The question of whether or how larger tires on a car or truck affect fuel economy is as old as the rubber tire and combustion engine. Some assert that larger tires are heavier and have more rolling resistance, decreasing economy. Others insist that larger tires effectively reduce the vehicle's numeric gear ratio, causing the engine to run at lower RPM and conserve more fuel. The truth of the matter is both simpler and more complicated. Like so many things in the automotive realm, it depends.
Though larger tires are indeed heavier than stock tires, this weight difference is less than 1 percent of the total weight of the vehicle, less difference than a passenger might make. The only difference the wheel's weight will make in terms of economy is in its rotating mass. Since the tire spins, it acts as a giant flywheel and takes more power to accelerate than a lighter tire. The truck with larger tires will generally get lower fuel economy in stop-and-go driving, but highway cruising will be unaffected.
Larger tires do indeed cause the engine to run at a lower RPM, which is why speedometers will usually read much slower than stock. This can be a good thing or a bad one, depending on your particular engine and vehicle combo. Generally, heavier vehicles with smaller displacement engines will suffer the most, since the numerically lower gearing makes it much more difficult for the engine to accelerate the vehicle. This means that in order to maintain stock acceleration, the driver must apply more throttle, thus using more fuel.
Vehicles with high-torque, large displacement engines like diesels are one of the few applications in which larger tires can actually improve fuel economy. Diesel engines generally produce a surplus of torque, and so may actually benefit from lower RPM.
Larger Tires are usually wider than stock tires. Combined with the fact that the larger sidewalls of bigger tires allow much more flex than stock, the part of the tire actually touching the road (contact patch) can be several times larger than stock. Though this does give more grip when needed, the molecular and mechanical bonding characteristics of rubber mean that you can expect these tires to "drag" more under coasting.
One of the more overlooked aspects of big tire installation are those involved with aerodynamics. Though some assert that the tire's larger cross-section presents and the increased resistance that accompanies is the major aerodynamic factor, the truth is much simpler.
Installing larger tires raises the vehicle, giving it a much larger frontal area relative to the ground. Tires that are 4" greater in diameter than stock will raise the vehicle by two inches, increasing frontal area by as much as 5% in some cases. Combined with the fact that more of the aerodynamically inefficient undercarriage is now exposed to the airstream, a drop in fuel economy can be expected.
In the best-case scenario, the only vehicle that could benefit from larger tires would be a diesel truck, whose body was lowered by half the diameter of the tire increase to maintain the same frontal area. All others can expect a drop in fuel efficiency.
- Photo Credit Image by Flickr.com, courtesy of doug wilson
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