Not many years ago, the concept of plus sizing was largely the province of autocrossers and road racers. That was before it spread to a new breed of vehicle owners who are collectively challenging tiremakers, wheel makers and even frame makers to fit ever-larger tires and wheels under automobiles and pickup trucks.
Today, it is not at all uncommon to talk about Plus 8 or Plus 10. New technology and science is employed to allow this sort of thing to happen.
To give you a sense of where we’re headed, you need to know that we are presently experiencing a 4% to 7% growth rate in technology. But, what does that really mean? It means that if technological growth remains at 4% until 2031, we will have compressed all of the technology we learned from 1880 to 2006 – 126 total years – into the next 24 years.
Let’s take that a step further. What if the technological growth rate is 7% between now and 2031? That’s like stuffing all the technology we have learned since Newton tossed an apple in the air in 1660 into 24 short years. Technology is growing at an exponential, mind-boggling rate.
So, what does all of this have to with plus sizing of tires and wheels? Just this: In the early 1980s, moving from a Plus 1 to a Plus 2 was a very big deal. Plus 3 and Plus 4 sizing (which was, in fact, just around the corner) was an otherworldly concept.
Today, we know that the owner of a pickup truck that comes with 17-inch OE tires won’t blink an eye to re-fit the pickup truck with a Plus 11 setup and 28-inch wheels. Imagine what could happen in the next two decades.
Elusive Answers
Even with today’s technology, dealers are constantly challenged to find the right inflation answers to solve customer desires. When it comes to serious plus-sizing, the answers are hard to find.
Don’t bother looking at the owner’s manual or even the vehicle placard. They don’t yet take into account such unexpected changes in tire and wheel sizes. Consider the Honda Civic.
The base OE tire size is 185/65R14 with a vehicle manufacturer recommended inflation pressure of 28 psi. This is a schoolteacher fitment. But, that teacher’s students will move ahead with a plus fitment of a 205/45R16 tire or a 205/40R17 size with a recommended inflation pressure of 36 psi.
Dialing that up another notch, those wanting a Plus 4 fitment on a Honda Civic may choose a 215/35R18 with a recommended inflation pressure of 34 psi. The load index for this size is 84, which offers 1,100 lbs. of load-carrying capacity. In the case of the Honda Civic, such fitment provides some load-carrying reserve at proper operating pressure.
(Here’s a guideline that you should share with customers: A Plus 4 size has a good potential for tire bruising because the tire pinches between the rim and the road. Further, the ride will be rough. If you notice any sort of sidewall bulge in a Plus 4 fitment, remove the tire immediately and replace it with a new one. You are looking at a bruise caused by a broken tire cord.)
In each of these examples, the overall tire diameter or OD has been closely maintained, as have the recommended inflation pressures. In other words, from Plus 1 through 3, the new tires are completely capable of carrying the load, as long as recommend inflation pressures are maintained.
When customers ask for such fitments, they want to look cool, and they want increased steering response and more cornering grip.
Here’s another rule of thumb: For every 1,000 pounds of static load, you need 114 millimeters of sidewall height for the best handling. A 255/40R17 offers a sidewall height of 4.5 inches, for example, and fits within the guideline.
There is more to consider. Take the case of 195/65R14 with an OD of 24 inches and a load index of 84. Note that there is little difference between that tire size and a 215/35R18 with an OD of 23.9 and a load index of 84. The recommended inflation pressure for the 195 size is 29 psi, capable of carrying a load of 1,234 lbs.
The recommended inflation pressure for the 215 size is 34 psi for a load-carrying capacity of 1,100 lbs. Follow these guidelines, and your customer can safely fall within the load-carrying capacities for the tires.
Going a step deeper, the 215 size inflated to 34 psi carries with it a built-in load-carrying reserve of 20% to 25%. That’s because the vehicle maker’s engineers have mandated that kind of safety for this size tire.
At the same time, the 195 size (which also starts with a 20% to 25% load reserve) inflated to 29 psi and fully loaded with four adults, luggage, etc., maintains a reserve of 10%.
Any time you allow a customer to leave your shop with less than a 10% reserve load-carrying capacity, you are making a mistake. The proper section height will not only handle the bumps, but it will crisp up the steering. In fact, some pickup and SUV owners report the ride is better with larger tires and wheels.
Turning to some of the more extreme tire size changes, a customer who moves from a 60-series tire to a 35-series tire is giving up a visual reference. You can sometimes see the sidewall deflection in a 225/60R16 size, but never in a 215/35R18 size.
What about SUV customers who come in with an OE size of 265/70R17 and want a much larger tire? Keep in mind that their OE tires came with a recommended inflation pressure of 35 psi and a load index of 113. You can safely take these customers to a Plus 7 305/40R24 size with a recommended inflation pressure of 35 psi and a load index of 117. Both carry the load.
Don’t make the mistake of installing a set of 215/35R19 tires with a load index of 85 on a full-size pickup. That type of load index is designed to handle a Honda Civic.
If you’re looking at a half-ton pickup, think about putting on 295/40R20s with a load index of 108. Now, you can tell those customers that they have tires capable of handling the load.
The Tweakers
More astute drivers – particularly sports car owners – may seek to tweak air pressures to carve a little extra out of their rides. Going off recommended pressures for improved handling, cornering, etc., has long been practiced, but it takes skill and an extra-sensitive backside to find the right balance of safety and desired performance.
For the most elite motorsports enthusiasts who are concerned about 1/100ths of a second at the track or autocross course, tell them to invest in a good inflation pressure gauge and a pyrometer. If the car is ‘pushing’ on course, bleed about 10% air pressure on the rear tires. This will make the car more balanced and cut down on ‘push.’
There are still people out there who play with +/-1 psi as they attempt to dial their vehicles in for a perfect day of driving. A dozen years ago, enthusiasts could fiddle around with inflation pressure changes of just 1 psi and get a “better ride.”
Today, thanks to advances in vehicle and tire technology, that thinking has changed a bit. As one tire engineer puts it: “It takes a 10% change in inflation pressure before the driver feels the difference. A 5% change in inflation pressure is for very fine tuning of the vehicle, tires and suspension.” This is probably most true for typical drivers.
Track time separates typical drivers from enthusiasts, and it is where the pyrometer comes in. Make sure the tires are nicely heated up, get your driver into the pits and measure the temperature spread across the face of the tire. The idea is to spread the temperature correctly across the tire tread.
The problem that crops up sometimes has nothing to do with tire engineering, but rather the car, the driver and the weather. Some drivers say a car is oversteering when it is really them causing the issue.
As you work with customers who prefer wider, low-aspect-ratio tires, be sure to remind them that a wider tire will hunt and wander as it grabs camber change on the highway. To help alleviate this problem, consider adding caster to increase the mechanical trail.
Finally, you are going to be seeing wilder and wilder tire fitments and wheel sizes over the next several years.
Enter the world of the so-called “donk” cars with 30-inch wheels and tires mounted on custom-built lifted suspension systems.
This is the point at which tiremakers seem to bow out of the conversation.
Why? Because such tire and wheel sizes are off the charts. All any sane person can do is ask the customer for the make of vehicle, model, year, OE tire size, original inflation pressure recommendation, replacement tire size and which model of tire came as OE.
Next, call the tire supplier for the best possible technical advice. There are some calculations that take reason into account to keep the owners of these vehicles safe.
One tire engineer, who wished not to be quoted, suggested that some of the most extreme fitments could simply use the maximum inflation pressure branded on the sidewall.
Is that really a good idea? No, but when radical fitments call for textbook guidelines that don’t exist, it’s all we really have to fall back on.
