What is 305 Tire Size?

When cars first populated public roads, they had comically narrow tires, similar to the ones we have on bicycles today. That is because tires were adapted to work on existing wheel profiles from horse carriages, but also because tire technology wasn't as advanced. In other words, it wasn't possible to make a structurally sound wide tire.

Today, though, that is not the case. In fact, you can put some exorbitantly large tires on your vehicle and make it look like a roller from behind. A popular width for modern tires is 305, which is close to "roller" territory and available for owners of large SUVs and trucks, but also modern supercars and muscle cars.

But what is a 305 tire size? What does it bring to the table, and what are the implications of putting such tires on your vehicle? These are some of the questions I will be answering in this in-depth article, where we will dive deep and unlock the mystery of the 305 tire size.

So, without further ado, let's decipher the 305 tire size!

Understanding Tire Size Notations

Tire size notations might seem complex at first glance, but they are quite straightforward once you know how to decipher them. Let's take the size 305/55R20 as an example.

This tire size is most associated with all-terrain and off-road tires due to its huge footprint and high sidewalls. It usually comes in LT (Light Truck) form, meaning it has internal reinforcements that allow it to carry heavier loads. Let's break it down!

• Tire Width: The First Number (305)

The first number in the tire size notation – 305 in our example – is the width of the tire measured in millimeters. A wider tire offers better traction and improved stability, especially at high speeds, but may affect fuel efficiency due to higher rolling resistance.

• Aspect Ratio: The Second Number (55)

The second number in the tire size notation represents the tire's aspect ratio, also known as the profile. This is the ratio of the tire's height to its width expressed as a percentage.

In our example, the aspect ratio is 55, which means the height of the tire is 55% of its width. If you do the math, you will see that the sidewall is 167.75 mm high, which is quite significant. So, with a rim diameter of 20 inches, the diameter of the tire will be 33.2 inches.

However, please note that narrower tires with 55 sidewalls, like, for example, 205 mm tires, will actually have a smaller sidewall because the number is the ratio of the width.

A lower aspect ratio (often seen in performance tires) leads to a shorter sidewall and provides better lateral stability, improving handling at high speeds. However, a lower aspect ratio might compromise ride comfort as it reduces the tire's ability to absorb road imperfections.

• Construction: The Letter (R)

You will see the letter R on most commercially available tires today because most of them have a radial construction. This indicates that the tire's ply cords – which give the tire its structure and strength – are arranged radially or perpendicular to the direction of travel. Radial tires are the most common type due to their durability, comfort, and efficiency.

The other tire construction is bias-ply, where the ply cords run at an angle. Bias-ply tires are usually stiffer but also much heavier. As a result, they offer shorter treadlife, worse stability, and less comfort.

• Wheel Diameter: The Last Number (20)

The final number in the tire size notation represents the diameter of the wheel that the tire is designed to fit, expressed in inches. In our example, the tire is designed to fit on a 20-inch wheel. The wheel diameter is crucial to ensure that the tire fits the wheel correctly.

As a context, a 20-inch wheel is a pretty substantial one and is mostly used on large trucks and SUVs.

Decoding Tire Sizes: The 305 Size Explained

Tire sizes can be a riddle because, as you have learned, some of the numbers are expressed in mm, others in inches, and then you also have ratios to worry about. The tire industry should have thought about better nomenclature, but I guess the current one just stuck with the audience, and they kept it.

But honestly, it would be much better if everything was expressed in inches (for countries that use the imperial system) and in mm (for countries that use the metric system).

With that said, in this case, we will mostly be talking about millimeters, which aren't as familiar to US citizens as inches. Don't worry, though, because I will convert them to inches to help you better understand everything.

1. The Meaning of the First Number: Tire Width

As you learned by now, 305 in a tire nomenclature means that the tread is 305 mm wide, from sidewall to sidewall. This dimension is substantial as it directly affects the tire's contact patch, which is the portion of the tire that is in contact with the road surface.

For context, 305 mm is equal to 12.0079 inches. You can quickly convert mm to inch by dividing the length value by 25.4.

2. Importance of Tire Width in Vehicle Performance

The tread width is perhaps the most important dimension when it comes to road performance because it directly affects grip. Namely, a wider tire would have a larger contact patch with the road, which increases friction and, therefore, grip. This is especially true in dry conditions, where the tire doesn't need to cut through water to keep its contact with the road.

However, a wider tire is also heavier and has a higher rolling resistance, which negatively impacts fuel economy. Not to mention, the heavy weight will also affect performance and agility, especially on tires with large sidewalls (like the aforementioned 305/55R20).

Another issue is that 305 mm wide tires won't fit most vehicles. In fact, you will need a fairly large SUV or truck with widened wheel arches to fit a 305 mm tire or some rare supercar. But let's have a closer look at where the 305 tire size is most commonly used.

Where is 305 Tire Size Commonly Used?

The 305 tire size is most common in heavy-duty vehicles, full-size trucks with special off-road lifted suspensions, and off-road-ready SUVs. However, 300+ tire sizes are also used in ultra-high-performance sports cars due to the overall grip they offer.

