This article offers an explanation of light truck tire sizes and construction, along with tips on plus-sizing, tire applications for both on and off-road, as well as lift kits and braking system upgrade tips to maximize vehicle performance.
First off, the tire size designation indicates not only tire dimensions, but the intended service application as well.
If the tire size designation begins with the letter “P,” this indicates that the tire is a P-metric size, which can include applications for passenger cars, minivans, SUVs and light-duty pickup trucks in the 1/4-ton and 1/2-ton load capacity ranges. If no letter precedes the size designation, the tire is a European metric tire (for passenger cars, minivans and SUVs). Euro-metric sizes are the same as P-metric sizes in terms of dimensions, but load carrying ranges will differ slightly.
If the size designation begins with “LT,” this indicates that the tire is designed as a light truck metric size, intended for heavy cargo loads and towing. This typically includes medium and heavy-duty trucks, SUVs and full-size vans rated in the 3/4-ton and one-ton ranges. LT tires are designed to provide a range of reserve load capacity. Example: LT245/75R16 (light truck application with a 245mm section width, 75 aspect ratio mounted on a 16-inch wheel).
If the tire size ends with “LT,” this means that the tire is either a numeric or flotation light truck tire, also designed for heavy loads. Flotation or wide-base tires are oversized in width, improving operation on loose surfaces such as dirt and sand. Applications include 1/2-ton, 3/4-ton and one-ton trucks and SUVs. Examples: 31x10.50R15LT (31 inch outer diameter, 10.50 inch section width, mounted on a 15-inch wheel); 8.75R16.5LT (8.75 inch section width on a 16.5-inch wheel).
Extreme traction LT tires
All-terrain (A/T) tires offer a closer compromise between on-road behavior and off-road use. Typical construction features will include wider shoulder grooves, an aggressive high-void tread design, a dual-compound tread, two full-width steel belts, a three-ply polyester carcass to guard against punctures and bruises under-tread and at sidewalls, a rim protector and a strong bead construction. They are aggressive enough for mild off-road conditions while offering paved road stability and acceptable noise levels.
Mud-terrain (M/T) tires are specifically designed to handle the worst-scenario off-road applications, rock crawling and soupy mud/sand conditions while maintaining an acceptable level of on-road drivability.
These tires typically feature a soft tread compound combined with a deep tread with very aggressive tread blocks and solid sidewall lugs.
Twin steel belts and four or more nylon sidewall plies are common. The heavy-duty casings are designed to help resist punctures and abrasions.
These tires are usually designed to flex to a greater extent, allowing the tire to conform to rough surfaces and to provide needed traction when aired-down. These tires often include a hefty rim protector (a substantially thicker rubber area adjacent to the rim) to further protect the tire and wheel rim in harsh impact and low-inflation situations.
Mud-traction tires may be branded with the M+S (mud & snow) designation, indicating that they are suitable for year-round traction in light snow, muddy and loose surfaces, while maintaining acceptable highway drivability. M/T tires generally provide superior overall traction in sand, dirt and snow and over rocks.
Rock crawler tires specifically for rock crawling activity offer super-aggressive deep-lug treads, soft high-grip tread compound for better grip on uneven surfaces, beefy, solid sidewall lugs for shoulder-contact over sharp rocks, rim protector rib and four or more sidewall plies.
Tire load ratings
Paying attention to a replacement tire’s load rating is critical. The tire must be able to safely support the vehicle weight. If the tire’s load rating is insufficient, the tire may become overheated and can self-destruct.
For example, if a passenger car performance tire is selected to replace an OE light truck tire on an SUV, the new tire should always exceed the requirement for the specific vehicle weight.
Generally speaking, consider the vehicle’s gross weight and divide this by four in order to roughly determine the load capacity for each individual tire. If the vehicle gross weight is 4,500 pounds, each tire should be able to safely support at least 1,125 pounds. However, you should never select a tire that only meets this minimum weight capability. Always select a tire that offers a greater, or reserve load capacity, which will help the vehicle handle and respond to higher-stress and higher-impact situations.
The tire’s load rating, or “Max Load,” indicates the individual tire’s safe maximum load-carrying capacity, when inflated to its recommended pressure.
Never exceed a tire’s maximum load rating (the limit that is molded into the tire sidewall, or the maximum vehicle load limit shown on the vehicle tire placard, whichever is less). The tire is designed and constructed to handle a specific maximum load, and overloading will result in a buildup of excessive heat in the tire which could lead to tire failure
The load index number on the tire sidewall is an assigned number that corresponds to the tire’s load carrying capacity.
Alpha-numeric tires will display an alpha code that indicates maximum load. The letter can range from “A” to “N.” The higher the letter, the larger the size and the greater the load carrying capacity at a given inflation pressure.
