There appears to be no disagreement that oversized tires and wheels affect a vehicle's performance, but to what extent they affect it is a matter of debate.
``Is this something to really be concerned about?'' asked Daniel Filiatrault, vehicle design and operations specialist for the Insurance Corp. of British Columbia. ``In the insurance industry, we don't have a classification for altered-height vehicles. In police reports, there's no category for policemen to tick off for altered height.''
There is still little concrete information on the effects of oversize tires and elevated vehicle height on vehicle and passenger safety, but there are intensive efforts to develop data, said Mr. Filiatrault, chairman of the Altered-Height Vehicle Working Group within the American Association of Motor Vehicle Administrators (AAMVA).
This group has been conducting a one-year study of the issue, including data collection that it plans to issue as a report in June, he said during an address at the Clemson Tire Industry Conference March 9-11 on Hilton Head Island.
Nowadays pickup trucks and sport-utility vehicles (SUV) are being fitted with tires with diameters as large as 49 inches, Mr. Filiatrault noted. This occurs despite strong warnings from the Rubber Manufacturers Association that replacement tires should be the same size as stated on the vehicle's tire information placard and that lift kits or other suspension alterations not approved by the vehicle or tire maker can hamper vehicle handling and stability.
Some of the effects of oversize tire installation are readily predictable, he added.
For instance, oversize tires on an SUV can raise the vehicle's center of gravity, increasing rollover risk, he said. A raised SUV's or pickup's bumper, in the case of a collision, can end up crashing into another vehicle's passenger compartment rather than the door or fender.
There's also the simple fact that a larger tire or wheel diameter creates longer stopping distances for vehicles, Mr. Filiatrault said. Part of that problem is the greater pedal force an oversize tire requires.
``There is a basic connection between pedal force and stopping distance,'' he said. ``Forty to 80 pounds of force are ideal. But with larger tires, you may need 200 pounds of force or more for a panic stop.''
Even with this information, Mr. Filiatrault said, there is no easy way to predict the effect of non-standard-size tires on such basic vehicle features as road handling, road holding and steering. The AAMVA study will attempt to address those questions, he added.