here is more info. from tire tech
http://www.tirerack.com/tires/tirete....jsp?techid=25
Plus Size Wheels and Tires
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Large diameter wheels and wide, low profile tires go together and show up everywhere from America's new car showrooms to its roads, tracks and trails. So whether the vehicle came from the assembly line or was upgraded after it was delivered, Plus Sizing (also called "Inch-Up" sizing in other parts of the world) probably played a role in its tire size choice. Plus Sizing allows tires and wheels to make a fashion statement while providing a functional improvement.
Plus Sizing dates back to the 1970s when Plus One and Plus Two fitments were the available choices. Drivers could upgrade their cars from relatively narrow 13-, 14- or 15-inch wheels and tires up to wider 14-, 15- or 16-inch combinations. While Plus One and Plus Two are still popular today, the starting point now typically begins with 15- or 16-inch wheel diameters and grows from there.
Plus Sizing supports the premise that it's important to maintain the same overall tire diameter whenever changing tires and wheel sizes to ensure sufficient ground clearance, appropriate driveline gearing and accurate speedometer readings. Large changes in overall tire diameter can alter the accuracy of the speedometer as well as the effectiveness of anti-lock braking system (ABS), traction control and vehicle stability system.
Plus Size Wheel and Tire Examples
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O.E. Size
16" x 7" WHEEL 205/55R16
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Plus Sizing wheels and tires is one of the easiest ways to improve the image of a vehicle. The visual appeal is obvious since alloy wheels are more attractive than tire sidewalls, and bigger wheels combined with shorter tire sidewalls produce a powerful image.
Using tires with shorter sidewalls also quickens steering response and increases cornering stability. Combining them with larger diameter wheels makes it possible to properly maintain the overall wheel and tire diameter, keeping odometer and speedometer changes negligible.
Plus Sizing's biggest risks stem from accidental encounters with potholes, curbs and debris on the road. Low profile tire sidewalls can be pinched more easily between the road and the rim because shorter sidewalls cannot accommodate impact as well as taller sidewalls. Once a vehicle has been Plus Sized, the driver has to try to go around obstacles, rather than run over them.
Additionally, wide tires tend to float on loose surfaces and cannot process water as quickly as narrow tires. This reduces snow traction and hydroplaning resistance when driving on water-soaked highways.
And finally, the maximum Plus Size applications for pickup trucks and sport utility vehicles result in wheel and tire combinations that are significantly heavier than the vehicle's Original Equipment (O.E.) tires and wheels. This increase in weight can lead to longer stopping distances, as well as increased suspension and brake wear.
Here's How We Do It!
We select O.E. equivalent tire diameters and load capacities by matching wider, lower profile tires with wider, larger diameter wheels. This maintains the accuracy of the vehicle's speed dependent systems, while reducing braking distances, improving responsiveness and increasing stability.
A Plus Sizing rule of thumb is to increase tire width by 10 millimeters and decrease sidewall height by 5 to 10 percent for each 1-inch increase in wheel diameter.
You will often find only +/- a few tenths of an inch difference in the overall diameter of the tires, as shown. This results in a negligible +/- four tenths of a mph speedometer variance.