Maximum load and inflation
The Max Load and maximum inflation numbers found on the tire sidewall indicate the maximum load that can safely be carried and the maximum allowable tire pressure. The construction of the tire (belts, bead, carcass, liner) dictates the tire’s ability to withstand pressure. The stronger the reinforcements, the greater pressure the tire can hold.
Most alpha-numeric tires feature a load range of B, which indicates that they are restricted to the load that can be carried at a maximum inflation pressure of 32 psi. C, D or E tires are capable of greater loads. Most load range C, D and E tires are intended for light truck applications.
Tire load-carrying capacity of P-metric tires is rated as either Standard or Extra Load. Standard Load tires are limited by the load that can be supported with a maximum inflation pressure of 35 psi. Extra Load rated tires are limited to the load that can be carried at a maximum inflation pressure of 41 psi. Generally, a Standard Load tire will not feature a special designation mark, while Extra Load tires will feature an “Extra Load” marking.
Extra Load P-metric tires will be branded as “Extra Load” and may be identified by an “XL” (for example: LT245/75R15 XL).
It’s important to note that a Standard Load tire (with a normal inflation pressure recommendation of 35 psi) may be marked with a maximum inflation pressure of 44 psi. This does not indicate an increase of the tire’s load carrying capacity, but indicates the tire’s ability to handle higher inflation pressure in order to accommodate special performance requirements.
A speed-rated tire’s sidewall markings will indicate size, followed by the load rating index and the speed rating. For example, a P195/60R15 87S indicates that this tire carries a load rating of 87 and a speed rating of S (this load rating is 1,201 pounds, and the speed rating is 112 mph). A 285/35ZR19 99Y tire carries a load rating of 99 and a speed rating of Y (this load rating is 1,709 pounds); and the speed rating is 186 mph).
Brake and suspension relationship
The tire, wheel and brakes (hub/rotor/caliper) are not supported by the vehicle suspension, and as a mass of weight are referred to as “unsprung” weight. This unsprung weight is basically being hauled around by the vehicle as ballast. The less unsprung weight, the quicker the steering and suspension will react, in terms of handling performance.
However, when you change to larger diameter and/or wider section width and the tire/wheel package becomes heavier (as is normally the case when larger diameter wheel and tire packages are installed on light trucks and SUVs), the unsprung weight increases. This places additional strain on the braking system, suspension and steering systems.
In terms of braking safety, many OE brake systems on light trucks and SUVs are marginal at best. When you install a heavier tire/wheel package, the OEM braking situation becomes even less efficient. Recommend to your customer that he should upgrade the braking system in conjunction with his purchase of a larger, heavier tire/wheel package.
A brake system upgrade would consist of larger diameter rotors, and larger surface-area pads in conjunction with multi-piston fixed calipers (the larger the better). For example, moving to four-piston fixed calipers in place of an OE single-piston floating caliper would substantially improve braking performance and shorten braking distance.
Performance aftermarket brake system manufacturers such as Brembo North America, Wilwood Engineering, Alcon Component Ltd., SSB (Stainless Steel Brakes Corp.), Baer Brake Systems and others offer braking packages (matching rotors and calipers) that have been specifically designed to upgrade all popular light trucks and SUVs.
As far as the suspension system is concerned, again, when you increase unsprung weight due to a heavier tire/wheel package, the suspension works harder in an effort to keep the tire contact patch on the road. Suggest your customer move to heavier-duty performance shock absorbers, possibly heavier anti-sway bars, and more resilient bushings for control arms and anti-sway bars.
A change from “rubber” bushings to a higher durometer urethane bushing material, for example, provides more precise and predictable suspension control and longer tire tread life as a result of diminished wheel angle deflection (toe, camber and caster).
How tire size affects gearing
With regard to off-road use, ground clearance is always an issue, especially in rough and rocky terrain. In addition to the option of raising the vehicle’s body and/or chassis via a lift kit, choosing a larger diameter tire will aid in increasing ground clearance. Note that we mention increasing tire diameter. For off-roading, you need to increase not only ground clearance but the level of tire “cushion” as well.
Basically, a taller section height creates more distance between the tread and wheel rim, providing more protection against the possibility of rim damage.
‘Plus’ sizing wheels and tires
If your customer’s goal is to obtain the ultimate in vehicle steering response and overall handling, one of the elements that needs to be addressed is the amount of deflection between the wheel and the ground as he attempts to plant the biggest footprint possible onto the pavement.
Reducing sidewall deflection will have a big impact on this. The most immediate way of reducing sidewall deflection is to use a tire with a shorter sidewall.
As a vehicle experiences lateral force (during a turn or steering maneuver), the tire sidewalls experience a degree of flex. That flexing movement creates a loss of energy, reducing the amount of grip placed on the road surface.