1. Popular Vehicle Types With 305 Tire Size

Sport utility vehicles (SUVs) and pickup trucks often utilize 305 tire sizes for added stability and better load-carrying capacity. You will also find 305 tire sizes frequently used in sports cars for enhanced traction and superior handling at high speeds. Chevrolet Corvettes, Dodge Challengers, and Ford Mustangs are a few examples of such vehicles in the USA, though supercars from other places also boast 305 mm wide treads.

2. SUVs/Trucks and 305 Tires: A Traction Perspective

Trucks and SUVs that come with 305 mm wide tires are usually fitted with special off-road suspensions and need added traction on slippery surfaces. For these vehicles, a 305/55R20 tire, for example, will provide better traction on gravel, dirt, sand, and large rocks.

In fact, tires that ate 305 mm wide can operate at very low psi when paired with beadlock wheels. Hence, they are often used for rock crawling (driving over huge rocks and rock formations), where thanks to the low pressure, the tire can conform to the irregular surface and provide grip.

Moreover, tires that are that wide are also used for driving over sand dunes because they offer higher flotation, i.e., they float over the surface rather than sinking into it. Again, you will need to operate the tires at very low psi for them to float, but the wider the tire, the better it will perform.

Still, 305 mm tires might not be the best for driving in mud. They provide good overall traction, but in mud, you will want the tire to sink into the surface to provide traction and not float over it.

With that said, there is no question that 305 tires will provide the large SUV/truck with higher road grip, particularly on off-road or all-terrain tires, which have high void area and, subsequently, a smaller contact patch. Also, tires that are that large also come with higher load ratings, which helps when towing and carrying heavy cargo.

3. Sports Cars and 305 Tires: A Performance Perspective

From a performance perspective, sports cars significantly benefit from the wider footprint provided by a 305 mm tire size. The increased surface area in contact with the road leads to improved traction, especially during acceleration and cornering.

In fact, on most sports cars with 600+ hp and a rear-wheel-drive configuration, wide rear tires are a must for a safe ride. Just lightly touching the gas pedal on these vehicles would transfer immense torque to the rear wheels. If they weren't wide enough, i.e., they don't have a high enough grip, the driver would have a hard time putting the power to the ground. Not to mention, the car would drift and slide uncontrollably in every corner and won't have enough lateral grip.

However, it is important to note that a 305 mm wide tire will only work on vehicles that were designed for it. Putting such wide tires on a car with 300 hp will have no effect on performance other than providing a higher lateral grip.

In fact, because the tires will be heavier, you can expect your sports car to feel more cumbersome to drive, and it will also be slower. In addition, you will never break traction on these tires, so using the throttle to aid turning will not be possible.

Furthermore, a wider tire like a 305 can also lead to increased rolling resistance, which might slightly compromise fuel efficiency. Overall, the performance benefits tend to outweigh this minor drawback in a supercar or muscle car context, but not in a lightweight sports car. Just have a look at most sports cars with up to 500 hp, and you will see that manufacturers put much narrower tires from the factory.

Pros and Cons of 305 Tire Size

305 mm tires offer distinct advantages and drawbacks since they are so out of the ordinary. Let's have a closer look.

1. Advantages of Using 305 Tire Size

• Enhanced longitudinal traction: a 305 mm wide tire would offer better acceleration (less slippage) and braking than a narrower tire, thanks to the larger contact patch with the road. However, it is important to note that, depending on the car, the acceleration might be worse due to the added weight (read the cons).
• Higher lateral grip: along with shorter stopping distances, 305 tires also offer much higher lateral grip than narrower tires. As a result, you can reach higher speeds in the corners, which might come in handy when driving on the track.
• Increased load capacity: the wide construction of a 305 tire size can contribute to better load distribution. This quality makes these tires an excellent choice for heavier vehicles like SUVs or pickup trucks, where the tires must support substantial weight.

2. Drawbacks of 305 Tire Size

• Decreased fuel efficiency: due to the larger contact patch with the road, 305 tires have higher rolling resistance. The added friction makes the engine work harder and translates into higher fuel consumption.
• Increased tire weight: apart from the added rolling resistance, the higher weight of 305 tires can also have a negative effect on fuel economy. However, the added weight can also affect straight-line performance, like acceleration and top speed. Not to mention, the handling might be less agile due to the higher unsprung weight, particularly on cars that weren't designed with wide tires in mind.
• A potential risk of hydroplaning: one less known drawback of wider tires, such as 305, is the lower hydroplaning resistance. Due to the large contact area, 305 tires won't cut through puddles of water as successfully, or in other words, they will float over it. The result of this is hydroplaning – a specific problem where the tires lose contact with the road because there is water that stands in the way. A car that hydroplanes is almost impossible to control, especially at higher speeds.
• Worse traction on snow: wider tires provide a higher grip on dry and damp roads, but they aren't particularly good for driving on snow. This is because, due to the larger footprint, the pressure they exert on the snow is dispersed. In other words, they won't dig through the snow as successfully, which limits traction.
• Less comfortable ride: larger, wider tires can sometimes lead to a less comfortable ride. The added stiffness required to support the tire's width can transfer more road imperfections to the vehicle, potentially leading to a harsher ride.

