There can be many reasons to change a car’s tire size. Perhaps you like the look of larger tires or want improved grip. However, the process isn’t as simple as it seems because the manufacturer calculated the existing setup to balance safety, performance, and fuel economy. Add a carefully calibrated speedometer and numerous sensors to the formula, and the slightest tire change can affect drivability and driver assistance systems. Under certain circumstances, using a different tire size can affect extended car warranty coverage.

Ultimately, you want to learn how tire sizes affect performance and why staying close to your car’s original specifications is critical to ensure smooth roads ahead (literally and figuratively). Continue reading as we explain how vehicle wheel size affects handling, braking, and gearing.

Why Tire Size Matters More Than Most Drivers Think

Whether you drive a fresh-from-the-factory automobile or something older, it’s easy to focus on what you can see. Yet lurking beneath that shiny exterior is a sophisticated chassis designed to work with a specific set of tires for a precise rolling distance and contact footprint. The same applies if you drive a Toyota Corolla or an Aston Martin Vanquish.

Any change to the tires’ diameter or width affects:

• The distance the vehicle travels per wheel revolution.
• The accuracy of the control module measurements for each rotation.
• The load is transferred to the suspension’s bushings and bearings.

Why does this matter? At the most basic level, a change in tire size translates into inaccurate instrument readings. For example, a larger rolling circumference results in fewer rotations per mile, leading to under-reported speed and mileage. Consider that in very practical terms, you might think you’re observing the speed limit, only to have a police officer tell you something different.

There are also technical implications. Fundamental safety systems, such as anti-lock brakes (ABS) and traction and stability control, rely on wheel-speed data to determine when to activate. When sensor signals aren’t transmitted correctly, these technologies can have mistimed reactions. Shifting also comes into play when the transmission control module incorrectly measures speed and throttle to determine gear selection and torque-converter function.

The less measurable impact of out-of-spec tire sizes includes changing how the car feels. Even changing just the sidewall height can alter cornering behavior and ride comfort. So, choosing a new set of tires requires careful consideration.

How Larger Tires Impact Wheel Alignment and Ride Stability

Your car’s alignment is determined by camber, caster, and toe. However, tire size impacts how these wheel angles contact the road surface. For instance, a wider tire can make the steering feel heavier at slower speeds (like during parking) or less responsive on the highway. A taller tire can cause the car’s weight to shift differently when turning or hitting a bump.

Tire Terms to Know

• Camber: Whether the wheels lean in or out when looking at your car head-on.
• Caster: Whether the wheels tilt forward or backward, which affects how the steering feels.
• Toe: Whether your tires point slightly toward or away from each other, like being pigeon-toed.

Even if camber, caster, and toe aren’t in your everyday vocabulary, these alignment angles affect how your car feels on the road. The vehicle may pull or wander, especially on grooved pavement. You may also encounter increased sensitivity in crosswinds because there’s less contact with the road. Still, it’s not unusual for an oversized tire to have increased wear on the edge.

It’s also worth mentioning that a larger wheel-and-tire setup adds weight. This can affect suspension responsiveness and cause the tires to be less planted on the pavement. In addition, the car may hop or bounce over rough surfaces and require longer stopping distances. It’s also not unusual for the rotors and pads to heat up during downhill braking.

The Link Between Tire Size and Transmission or Drivetrain Strain

Larger tires enable a car to travel further with each wheel turn. While that might appear to be an advantage, the aftereffects of such a switch are anything but. At a minimum, the new setup may make the vehicle slower to get into motion, requiring the engine and transmission to work harder. This additional effort can translate into weaker acceleration and worse fuel economy. The transmission may shift more frequently, especially during downhill travel, which reduces fuel efficiency and increases stress on the gearbox.

The impact of larger tires on the transmission is equally significant with all-wheel-drive (AWD) or four-wheel-drive (4WD) cars. In this case, the driveline is required to work in ways the system wasn’t originally engineered for, causing premature wear. Furthermore, if any oversized tires experience uneven wear in an AWD/4WD vehicle, the drivetrain has to spin the wheels at speeds to equalize traction and power, also taxing the system.

Common Sensor Errors Caused by Improper Tire Sizing

Most modern vehicle safety systems (ABS, stability control, and traction control, to name a few) rely on wheel-speed sensors to operate correctly. Changing the tire’s size may cause these sensors not to send accurate information and reduce safety. Under these conditions, the ABS might engage sooner than expected on a slippery surface because the system thinks a wheel is decelerating more rapidly than it really is. Stability control can suddenly cut in during a turn if it thinks the car is sliding when it’s not.

Accurate speed information is equally critical to adaptive cruise control and collision-mitigation systems. This data helps these technologies determine the following distance and closing rate, which is essential for determining when these driver aids are activated. Meanwhile, a car’s tire pressure monitoring system (TPMS) can become temperamental when air volume and load ratings differ from the manufacturer’s specs. You’ll get dashboard warning lights that may go ignored, causing a problem when tire pressure actually drops.

Real-World Warranty Denials Linked to Tire Modifications

Legitimate extended warranty companies play by the book when it comes to approving or denying claims. If a covered component fails, repairs will be authorized according to the terms of the contract. However, a claim caused by an aftermarket vehicle modification can be a reason to deny coverage. Check the fine print if you have an extended warranty or are thinking about one. Most plans say they can deny claims if the car has been modified, including using oversized tires.

We’ve covered how adding bigger tires to your car can affect performance, safety, and longevity. Let’s see how that translates into potential real-world scenarios that could involve the denial of a warranty claim:

• Transmission Trouble: The gearbox in an SUV fitted with oversized tires starts slipping gears. The warranty inspector traces the failure to extra stress placed on the transmission from the larger tires and denies the claim.
• Drivetrain Wear: A 4WD truck develops differential problems following a switch to out-of-spec tires and wheels. The repair shop identifies the issue as related to uneven rotation speeds, and the warranty turns down the repair request.
• Sensor Errors: A sedan’s ABS light comes on after larger new tires are installed. The shop traces the cause to inaccurate wheel-speed readings, and extended warranty coverage is declined.
• Wheel Bearing Failure: A van with larger, heavier tires develops a noisy wheel bearing. The warranty administrator rejects the claim due to the non-standard setup.

How to Choose the Right Tire Size to Stay OEM-Compliant

Selecting tires that are compliant with warranty coverage and automotive industry guidelines isn’t complex. Most tire retailers recommend keeping the overall diameter within three percent of factory specifications. Tire dealer Les Schwab has an online calculator that lets owners determine what options stay within automaker guidelines. There are numerous similar tools available from other tire companies. Alternatively, a professional tire shop can provide recommendations if you’re uncomfortable determining the details independently.

Regardless of the approach, finding size-friendly tires starts with locating the correct tire specification information for your vehicle. Usually, these details are on a placard on the driver’s door or door jamb. Here, you’ll find the size, load index, and speed ratings for all four corners.

Why Plans May Exclude Coverage for Improper Modifications

Larger tires increase the risk of mechanical failures in numerous vehicle systems. Vehicle service contract (VSC) providers, such as Endurance Warranty Services, have coverage exclusions for non-standard vehicle components, including aftermarket tires or lift kits, that may cause unintentional damage to your vehicle.

Customized Coverage with Endurance

Whether you’re already driving a car with larger-than-standard tires or prefer a ride that matches factory specifications, Endurance has a wide range of vehicle protection plans that can protect against breakdowns and surprise repair bills. Best of all, you can use any licensed facility for covered repairs.

Learn more about your coverage options by calling (800) 253-8203 for a FREE quote. You can also visit the Endurance online store to see your price and plan recommendations instantly.

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