The answer is to maintain the original overall height of the tire, while reducing the sidewall height. If we do that, we create a larger hole in the center of the tire, which we then fill with a larger diameter wheel.
This plus-sizing is a simple and effective way to enhance both performance and appearance. In addition to reducing sidewall height, a plus move allows us to use a wider tread and section width. The resulting combination of less flex and more rubber on the road is the ideal enthusiast scenario.
The plus concept is used as a guideline in altering wheel and tire sizes. Very simply: Within the plus concept, you retain the stock outside diameter of the OE-sized tire, while improving the vehicle’s handling by lowering the tire’s aspect ratio (going to a shorter sidewall).
Whatever changes take place, the one constant will always be the tire’s outside diameter.
Plus-sizing theory and practice
Let’s say the vehicle was originally factory equipped with 15-inch wheels. A Plus-One move would mean changing to a 16-inch wheel.
Once the new wheel has been selected, you’ll need to know outside diameter of the original tires.
For the sake of argument, let’s say that’s 30.7 inches. Now all you have to do is to locate a new tire that features a 30.7-inch O.D. and will accept the 16-inch wheels the customer has chosen.
Here’s an example: Let’s say the original wheel is a 15-inch diameter and the OE tire was a 265/75R15.
You could make a “Plus-Zero” move by keeping the stock size wheel and changing to a 315/60R15 tire, or go “Plus-One” by changing to 16-inch wheels mounted with 265/70R16 tires; or go “Plus-Two” by moving to 17-inch wheels with 265/65R17 tires. It’s all a balancing act.
Generally speaking, as you increase rim size by one inch in diameter, the tire size will decrease in aspect ratio by a factor of 10 and the section width will increase by a factor of 10.
This generalization is just that — a generalization. You must always keep clearances in mind, and that includes wheel well lips, struts, coil springs, steering knuckles and inner wheel well housings.
As long as the new tire’s section width (width of the tire’s body when mounted) won’t rub on the body, brake calipers or suspension parts, you’re customer is home free.
Keep in mind that you may not be able to match O.D. exactly from the OE tire to the new tire. All you can do is get as close as you can.
To select a tire for a Plus-One alteration
Theory —
When you increase the wheel diameter by 1 inch:
• Increase the tire section width by 10mm.
• Decrease the tire aspect ratio by 10 points.
Practice —
For instance: If the OE size was 265/75R16 (listed as having a 31.7-inch O.D.), the Plus-One size would now be 275/65R17. If that size is not available, move the customer into a 285/70R17, which offers the same O.D.
To select a tire for a Plus-Two alteration
Theory —
When you increase wheel diameter by 2 inches:
• Increase the tire section width by 20mm
• Decrease the tire aspect ratio by 20 points
Practice —
For instance: If the OE size was 265/70R16 (30.7-inch O.D.), the new Plus-Two size would theoretically be 285/50R18. However, since that size may not be viable, you would continue to search for an 18-inch tire that offers as close to a 30.7-inch outside diameter as possible. The size 285/55R18 is listed as having a 30.3-inch O.D., so providing the section width will clear the wheelwell and suspension, that would be the most logical choice. ■
Plus-sizing tips
Plus-Zero: Uses the OE wheel diameter, but makes use of a tire with a wider section and a lower aspect ratio. The customer derives the benefits of a wider footprint without changing wheel diameter. However, in order to make room for the wider tire in the wheelwell, your customer may have to use an aftermarket wheel with an offset that accommodates the positioning of the wider tire.
Plus-One: Uses a one-inch-larger diameter wheel than OE. The tire size will probably be one-step-lower in aspect ratio and, generally speaking, one or two steps larger in section width.
Plus-Two: Uses a two-inch larger diameter wheel than OE. The tire size will probably be two-steps-lower in aspect ratio and, generally speaking, one or two steps larger in section width.
Plus-Three: Uses a three-inch larger diameter wheel than OE. The tire size will likely be three steps lower in aspect ratio and 30mm larger in section width.
Be aware that there’s no magic formula for determining the exact plus size that will suit every single situation. You can use manufacturers’ printed guides to get close, but you should always double-check by determining the O.D. and section width of the target tire.
Tire dimension and performance formulas
FINDING TIRE SECTION HEIGHT
Section width x aspect ratio % = section height
(8.5 inches x 60%, or .60 = 5.10 inches)
FINDING TIRE SECTION WIDTH
Section height divided by aspect ratio = section width
(5.10 inches ÷ by 60% = 8.5 inches)
FINDING TIRE OVERALL DIAMETER
Section height x 2 + wheel diameter = overall tire diameter
(5.10 inches x 2 + 15 inches = 25.2 inches)
