A tread gauge is a simple tool to measure tire tread depth in increments of 1/32″ or 1mm. Most use a probe inserted into a tread groove to measure the distance between the top of the tread and to bottom of the grooves.
Safety on wet roads is the main reason for keeping an eye on tread wear, as well as knowing when to have your tires replaced.
How To Measure Tread Depth With A Gauge
- Insert the probe of your gauge in one of the main grooves in your tire tread design pattern.
- Push down on the top of the gauge until the shoulders of the gauge come to rest on top of the tread blocks and the probe contacts the bottom of the tread groove.
- Remove the gauge from the top of the tread, being careful to not nudge the probe, to read the measurement.
- Repeat the process at several points around the circumference and width of the tire.
The key to understanding how to use a tread depth gauge is knowing what the measurements mean. Each increment of tread wear reduces tire performance in the rain and increases stopping distances and the chance of hydroplaning.
We’ll cover some of the common types of tread gauges and explain the difference between the tire wear ranges.
Let’s take a closer look.
Tire Tread Measuring Tools
There are many types of tread depth gauges on the market. Some are far more complicated than others, but they all do the same basic thing – measure tread depth in inches or millimeters.
Some will provide additional features while others are easier to use.
Let’s review each of the different types of gauges.
Plunger Type Tread Depth Gauge
The most common, and usually least expensive tire tread depth gauge is the plunger type gauge. These simple tools rest on the top of the tread blocks and you press a plunger that inserts a probe into the tread groove.
When the plunger stops it will display the depth of the tread at the bottom of the gauge measurements. It works by simply sliding a stick, similar to a ruler, out of a tubular housing. The edge of the housing is the point on the stick that the depth of the tread equals.
Dial Tread Depth Gauge
My personal favorite type of tread depth gauge is the dial gauge. This gauge works identically to the plunger-type gauges, but it displays tread depth on a dial with a needle instead of a ruler-type stick that protrudes out of the housing.
It really isn’t any easier to use, but it is easier to read for not much difference in cost. If you’re a professional looking to pick up a quality depth gauge or just a consumer that’s serious about regularly checking tire wear, I highly recommend the dial-style gauges.
Digital Tread Depth Gauge
Digital gauges for measuring tire wear are my least favorite for a few reasons, but they are perfectly good tools.
Digital gauges work the same as the plunger and dial gauges with a probe that you insert into the tread groove, but instead of an analog display, it has a digital display.
A digital gauge will display tread depth on a digital display, usually to an unnecessarily accurate scale. Often, this extra resolution fluctuates slightly if you press a little harder or softer due to the rubber giving underneath the probe.
Digital gauges typically toggle between metric and standard units, however, they display decimals for the inch measuring scale instead of the commonly used fractions.
A digital gauge also relies on a battery that will eventually need to be replaced, which will undoubtedly be discovered at an inconvenient time.
I don’t find there to be any advantages to the digital gauge aside from possibly being easier to read for some. The main downside is reliance on a battery. For these reasons, I don’t recommend them.
Where To Measure Tire Tread Depth
You should measure tread depth in the deep grooves between tread ribs. Do not try to measure smaller or narrower tread grooves since these don’t always extend as deeply or they are too narrow for the probe and they won’t produce an accurate reading.
You should take multiple measurements around the entire circumference of the tire and across the width of the tire. This will ensure that you’re able to accurately read the overall depth of the entire tire.
Taking multiple measurements will help you identify uneven tire tread wear patterns. Also, the most shallow tread depth should be considered the actual tread depth to ensure safe driving in wet conditions.
How To Read A Tire Tread Gauge
All tire tread depth gauges measure the tire’s tread depth in inches or millimeters. Here in the United States, tread depth is most commonly measured in thirty seconds of an inch.
If you’re using a digital gauge in inches, you’ll have to do a little math in your head to convert fractions to decimal increments. Another reason I don’t like to use a digital gauge.
Tire Tread Gauge Chart
New Tire Tread Depth
11/32″ – 10/32″ (9mm to 8mm)
Most new tires for passenger cars and trucks will have a depth of 10/32″ or 11/32″. Some off-road truck tires will usually have much deeper tread depths. But road-legal tires, when new, will have at least this much tread depth.
Safe Tire Tread Depth
9/32″ – 5/32″ (8mm to 4mm)
A common question that I’m often asked is what is a safe tire tread depth. Assuming your tires aren’t brand new, anywhere from 9/32″ down to 5/32″ of tire tread is a good range.
Good Tire Tread Depth
4/32″ – 3/32″ (4mm to 2mm)
Tire tread depths between 4/32″ and 3/32″ are considered to be still good and above the legal limit. Most tires that have reached this depth should be considered worn out and ready to be replaced, but they can still be used if care is taken in rainy weather.
Minimum Tread Depth
2/32″ (1.6mm)
The legal minimum tread depth measurement is 2/32″. Tires that have reached this low depth have very little grip in wet weather and stopping distances in the rain will be much farther than deeper tire tread patterns.
This is also the point at which your tires will have worn down to the level of the wear bars. Wear bars are the raised portion in the bottom of the main tread grooves that are designed to indicate when you need to replace your tires.
Tread Depth To Replace Tires
When a tire’s tread reaches 4/32″ it’s time to consider tire replacement. Tread wear down to this level gives very little room between the tread blocks to allow water to flow out from under the contact patch in wet conditions.
This is well above the minimum tread depth but still quite low and will have allowed you to get the majority of the value out of your new tires.
Pushing to a depth of 3/32″ is not unreasonable but you are reducing the safety margin in rainy conditions even further.
When tread wear reaches 2/32″ it is legally time to replace tires. At this point there is precious little left of the tread groove and water can only escape from underneath the contact patch in the mildest of wet weather conditions.
Risks of hydroplaning are very high at this level and stopping distances will have become dramatically longer than those of new tires.
How Deep Should Tread Depth Be?
The deeper your tread grooves the better. It’s commonly recommended to maintain 4/32″ or more tread depth for your safety. This will ensure adequate traction and handling performance in rainy weather conditions.
Allowing your tires to wear below 4/32″ becomes more and more dangerous and traction and handling performance on wet road conditions begin rapidly dropping.
The difference between 5/32″ and 4/32″ is much smaller than the difference between 4/32″ and 3/32″.
Stopping Distances On Wet Roads
Safety is the main reason to have your tires replaced when they become worn. Tire groove size determines how well your car or truck will stop on wet surfaces and can help prevent hydroplaning.
Tire grooves channel water out from underneath your tire’s contact patch and allow it to gain traction and grip the pavement when road conditions are rainy.
The following graph shows test results of stopping distances at different tire wear levels in moderate rain conditions from 70mph.
Other Problems And Considerations
Off-road and winter tires have different tread depths and don’t fall into these ranges. Off-road tires have deeper grooves and winter tires need at least 5/32″ of minimum tread depth to get traction in snowy and slushy conditions.
Also, uneven tire wear or damage due to curb strikes, potholes, or foreign objects can obviously make a tire with an otherwise good life left, dangerous or unable to hold air pressure.
Resources
Below are some links you may find helpful when learning about tires
Final Thoughts
Tread wear will ultimately determine the performance of the tires equipped on your car or truck on wet road surfaces. The more worn your tires are, the less ability they will have to stop and reduce the chances of hydroplaning.
It’s important for you to understand what the effects of tire wear are so you can make an informed decision about when to replace your tires.
Good luck and happy motoring.
