If you buy only two new tires, consider putting them on the rear axle. Purchasing four tires is the ideal, but as a cheapskate, I understand the pain of tossing two half-worn tires when the other two are completely worn out. I’m here to help fellow tightwads stay safe.
Stop with the “even ifs.” No matter if your vehicle is front-, rear-, or all-wheel-drive, a pickup, a sporty car, or an SUV: If you buy only two new tires, you should put them on the back, according to tire manufacturer Michelin North America.
The short answer: On a rainy day (or if a sprinkler system is irrigating the pavement), even a small puddle could cause your car to spin out if you have worn tires on the back and new tires on the front.
Worn tires will hydroplane well before new ones. The water in wheel ruts found on older highways can be enough to literally lift the worn tires completely off the pavement. If you have new tires in front and old tires in the back, the worn rears are floating while the deep grooves of the new fronts easily cut through the water. Water is not compressible: It either flows through the tire’s grooves or lifts the rubber from the road.
Here’s why that’s bad: Rear tires provide stability. If the worn rear tires are riding on top the water, they can’t offer stability — even if the new fronts are providing plenty of steering ability. So, you can easily end up spinning out.
However, if the new tires are on the rear, the fronts will lose grip before the rears – which can be an easier situation to cope with. Release the accelerator, leave your hands where they are, and wait for the traction to return. Avoid turning the steering wheel more or applying the brakes.
Check out Michelin’s video on the subject:
I have ridden with thousands of drivers in demonstrations like the one featured in the video above. Almost all spun out when the car had newer tires on the front and half-worn rubber on the rear. (The rare exceptions were mainly dirt-track racers.) With newer tires on the rear and more-worn tires on the front, no one lost control.
Front-wheel drive (FWD) is usually why front tires wear out before the rears. The reason: On FWD cars, the fronts carry two-thirds of the vehicle’s weight, do all of the steering and acceleration, and transmit almost all of the braking force. Sometimes, people fail to rotate their tires and discover the fronts are almost bald while the rears appear to have plenty of tread.
A common question: What difference in tread depth can lead to instability? I’ve experienced it at less than 2/32nds of an inch. If you can tell a difference when you stick your finger into the tire grooves, the tires with the most tread should be on the rear axle. Even electronic stability control – a system in your car that can help to automatically bring you out of a spin, in certain situations — can’t help if the rear tires are completely hydroplaning.