The ABCs of Plus Sizing
Changing a Vehicle's Look Can Change Its Performance
Joe Findeis, publisher of The Ultimate Wheel & Tire Plus Sizing Guide, has provided expert advice on plus sizing tire and wheel packages for more than 40 years - both at the retail level and as a consultant to the tire industry.
He’s the first to admit that custom tire and wheel packages are “wants,” not “needs.” But that doesn’t absolve tire dealers and other tire retailers from the duty of making sure that mounting a spectacular-looking aftermarket tire and wheel combination doesn’t negatively impact vehicle performance and safety.
Findeis defines plus sizing as “the act of installing larger-diameter wheels with lower-profile tires to improve and enhance the handling and appearance of one’s vehicle.
“With the correct application, plus sizing can enhance performance. The lower aspect ratio (shorter sidewall) and increased contact patch (larger footprint) can improve overall handling and appearance.”
However, this does not give shops a free hand to sell and install any tire and wheel combination that a customer wants to buy, he notes.
Never install a tire with lower load and performance ratings than what the vehicle manufacturer recommends.
Findeis calls this “basic common sense. When we’re figuring plus sizing fitments for our database, the first thing we look at is load rating. You can get in trouble by messing with load ratings.”
Never set an inflation pressure level that is lower than what the vehicle manufacturer recommends. If different inflation levels for front tires as opposed to rear tires are specified, always follow the same relative pressure difference.
Always verify adequate body and chassis clearance before installing a tire and wheel package.
In addition, make sure that replacement tires have the same overall diameter/rolling circumference as the vehicle’s original equipment tires. Verify the original tire size using the vehicle’s door placard.
“Keep an eye on overall tire diameter,” says Findeis. “In the old days, the only thing you had to worry about was your speedometer being off” if the overall diameter of the tire changed.
“It’s more of a concern now,” due to the prevalence of ABS, traction and stability control systems and advanced driver assistance systems, which have become more commonplace on vehicles. However, it is widely accepted that a variance of 2% to 3% or less should pose no adverse effects on vehicle systems, he says.
Be aware of unsprung weight, which Findeis classifies as “anything that’s not being absorbed by the vehicle’s suspension.”
And keep in mind that plus sizing - particularly extreme plus sizing - can have a negative impact on OE brake performance.
“Back when people were putting 24-inch tire and wheel packages and the extreme 26-inch and 28-inch packages on SUVs, all that extra weight increased their stopping distance and also burned the heck out of those vehicles’ brakes.
“A conservative and correct package is better than going too big, which can create future problems.”
The shorter sidewalls found on high performance (HP) and ultra-high performance (UHP) tires are more susceptible to damage from road hazards like potholes. (For example, a plus-one fitment places the rim one-half inch closer to the ground and a plus-two fitment places the wheel an inch closer to the ground. This creates a less-forgiving situation.)
Dealers who are newer to plus sizing might be tempted to base their recommendations on what’s currently on a customer’s vehicle. Don’t make this mistake, says Findeis.
“They will run out to the parking lot, look at the tire on the car and prepare a plus size tire and wheel package based on what is currently on that car. But whoever put that tire on first could have installed the wrong tire. I have an old saying: ‘When in doubt, leave it out.’ It’s better to be safe than sorry.”
Proper plus sizing can enhance a vehicle’s performance, according to Findeis. Low-profile sidewalls, in addition to providing a wider contact patch, can increase vehicle handling and stability. And a stiffer sidewall can enable faster steering response and shorter braking distances.
Customers who want to dress up their vehicles with HP/ UHP tires and custom wheels are looking to tire experts for advice, says Findeis. That’s why salespeople should be equipped to explain different scenarios and benefits.
“First, determine the customers’ expectations and of course, the customer’s cost range,” he explains.
“The package a customer might have in mind may cost $4,800, but he might only want to spend $1,200. You also need to find out what the customer is expecting” in terms of vehicle performance.
“It’s not always positive to start the sales process with some negatives, but realistic expectations - even if they are perceived by the customer to be negative - need to be worked into the conversation somehow.”
The goal is for the customer to drive away happy and safe, says Findeis.
“Plus sizing should be a positive experience and one that enhances a customer’s enjoyment of his or her vehicle. But changing the look of a vehicle can change its performance.”
Brushing up on the basics of plus sizing can prevent unpleasant surprises and promote customer satisfaction, he adds.