Tread Void Ratio: Off-Road vs Highway Use

What is tread void ratio?

Tread void ratio is the proportion of the tire's footprint that consists of open space, grooves, and sipes rather than solid rubber. A tire with a high void ratio has large, widely spaced tread blocks with deep channels between them. A tire with a low void ratio has continuous ribs or tightly packed blocks with narrow grooves.

The federal motor vehicle safety standards define a groove as the space between two adjacent tread ribs, and tread ribs as tread sections running circumferentially around a tire. These are the voids that determine how much rubber contacts the road. The tread depth and the width of the grooves together set the void ratio for a given design.

Why high void ratio helps off-road

In mud, sand, gravel, and snow, a high void ratio serves two functions. First, the open channels give debris somewhere to go. When a tire rolls through mud, the tread blocks bite into the surface while the voids fill and then release the material as the tire rotates. Tires with too little void space pack up with mud and turn into slicks.

Second, the edges of the tread blocks themselves provide the biting edges that generate traction in loose material. More, larger blocks mean more edges to dig in. This is why mud-terrain tires are built with aggressive, high-void patterns that look nothing like a highway tire.

For serious off-road work, the best mud terrain tires and best all terrain tires use void ratios tuned for the conditions they will face. When you get into deeper mud, you generally want more void. For mixed use that includes daily driving, a more moderate void ratio keeps road manners acceptable.

What high void ratio costs you on the highway

Every groove in a tread pattern is rubber that is not touching the road. On dry pavement, a tire with a low void ratio can put more rubber in contact with the surface, which helps with braking and cornering stability. A high void ratio tire, by contrast, has less contact area and the large blocks can squirm under load.

Noise is another consequence of high void ratios. Open tread patterns trap and release air as the tire rolls, generating a whine or hum that becomes louder with speed. The large tread blocks also pound the pavement, adding to the noise. For long highway trips, this can be fatiguing.

Wear can also be uneven. The big, unsupported blocks of an aggressive off-road tread tend to flex more on pavement, which can lead to irregular wear on the rear edge of the blocks. This is one reason a mud-terrain tire may not last as long when used mostly on roads.

If most of your driving is on paved roads, consider highway terrain tires or a set from the best car and SUV tires guide. You will give up some off-road capability but gain a quieter, more stable ride.

Matching void ratio to your driving

The right void ratio depends on where you drive most. If you spend the majority of your miles on highways and city streets, a low void ratio tire with continuous ribs or tightly packed blocks is the right choice. These designs prioritize stability, low noise, and even wear.

If you drive on gravel or dirt roads occasionally but still commute daily, a moderate void ratio tire, such as a light all-terrain, balances traction with comfort. These tires use smaller, more closely spaced blocks that still provide some off-road bite without the worst highway penalties.

If you regularly drive in deep mud or over loose rock, a high void ratio mud-terrain tire is appropriate. You will accept the noise and stability tradeoffs because the tire is doing the job it was designed for.

Vehicle type also matters. A heavy truck or SUV with a high void ratio tire can feel vague in lane changes at highway speed. The same tire on a lighter vehicle may feel less planted. If you need to carry significant loads, check the tire's load range. The tire load range for light trucks guide explains how load capacity interacts with tread design.

For a full comparison of tread families, see mud-terrain vs all-terrain tires and all-terrain vs hybrid tread off-road.

Typical void ranges by tire type

Tire makers do not publish a standardized void ratio number, so you cannot compare models on that spec alone. You can, however, judge void ratio visually. The photos on tire manufacturers' spec pages or the detailed images on our size guides show the tread pattern clearly.

Highway tires usually have the lowest void ratio, with nearly continuous ribs and narrow circumferential grooves. All-season passenger tires follow, with some lateral slots added for wet grip. All-terrain tires step up the void ratio with larger blocks and wider channels. Mud-terrain tires sit at the top of the range, with extremely open patterns and deep voids.

The way to read a tread pattern is to look at the area of the footprint that would touch the road. If you draw a rectangle around the contact patch, what fraction is rubber versus groove? That visual is your void ratio estimate. For wet weather grip, you want some voids to channel water away. The tire grooves and hydroplaning article explains this tradeoff.

How tread depth changes void ratio

Void ratio is not fixed for the life of the tire. As the tire wears, the grooves become shallower. A worn tire has a lower effective void ratio at the contact surface than the same tire when new. This is one reason a new mud-terrain tire performs better in mud than the same tire at half tread depth.

It also means a worn highway tire handles differently in the rain. The grooves are what displace water from under the contact patch. When they wear down, the risk of hydroplaning goes up. Federal standards require a tread wear indicator that gives a visual indication when the tire has worn to 1/16 inch remaining depth. At that point, the void ratio has dropped significantly.

If you do the majority of your driving on wet highways, a low void ratio tire with deep, effective water channels is safer than a high void ratio off-road tire. The tire tread depth for rain article has more detail. For off-road driving, the same wear that reduces void ratio also reduces the bite you get in loose material.

What to pick for your use

Questions

Does a higher tread void ratio always mean better off-road traction?

No. A higher void ratio helps in mud and loose soil because the tire can clear debris and the block edges bite in. But on hard-packed dirt or rock, too much void reduces the contact patch and can hurt traction. Some off-road conditions, like rock crawling, favor a more moderate void ratio with large, stable blocks.

How does tread void ratio affect highway noise?

High void ratio treads are generally louder because the open spaces trap and release air as the tire rolls, and the large blocks pound the pavement. Low void ratio tires with continuous ribs are quieter. Noise also depends on the tread pattern and how the blocks are arranged, not just the void ratio.

Is a low void ratio tire safer in the rain?

Not necessarily. A low void ratio tire needs effective grooves to channel water away and prevent hydroplaning. A tire with a moderate void ratio but well-designed water evacuation can be safer in heavy rain than a low void ratio tire with shallow grooves. The key is the depth and design of the water channels, not just the total void.

Will a high void ratio tire wear out faster on the highway?

It can. The large tread blocks of a high void ratio tire flex more on pavement, which can generate heat and lead to irregular wear, especially on the rear edge of the blocks. Highway driving also tends to be at higher speeds, which aggravates this. If you use a mud-terrain tire mostly on roads, you may see shorter tread life.

Can I judge void ratio without a published spec?

Yes. Look at the tread face from above and estimate the fraction of the footprint that is groove versus rubber. A highway tire has mostly continuous rubber with thin grooves. A mud-terrain tire has large, widely spaced blocks with deep channels. The contrast is obvious in person or in detailed product photos.