Selecting the Right Tire Size for Your Vehicle

Choosing the right tire size for your vehicle isn't a decision you should make lightly. Sure, if you listen to enthusiasts, who dominate online forums, going wider will always be beneficial.

However, while a wider tire does bring some benefits, it can also disturb your vehicle's optimal balance between performance, handling, comfort, and fuel economy. Not to mention, wider tires are also heavy on your wallet.

1. Factors to Consider When Choosing Tire Size

Before you even think of switching to wider tires, you should see whether they will fit your particular car. Consult your vehicle's manual or the placard on the driver's side door jamb for the manufacturer's recommended tire size, and see whether a 305 mm size is included.

If it's not included, your best option would be to visit online forums and read about the experiences of other owners (of your car model) that installed wider tires. Look for things like fender rubbing, suspension geometry, and just the general subjective evaluations of each driver.

Furthermore, you should think about the driving conditions in your area. Namely, if it often rains and the streets are covered with standing water, putting 305 tires might not be a good solution due to hydroplaning issues. It's the same for driving on snow – a narrower tire would cut through the snow more easily, providing you with the necessary traction.

Lastly, be prepared for higher running costs – wider tires tend to decrease fuel efficiency due to increased rolling resistance. Hence, if fuel economy is a priority, you might want to stick to the narrower tire size recommended by your vehicle's manufacturer.

2. What to Consider When Switching to a 305 Tire Size

If you're contemplating switching to a 305 tire size, remember that it is designed for special vehicles. It would fit off-road trucks with lifted suspensions, very powerful supercars and muscle cars, and commercial vehicles.

Thus, it is crucial to make sure that it fits your car before splurging the cash. The wheel wells on your vehicle should have enough space to accommodate these massive tires, or the tires will rub against the fender.

If you really insist on accommodating 305 tires on a car with small wheel wells, you will need to do some fender rubbing or cut large portions of the fenders. This is a job that some car body shops do successfully, producing visually impressive widebody cars. However, do note that such modifications are very costly.

Furthermore, changing tire size can affect your car's speedometer accuracy. A wider tire might not necessarily mean a larger tire (in diameter), but because the sidewall is actually the aspect ratio of the tread width, going wider usually also means going larger.

When you increase the tire size, your speedometer may read slower than your actual speed. This discrepancy happens because your wheels are covering more ground in a single revolution. You might need to recalibrate your speedometer after changing the tire size. Otherwise, you risk your safety because you will be driving faster than you think, and also, let's not forget that you can get a ticket more easily.

Caring for Your 305 Size Tires

Wider tires require similar maintenance to narrower tires, though perhaps a more regular one. Moreover, due to their huge section width, 305 tires suffer from issues like irregular wear more easily.

1. Maintenance Tips for Larger Tire Sizes

Ensuring that 305 tires are properly inflated is even more crucial than on narrower tires. Thus, you should check the tire pressure at least once every month or even more often.

Over or under-inflation can lead to uneven wear, reduced fuel efficiency, and potentially dangerous blowouts. Also, since irregular wear is a bigger issue on 305 mm tires, I highly recommend sticking to a strict rotation schedule, like every 5,000 miles.

Lastly, visual inspections can reveal quite a lot about numerous issues with your tires, like signs of wear, damage, or any objects lodged in the tread. The fact that 305 tires are wider makes them a larger target for road debris, so make sure nothing is stuck inside the tread grooves, or it might damage the tire.

2. Wheel Alignment Considerations for 305 Size

The 305-size tire's wider contact patch can put more stress on your vehicle's suspension components, possibly leading to more rapid misalignment.

That is especially true on cars that weren't designed with wide tires in mind, where the more fragile suspension components will change their geometry. This comes from the fact that 305 tires are heavier, which adds unsprung weight.

However, the higher lateral grip of 305 tires also affects the suspension components. Namely, such tires would bottom out passenger car suspensions in the corners because the springs and dampers will be subjected to higher lateral loads than they were designed for.

Therefore, regular alignment checks are vital. Incorrect alignment can lead to uneven and accelerated tire wear, negatively impacting your vehicle's handling and safety.

Conclusion

Now that you learned everything there is to know about tires that are 305 mm in wide (and I really hope I answered each one of your questions), what is your opinion? Will you put such large tires on your car or truck, or will you stick with the current size?

My advice would be: unless you are ready to live with the compromises of huge tires to get that extra performance, stick with the factory options. In other words, if you are building a lifted rig for off-road adventures, then you should think about putting in 305 tires.

Also, for those that want a widebody sports car or show car that would impress people around, 305 tires are also a good option. Still, you can go narrower and have a similar effect – I've seen some impressive widebody cars with 275 mm or 285 mm tires!