includes 90-degree street corners, a 5-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.
All four of the tires in our test proved you don't have to give up performance to get a run-flat's extended mobility. The Pilot Sport PS2 ZP really came alive when driven at the limit, displaying very responsive steering, excellent braking traction and stable cornering. It almost felt a little more composed and capable than we remember of the non-ZP version tested a few years ago. Not all that far behind was the Potenza RE050A I RFT, also feeling well-balanced, and obviously well-tuned for this car by the Bridgestone and BMW engineers. Subjectively, the Ecsta SPT XRP followed with good steering response and rear stability during rapid transitions. Feeling a little less precise was the Cinturato P7 Run Flat, which was still able to lap our course a bit faster than the more predictable-feeling Ecsta SPT XRP.
In the wet, the Pilot Sport PS2 ZP again lead the way thanks to its good grip level and sure-footed feel with just a hint of modest understeer. The Cinturato P7 Run Flat was able to lap our track almost as quickly as the Michelin, but didn't feel nearly as stable or predictable. The Potenza RE050A I RFT trailed the Pirelli in lap time, but subjectively was more balanced and easier to control at its somewhat lower limit. Rounding out the group was the Ecsta SPT XRP which felt a little more edgy at the limit, making it a bit harder to control.
Our Real World Road Ride features a relatively flat 6.6-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 approximately 500 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.
@ 15,000 Miles
|% vs. Most Efficient|
|Bridgestone Potenza RE050A I RFT||26.7||561.8||-3.37%|
|Kumho Ecsta SPT XRP||27.6||543.5||---|
|Michelin Pilot Sport PS2 ZP||27.3||549.5||-1.10%|
|Pirelli Cinturato P7 Run Flat (W- or Y-Speed Rated)||27.3||549.5||-1.10%|
|*Our evaluation used Linear Logic ScanGauge II automotive computers to record fuel consumption and Race Technology DL1 data loggers to record true distance travelled.|
While none of the tires in this test were designed with low rolling resistance as a high priority, we did find a small difference in observed vehicle fuel economy. Based on our results, the 0.9 mile per gallon difference between our lowest and highest observed fuel economy would result in an annual difference of just over 18 gallons of premium gasoline. At the current cost of $4.00/gallon, it would amount to an annual difference of about $73 for drivers traveling 15,000 miles a 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.
While many Original Equipment run-flat tires have a poor reputation with consumers, our test shows that some of the performance designs can deliver excellent handling with reasonable road manners.
The Michelin Pilot Sport PS2 ZP seems to do it all with crisp handling, good traction and reasonable road manners. The Bridgestone Potenza RE050A I RFT is one of a wide array of O.E. run-flat designs from Bridgestone, and on the BMW 3 Series at least, drives pretty well overall. The Pirelli Cinturato P7 Run Flat offers a well mannered alternative to O.E. run-flats, and the Kumho Ecsta SPT XRP leads the value proposition with a reasonable price and good fuel economy.
Bridgestone Potenza RE050A I RFT (Max Performance Summer): The Potenza RE050A I RFT (Run-Flat Tire) is Bridgestone's Max Performance Summer run-flat tire developed exclusively for selected BMW Z4 vehicles, as well as 3 Series cabriolets, coupes and sedans equipped with BMW's sport package. It is designed to provide temporary extended mobility for a distance of 50 miles at up to 50 mph even after a puncture has allowed complete air pressure loss. Developed to combine responsive and predictable handling with good traction on dry and wet roads, it is not intended to be driven in near-freezing temperatures, through snow or on ice. Read more.
Kumho Ecsta SPT XRP (Ultra High Performance Summer): The Ecsta SPT XRP (KU31) is Kumho's Ultra High Performance Summer self-supporting run-flat tire developed for sports cars, coupes and sedans to provide temporary continuing mobility (up to 50 miles at up to 50 mph) in the event a puncture allows complete air pressure loss. While Ecsta SPT XRP (Extended Runflat Performance) tires are designed to combine performance, comfort and good looks with the convenience run-flat mobility provides, they are not intended to be driven in near-freezing temperatures, through snow or on ice.
Michelin Pilot Sport PS2 ZP (Max Performance Summer): The Pilot Sport PS2 ZP (Zero Pressure) is Michelin's Max Performance Summer run-flat tire developed for selected performance sedans and sports cars. Initially introduced as Original Equipment (O.E.) on BMW 3 Series sport package cars, the Pilot Sport PS2 ZP has recently expanded its O.E. applications to include Michelin's first Corvette fitment on the highest performing production supercar in Chevrolet's history, the 2009 Corvette ZR1. The Pilot Sport PS2 ZP is designed to combine world class dry road traction, handling and cornering, however it is not intended to be driven in near-freezing temperatures, through snow or on ice. Read more.
Pirelli Cinturato P7 Run Flat (W- or Y-Speed Rated) (Ultra High Performance Summer): The Cinturato P7 Run Flat tires featuring W- and Y-speed ratings are Pirelli's Ultra High Performance Summer run-flat tire developed for the drivers of sport coupes, sedans and crossover vehicles. Cinturato P7 Run Flat tires feature Pirelli's 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's 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. Cinturato P7 Run Flat tires, like all summer tires, are not intended to be driven in near-freezing temperatures, through snow or on ice. Read more.
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