Your Tire Tread Might Leave Marks On Your Wallet
What we do to increase trail performance often decreases fuel economy on the street. With the help of Michelin, we undertook to study that situation from the tire's point of view, and see what effect tread pattern had on real-world fuel economy in a controlled situation.
Rolling Resistance 101
It takes power to push your truck down the road against mechanical friction, wind resistance and tire rolling resistance. At low speeds, the tires and drivetrain are the primary resistance. Aerodynamics takes ever-increasing amounts of power at speeds above 50 mph. According to various studies done since the '70s, tires take 15 to 25 percent of the mpg bite at low speeds and 20 to 35 percent at freeway speeds. Gnarly off-road tires are known to take an even bigger chunk of power in all situations than street or all-terrain tires, but we could not find any specific testing done on them.
A big part of rolling resistance comes from hysteresis, a three-dollar word for the elastic properties of a tire that lets it conform to the road surface according to load, internal air pressure, and temperature. The more it conforms, the more power it takes to roll. Hysteresis is shown by how much more effort it takes to push a truck with the tires aired down than with them aired up. According to industry info, the tire tread typically accounts for 65 to 70 percent of total rolling resistance, the body 30 to 35 percent. Rolling resistance can be all but eliminated, such as by going to very stiff sidewalls, harder rubber compounds, or even solid tires and minimal tread (e.g., racing slicks), but hysteresis and tread design are what makes the tire deliver grip and traction. Tire design is always a compromise between traction qualities and rolling resistance, but technology, especially in the areas of tread compounds and body design, is bridging some of the gaps.
You can find rolling resistance data listed two ways: In pounds of force (#F) and in a rolling resistance coefficient (RRC). RRC is best used to compare two tires of the same size. The #F is the raw force required to move the tire under a given load. Typically the RRC measurements are taken using two SAE protocols, one at a fixed 50mph speed and the other at varied speeds at a specified load.
Rolling resistance data for car tires is readily available but, thus far, testing on LT and flotation truck tires has been limited. The available material shows a big difference in rolling resistance between Light Truck (LT) and Passenger (P) rated tires. You can tell them apart by looking at the size in the specs or on the sidewall: P235/75R15 versus LT 235/75R15, for example. The differences are in the construction of the tires. LT tires are considerably heavier, but they need to be, and that's apparent when you compare load capacity. If load capacity matches your truck, a P-metric tire can be a slight to moderate mpg boon versus an LT. Bear in mind, however, that the LT tire is more durable, especially in a hauling or towing situation. For this reason, Federal Motor Vehicle Safety Standards prescribe reducing the sidewall load capacity by 10 percent when using P-metric tires on light trucks.
Flotation tires, e.g., the sizes rated in inches (35x12.50R15), are a mix of characteristics. Some are LT-rated and some are not. In these cases, the Load Range C tires are generally closer to the P-Metric, and the D- and E-rated tires more like a LT. Some are actually marked "LT." We were not able to find a single rolling resistance test on flotation sizes.
We started with a late model Ford F-150HD. It had recently been used to study fuel economy changes with various bolt-on engine mods, so we knew how to drive it consistently. We started with two sets of tires in the same size class, namely LT285/70R17 (33-inch equivalent), and mounted them on two sets of OE steel wheels.
With tire pressure an mpg consideration, we used a formula that converts the truck's original 245/70R17 factory pressures to the increased volume of the bigger tires. This formula can be used by anyone doing a tire upgrade, and you can see it in the nearby sidebar. In addition, we corrected tire pressures to a standard temperature and made sure we marked and reinstalled the tires in the same positions where they were broken in
We chose a 182-mile route of mixed city, country and freeway driving and picked a warm, temperature-stable day in which to run both sets of tires. It was within about nine degrees ambient temp at the end of each test, and the truck got a 6-mile warm-up before the start point of each test. We spot-checked to make sure the engine, trans and diff oils were warmed up to approximately the same point at the start of each test. In addition, gas from the same station and the same pump was used for fill-ups.
We preran the route and picked a weekday when the traffic would be consistent. During the drive, we studiously observed the speed limits, made full stops at all stop signs, and used the cruise control and the engine load feature on the Gryphon programmer to make our acceleration as consistent as possible.
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