Most modern cars come equipped with 15 to 18-inch stock tires, whereas SUVs and larger vehicles have stock wheels from 19 to 22 inches. Some hobbyists modify trucks and other vehicles for looks or enhanced performance and may switch to tires as large as 35 inches. It’s not advisable to make these changes without knowing how a vehicle’s drivetrain works because large tires can affect multiple mechanical components of a car, including the transmission.
Thirty-five-inch tires won’t directly ruin a transmission, but they can cause premature failure. Larger tires cause the transmission to work harder, as it requires more force to rotate the axles. Re-gear the transmission and recalibrate the speedometer when making substantial tire size changes.
When modifying a vehicle, owners should know what they’re doing to prevent the early breakdown of a car’s critical mechanical systems. This article explains how tire size affects a car’s drivetrain system and whether you should re-gear the transmission to accommodate a larger tire size. Read on to learn more.
Does Tire Size Affect Transmission?
A vehicle’s drivetrain is made up of several systems that work together in conjunction with the engine to get a car moving — the drivetrain is not a single part. The transmission and wheels are both parts of the drivetrain, as well as the following:
- Constant Velocity (CV) Joints
When one or more parts of the drivetrain experience damage or other significant changes, all parts are subsequently affected in some way. So, while tire size won’t directly damage a car’s transmission, it does affect it and can potentially cause trouble in the long run. And adding 35-inch tires to a vehicle with a drivetrain initially designed to handle 19-inch tires completely alters the overall balance of the system.
With that said, it is possible to add 35-inch tires to a vehicle without negatively affecting the transmission; you simply must make other adjustments to accommodate the larger tire size to reduce its harsh impact on the drivetrain system.
If you switch to 35-inch tires without any other alterations, you put enormous stress on the drivetrain. This causes all components to work harder and wear down more quickly, potentially leading to the premature failure of one or more vital systems.
How Transmissions and Tires Work Together
A car’s engine supplies power to the vehicle, and the transmission takes power from the engine and transfers it to the axles to rotate the wheels. The amount of energy transferred is determined by the given speed.
With stock tires, a certain amount of energy is required to rotate the tires. When increasing the size of the tires, the amount of power necessary to turn the axles becomes greater. It takes more energy to make a single revolution because the tires are bigger and move the vehicle slightly further.
So, the transmission must transfer more energy to the axles, thus increasing the work. When work increases, so does the force. Increased force means increased stress on the transmission.
Re-Gearing and Recalibrating for 35-Inch Tires
The most significant problem associated with tires of a larger circumference is differences in drive ratio — the bigger the tire, the greater the drive ratio. When switching to 35-inch tires, you must re-gear the transmission to avoid unwanted stress on the axles and drivetrain due to an increased drive ratio.
Large Tires and Shifting Issues
If you don’t re-gear, you’ll likely begin experiencing serious shifting issues as a result. Though these issues may start as minor inconveniences, over time, they’ll become more frequent, eventually leading to slipping of the transmission and, quite possibly, total failure.
In manual transmissions, you’ll notice that you’re regularly downshifting to maintain speeds. With automatic transmissions, you may find the vehicle shifts at non-optimal speeds, leading to reduced fuel efficiency and other performance issues.
Unfortunately, the longer the issues persist, the more stress you put on the vehicle and the entire drivetrain system. So, do not delay! Get to re-gearing the transmission and recalibrating as soon as possible.
Re-Gearing a Transmission
Swapping out differentials on an automatic transmission is fairly simple, but total re-gearing on an automatic is practically impossible without knowledge of total rebuilds. Therefore, the best way to fix transmission issues on automatics after tire changes is to adjust shift points in the computer using an electronic engine tuner.
When preparing to re-gear a manual transmission, you’ll need to consult a gear selection chart for your specific make and model of vehicle. Make sure to have your car’s current rear axle gearing, the diameter of your current tires, and the diameter of the tires you intend to install.
Also, keep in mind that when re-gearing a four-wheel drive, you’ll need to re-gear both the front and rear differentials to the same ratio to prevent axle binding.
In addition to re-gearing after switching to 35-inch tires, you’ll need to recalibrate the speedometer. If you skip this step, you’ll receive inaccurate speed readings and inaccurate mileage readings on your odometer. The faster you drive, the more your speedometer reading will be off.
A tire’s circumference calculates the vehicle’s speed. Any deviation in size from the factory tires affects the speed and odometer readings. With larger tires, the speedometer and odometer read slower than with stock tires. As a result, you’ll notice inaccurate transmission shift points (which is why you must reprogram shift points in the computer on automatics) and possible issues with traction control and automatic brake system (ABS) functionality.
Fortunately, most newer vehicles utilize electronic speed sensors that allow users to recalibrate the speedometer using programmable tuners. Older vehicles, however, use cable-and-gear-driven speedometers, so they require manual recalibration.
Switching tires on a vehicle can lead to improved aesthetics, better fuel economy, and enhanced performance. However, if you’re not sure what you’re doing, you could damage your car. Car owners looking to change from stock tires to bigger wheels should understand the basics of drivetrain systems and gear and drive ratios. Additionally, they should know what they’re doing to prevent damage and the premature breakdown of vital vehicle systems.
- Universal Technical Institute: What Is a Powertrain?
- AAMCO: Increasing The Tire Size Can Affect You
- National Academies Press: Cost, Effectiveness, and Deployment of Fuel Economy Technologies for Light-Duty Vehicles
- Transmission Digest: Transmissions & Oversized Tires
- Expedition Portal: Understanding Gearing and Why You Want To Re-Gear
- RV Magazine: Transmission Tuners and Programmers
- The Art of Manliness: How Manual Transmission Works in Vehicles