March 27, 2023 8 min read 1 Comment
Tires are the most important component on your vehicle. No system, not your four-wheel drive or your antilock brakes, or anything else, is able to work except through them. But, most trucks and SUVs are fitted with inadequate tires from the factory. How can you upgrade or replace them without sacrificing performance?
4x4 tires come in two types of weight ratings, P-Metric or LT.
The P in P-Metric stands for passenger car. LT stands for light truck which, in this parlance, means a full-size, non-commercial pickup like a Ford Super Duty.
You need to select a tire with a rated capacity appropriate for the weight of your vehicle. For most drivers, this is as simple as looking up the Gross Vehicle Weight Rating (the weight of your vehicle plus the maximum amount of weight it's designed to carry), then dividing that number by four. If you’re towing extremely heavy trailers or hauling a huge amount of weight in your bed (such as with a slide-in camper), you may want to look up your rear axle weight rating, divide that number by two, and go off that instead.
Beyond safely supporting the weight of your vehicle and its load, a tire’s weight rating will determine many other performance metrics.
In this photo of the Toyo Open Country ATIII range, some of the differences between different types of tires are obvious. P-metrics have the tightest tread pattern, and flotations are the most open. Photo: Toyo.
A stronger tire is also a heavier tire. And one which must be inflated to higher pressures.
Weight is an incredibly important factor for two reasons. First, a tire's weight resists both your engine and brakes every time you speed up or slow down. So, a heavier tire will reduce acceleration and extend braking distances. It will also reduce your fuel economy. Second, a tire’s weight impacts your suspension’s ability to control its movements. A heavier tire will move up and down with more momentum, and that movement will have greater influence on the rest of your vehicle as a result.
Combine the momentum of a heavier tire with a stiffer carcass inflated to a higher pressure, and you can see dramatic impacts on ride quality. That stiffer tire will be less able to absorb bumps on its own, transmitting more forces to the suspension, and due to the increased weight, those forces will be higher.
Tire makers also tailor rubber compounds, tread patterns, and ply strength to suit different use cases. P-Metric tires prioritize on-road use with higher proportions of silica in their compound, tighter tread patterns, and more siping (thin, squiggly lines in the tread that mechanically key with the surface you drive on). This gives them more grip on pavement in inclement weather, and makes them run quieter. LT tires feature a higher proportion of natural rubber, and use more open tread patterns with less siping. This makes them less prone to chipping on gravel, and enables them to better grip loose and slippery off-road surfaces.
It’s common to see LT tires available in larger sizes.
From left: an all-terrain, highway-terrain, and mud-terrain. Photo: BFGoodrich.
4x4 tires are available in six different designs, intended for different use cases.
Highway Terrain: What most vehicles come with from the factory. Because the vast, vast majority of drivers rarely, if ever, venture off-road, and purchase trucks with the expectation that they’ll ride and handle like a luxury or sports car, automakers prioritize that use case when selecting the tires they fit to their vehicles. While highway terrains will make little noise and should perform pretty well in rain, their primary advantage is that they’re cheap. Tight tread patterns will struggle to find grip on both off-road surfaces and in winter conditions, and thin, lightweight carcasses can be punctured by something as flimsy as a cactus barb. If you plan to take your truck camping, use it in inclement weather, or travel through rural areas, then you should plan on replacing your highway terrains at the earliest possible opportunity.
All Terrain: These tires attempt to strike a compromise between on-road handling, off-road traction, and performance in inclement weather. All terrains excel in no specific condition, but work reasonably well in most. As such, they’re the right choice for the vast majority of 4x4 drivers. All terrains also come in the widest variety of sizes and weight ratings, allowing you to tailor performance to your specific needs. Drivers who plan only limited modifications to their vehicles and spend most of their time on pavement will be best suited by a P-Metric all terrain tire in a size close to stock. Drivers with significant off-road needs driving heavily modified vehicles, but who still need to handle long highway drives, may prefer an upsized LT all terrain.
Rugged Terrain: If you want to prioritize traction on loose surfaces while retaining some highway manners, then consider an R/T tire. They’ll work better in mud than an equivalent R/T thanks to a tread pattern that is typically deeper and more open, but won’t take those merits quite as far as a mud terrain.
Mud Terrain:Mud terrains will have the deepest, most open tread pattern of any off-road tire. As the name suggests, that helps clear mud so the tire can continue to provide grip on really slippery surfaces. Those large tread voids are also very good at mechanically keying with jagged rocks. The tradeoff is most mud terrains are going to be very noisy on the highway. Soft rubber compounds used to create mud terrains actually grip both pavement and deep snow well, but the flexibility created by that compound along with the large, widely spaced tread blocks will make steering feel imprecise, and may key with rain grooves on highways in the southwest, reducing high speed stability.
Flotation: A speciality tire designed for highly modified 4x4s only, flotation tires may look similar and be branded alike with both all terrain and mud terrain stablemates, but are typically very wide, and, as the name suggests, are designed to float on top of loose surfaces like sand, mud and snow. In addition to that width, flotation tires feature very strong, reinforced sidewalls that allow them to be reliably operated at single digit PSIs, maximizing their contact patch. While all that may sound like a great option, these things are the heaviest tires here, and the ones with the most egregiously poor road manners.
