How to measure tire tread
When tires last for 60k miles or more and wear is so gradual, it’s pretty easy to just take them for granted and not even give much thought to the minimum tread depth to replace tires.
Worn tires are a problem, though, and can have a drastic effect on your vehicle’s braking performance, cornering, ride quality and noise level --- and in wet weather, they can be downright dangerous. We’re going to talk a little about how to take stock of your tires’ condition, as well as ways to get the most miles out of your tire investment.
How to check tire tread depth
If you’re wondering how to measure tire tread depth, there are a few different methods of going about this, and they’re all pretty accurate and simple. Tread depth card: Like the name suggests, this is a card that’s marked with fractions of an inch. Just insert the card into your tread grooves and read the depth on the increments on the card.
Tread depth gauge
This little device puts a probe into the base of the tread and gives you a reading in millimeters or fractions of an inch; it’s more like the type of thing you’d see in a tire shop or state inspection station, but you can get readings that are just about as accurate with other methods.
Tires are designed with “wear bars”that are molded into the base of the tread at a right angle. The wear bars are 2/32” tall, so if the wear bars are flush with the tread face it’s definitely time for new tires.
Penny test/quarter test
You may not have a tread depth gauge or card, but you probably have 26 cents! Here’s a handy lesson on how to check tire tread with a penny. Insert the penny into the tread groove, Lincoln head down. If the rubber barely reaches Lincoln’s head, your tires are at 2/32” depth (more on that in a minute). Try again with a quarter, Washington’s head down. Does the rubber reach George’s head? Then your tires are at a depth of 4/32”. One more time with the penny; if the rubber reaches the Lincoln Memorial, your tires are at a depth of 6/32”.
What’s the big deal about 2/32”?
In every state, 2/32” is considered to be the minimum safe tread depth in order to pass a state safety inspection. Our informal advice here, however, is that 2/32” is really too worn to still be safe. A road test from Consumer Reports showed a considerable drop-off in braking, cornering and especially wet-weather performance well before tires were worn down to 2/32”.
Wet weather is a special concern when it comes to worn tires. On a wet road, a wedge of water accumulates in front of your tires and the grooves in your tread need to be able to channel that water and direct it behind the tire’s contact patch. Tires that are excessively worn can’t do that anymore, leaving your tires to skate along on a film of water that takes you out of contact with the road. That’s what hydroplaning means, and a hydroplaning vehicle can get away from you very quickly.
Other things to check your tread for
Other than tread depth, there are a number of other things you should be mindful of when inspecting your tires.
A lot of people may not be aware of it, but tires have a life expectancy that has nothing to do with the number of miles on them. Exposure to the sun’s UV rays and to the elements can cause dry rot, where the tire’s rubber actually starts to break down and slowly disintegrate. You can recognize dry rot by a network of fine cracks along the sidewall and shoulder, with the tire’s coloration fading from black to a dull gray. Dry rot can be very dangerous since it can cause a blowout and complete tire failure with little warning.
This is similar to what happens with dry rot, but it’s more of a function of age. Tires can start showing cracks at the shoulder and sidewall (and even across the tread) due to exposure to ozone from electrical fields, chemicals like tire dressing products or oils, low tire pressure or even manufacturing defects. Superficial hairline cracks aren’t necessarily something to worry about, but severe cracking means tires should be replaced.
Keep an eye out for nails, staples, broken glass and other junk embedded in a tire’s tread; these can all lead to tire failure or at least can necessitate a repair.
A bulge or bubble in your tire’s sidewall is a bad sign; it means something is about to give way internally in the tire’s structure and you’re going to need to replace that tire ASAP.
Cord and belts
A tire that has its internal steel belts or fabric cords exposed is worn to the point of imminent failure. Get rid of that tire and replace it ASAP!
Uneven tire wear
Tires that are worn unevenly can be an indicator of other issues with your vehicle, if you know what to look for:
Excessive wear at center of tread
Tires that are worn more at the center than the edges might be overinflated, as over-inflation changes the tire’s shape and its contact patch, with more of the vehicle’s weight riding at the center of the tread.
Excessive wear at edges of tread
This is a sign of underinflated tires; again, this can change the tire’s shape and profile. Underinflation also generates excessive heat from its added rolling resistance, hurts fuel economy and affects braking and handling.
Wear at one front tire’s outside edge
This type of wear is almost always a sign of alignment problems. Your front wheels are set for specific steering angles (called toe-in and toe-out) as well as an inward/outward tilt, as seen from the front (called caster/camber). Excessive wear at an outside edge is a sign of an alignment problem, especially if you’ve noticed the steering wheel pulling to one side. The tire in question has been trying to steer the vehicle in another direction and is being “dragged along” by the other tires, scrubbing tread off unevenly at the edge.
Uneven,“cupped” wear can point to ball joints, shocks, tie rod ends or other front-end parts that are too worn to keep the tire in good contact with the road, causing a patchy wear pattern.
Run a hand lightly along the tread surface. If you can feel that tread ribs are higher and sharper at one edge than the other, this can also be a sign of improper toe or caster/camber.
Tires in front worn more than rear tires
Your tires should be rotated by switching their positions on the vehicle every 5,000 miles or so. This is to even out wear patterns, since weight distribution and inertia from cornering or braking will wear front tires more quickly. When front tires are more worn than rear ones, it’s probably due to a failure to rotate them.
It might seem irrelevant since sidewalls aren’t in contact with the road, but have a look at them and keep an eye out for cuts, deep curb scuffs, bulges or indentations (as well as dry rot and cracks mentioned above). Take a look at the bead where the sidewall is mounted to the rim as well, and remember that a puncture or cut on a sidewall that causes tire failure can’t be repaired.
A hard hit on a curb or railroad tracks can be enough to knock a vehicle’s front end alignment out of spec. If you’ve noticed this kind of tire wear, a pull to one side and a tendency for the steering wheel to not readily center itself after rounding a corner, you should probably have your alignment checked and reset if necessary.
Getting the most miles out of your tires
A few changes in driving habits can help extend your tire life a bit:
Keep your speed down while cornering or negotiating an onramp or exit ramp (even if hard cornering is fun)
Avoid hard stops and jackrabbit starts
Go easy on rough roads and over railroad tracks, potholes and gravel
If you have to tow or haul heavy loads, keep your speed down and be mindful of your tires’ maximum weight rating (stamped on the sidewall)
When and where you should check your tires
It’s a good idea to check inflation once a month, since rubber is slightly porous and air molecules can migrate their way through the sidewalls and valve stem, costing you about one PSI of air pressure monthly. While you’re using the tire gauge and topping off the tires (remember to only do this when the tires are cold), take a minute and do a quick inspection of each tire, looking for everything we mentioned above and running your hand lightly over the tread surface to feel for irregularities.
Tires have a hard job, and too often they don’t get enough consideration from drivers. With any luck, your tires will last through their whole treadwear warranty phase, but it’s important to know when your tires are too worn out to be safe any longer. With what we’ve talked about here, you should now have the know-how you need to check out your tires thoroughly and know when they’ve gotta go!
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