Winter:Winter tires are constructed using a rubber compound designed to remain flexible at extremely low temperatures, and which includes microscopic pores. As you drive over ice, the weight of your vehicle will melt a thin film of water, which is what eliminates traction. A winter tire’s pores are designed to wick that water out from between the tire and the ice, allowing that soft, flexible compound to grip that surface. Winter tires also feature tread patterns specifically designed to cling to snow, and are jammed full of siping to maximize mechanical keying with ice’s texture. Some winter tires also feature studs. Studs are designed for use in northern European countries, where roads are allowed to accumulate packed snow, and drivers almost never encounter bare pavement during winter months. While studs may grip ice well when new, we plow our roads in North America, and studs will rapidly wear beyond the point of usefulness if used on bare pavement. If you hope to safely drive in winter conditions, you need to run winter tires. Keep them on a spare set of wheels, and swap them with your all-terrains every April and November.
The weight capacity of an LT tire will be listed as a ply equivalent letter. Those range from from C to F. Back in ye olde past, tires were constructed using layers of cotton fabric inside the rubber. The number of those plies used in a given tire determined its strength. Today, cotton has been replaced by steel, kevlar, or other modern materials. Because those materials are vastly stronger than cotton, you don’t need 10 layers of them to deliver the same strength as 10 layers of cotton. So, we now use ply equivalent ratings. All you need to know is C will be the least strong, most flexible LT tire, and F the strongest, least flexible.
Think of a tire like a balloon. Inflating a small, thin balloon is easy. Blowing up a very large, very thick balloon is harder. The more weight a tire is designed to support, the more air it will need to contain. Stronger tires are also heavier tires.
A tire with a high weight rating is not necessarily more puncture resistant than one with a low weight rating. Puncture resistance comes from the thickness of the rubber (indicated by the depth of a tread), and the construction of the plies. Toyo, for instance, folds the plies on its Open Country range back around the bead, to cover the sidewall in two layers. That feature is not indicated by the ply equivalent rating.
It should come as no surprise that government regulations are vastly outdated, and carry no merit in the modern world beyond simply being a regulation. Tires may feature two types of sidewsall stamp, allegedly indicating winter capability. Beyond satisfying some legal requirements, these stamps are otherwise meaningless.
M+S: Mud and snow. To earn this stamp a tire’s tread pattern must be made up of at least 25 percent void. California and some other western states require that drivers of 4WD vehicles hoping to forego snow chains on mountain passes be equipped with M+S tires. This in no way indicates that a tire is actually designed to perform in mud or snow.
3PMSF: Three Peak Mountain Snowflake. To earn this stamp, a tire must demonstrate acceleration performance on packed snow that’s 10 percent better than a reference tire. That reference tire is a cheap Firestone highway passenger tire designed in 1993. No braking or lateral traction is tested, nor is any condition other than packed snow.
Larger tires will more easily roll over larger obstacles off-road, and will improve a vehicle’s approach, breakover, and departure angles. Aired down to low pressures, larger tires will also have longer contact patches. Larger tires also look cool. That’s the end of the list of pros. Cons include factors we’ve already discussed like increased rotational mass and unsprung weight, reduced performance and fuel economy, and impaired ride and handling. There’s also one other big factor we need to discuss: effective gearing.
It’s the job of gears to multiply the force your engine is able to exert over the tires. That’s why your vehicle accelerates fastest in the lowest gears, then achieves the greatest fuel economy in higher gears.
By fitting larger tires, you will be reducing the force your engine is able to apply. This further reduces performance beyond the effect of rotational mass, and will drastically impair your fuel economy. Off-road, it may actually reduce a vehicle's ability to safely climb or descend large obstacles.
When paired with locking axle differentials, the force larger tires may apply to your drivetrain may also exceed safe limits, causing components like CV axles to fail, and shortening maintenance intervals for wear items.
Larger tires also alter complex suspension geometry, which is what often causes modified 4x4s to wander rather than drive in a straight line at highway speeds, can reduce vehicle stability while braking, and will compound wear to components like wheel bearings.
Because a vehicle’s front tires must also turn left and right while moving up and down, it’s also very common for upsize tires to come into contact with important components like your truck’s frame, crash absorbing structure and body, necessitating removal or modification of those things, which are again important.
For those reasons, most drivers will be best served by a tire close in weight and size to what comes stock. Just in a flavor actually capable of gripping off-road surfaces, remaining safe in inclement weather, and which resists punctures.
Some of that very long, very serious list of downsides can be minimized by re-gearing your axle differentials to restore your engine’s ability to exert force over your tires. Budget at least $3,000 for parts and labor if you plan to do this.
But, we get it. Not only do big tires look really cool, those approach, breakover, and departure angles, and contact patch are some of the most important factors in determining the outright off-road capability of your vehicle. Want to determine the most effective tire for your truck, with the least compromise, and figure out how to make that tire fit? Talk to the shop where you had your GFC installed, they’re versed in making all these complex factors add up to function. You can also learn from the experience of other GFC owners on our forum.