235 40r18 vs 235 45r18 Tire: Differences & Similarities

235 40r18 vs 235 45r18 tires are two of the most sought-after tires when it comes to installing new ones in cars. They are reliable, flexible, of great quality, and offer durability.

However, because they are quite similar, you might get confused about which one to get and what to leave behind for a safe and efficient drive.

But then again, we have the right solution for you. Below, we have discussed 235 40r18 vs 235 45r18 tire sizes so that you know their difference and similarities and pick one accordingly.

What Do 235 40R18 and 235 45R18 Mean on Tires?

Even if you are not a newbie, the numbers on the 235 40R18 and 235 45R18 tires might confuse you. To make sure you are not confused again, let’s explain what these numbers mean on the tires.

The “235” on both tires means that they both are 235mm wide. However, the “40” on the 40R18 tires explains that they have an aspect ratio of 40% while the “R18” means that the rim size is 18 inches.

The same goes for the 235 45R18 tires as well. While the “45” indicates the aspect ratio, the “R18” is the rim size.

What Are The Differences Between 235/40r18 vs 235/45r18 Tire?

235 40r18 vs 235 45r18

While talking about 235 40r18 vs 235 45r18 tires, the first thing that comes to your mind is their differences. Below, we have discussed the differences between these tires to make sure that you know them better and make a purchase decision accordingly.

Features  235/40r18235/45r18
Diameter25.4 inches26.33 inches
Sidewall Height3.7 inches4.16 inches
Radius12.70 inches13.16 inches
Tire Circumference79.8 inches82.71 inches
Revolutions per mile793 RMP766 RMP
Max PSI42 psi (2.9 bar)36 psi (2.5 bar)
Load Index Rating95.094.0
Load Capacity1521 Pounds 1477 Pounds
Max Speed168 mph186 mph

A Brief Explanation of 235 40r18 vs 235 45r18 Tires Differences

Even though it is very evident from the table that the 235 40r18 and 235 45r18 tires differ in more ways than we thought, let’s discuss them in detail for an even clearer idea.


From the table, it is very much apparent that the overall diameter of these tires (235 40r18 and 235 45r18) is not the same. While 40r18 has a diameter of 25.4 inches, the 45r18 tires are 26.33 inches.

And even though the difference might not seem too big, when it comes to car tires, it is something you should consider.

Such a small difference can make a relatively big impact on how you control your car on the terrain.

The tire with the larger overall diameter will definitely make your vehicle feel more stable and in control on the road compared to the ones with a smaller size.

Sidewall Height

Before getting a tire for your car, remember that the taller the sidewall height, the smoother your ride will be. And the shorter the sidewall height, the better handling you will enjoy.

As the 235 40r18 has a sidewall height of 3.7 inches it will give you more handling power compared to the 4.16 inches sidewall height of the 235 45r18 tires.

On the other hand, because of more height, the 235 45r18 tires ensure a better and smoother ride.

Therefore, when picking them up, get the ones that suit your needs completely.

Tire Circumference

The circumference is the outer perimeter of the tire and is measured in inches. Because both the 235 40r18 and 235 45r18 tire sizes differ, their circumference differs too. And this difference affects your driving as well.

Just like the diameter, tires with larger circumferences also provide stability and ensure a safer ride on the roads.

Revolutions per mile (RPM)

In simple words, the tire revolution or revolution per mile (RMP) is the number of times a tire revolves while covering a distance of 1 mile. As the tire size of both 235 40r18 and 235 45r18 differs greatly, their RMP also differs.

Because the 235 40r18 tires are smaller, they have an RPM of 793 compared to the RPM of 766 of relatively bigger 235 45r18 tires.

Even though the difference is not that considerable, it makes a great impact on how the car responds. Tires with higher RPM will make the vehicle more responsive and give you more control over it, especially in dry conditions.


Maximum PSI is the highest amount of air pressure you can insert in your tire to make it run with its maximum capacity.

While the max PSI of 235 40r18 tires is 42 psi, it’s much less for 235 45r18 tires. This means that you would require less air pressure to operate these tires at full capacity.

Load Index Rating

The load index rating shows the maximum weight that a tire can support when properly inflated. While 235 40r18 tires have a load index rating of 95.0, it is 94.0 for the 235 45r18 tires.

And as the higher the rating, the greater load it can carry, the 235 40r18 can carry more weight than the 235 45r18 tires.

Load Capacity

The tire load capacity of the tire is the amount of weight (or load) it can bear safely. As 235 40r18 have more load index, they can bear more load (1521 Pounds) than the 235 45r18 tires (1477 Pounds) at maximum air pressure.

Max Speed

In general, the maximum speed of the tires is the highest speed a tire is legally approved for. If you get the 235 40R18 tires for your car, you can drive it at 168 mph while this range is much higher (186 mph) for the 235 45R18 tires.

Similarities Between 235 40r18 and 235 45r18 Tire Sizes

Just like differences, there are more than a few similarities between the 235 40r18 and 235 45r18 tires. And just as we discussed the differences, let’s discuss the similarities between these tire sizes for your easy understanding.

Features  235 40r18235 45r18
Tire Width9.25 inches9.25 inches
Construction TypeRadial-Ply TireRadial-Ply Tire
Rim Size18 inches18 inches

A short discussion of 235 40r18 and 235 45r18 Tire Similarities

We have talked about the similar aspects of the 235 40r18 and 235 45r18 tires to ensure you know about them better.

Tire Width

The tire width is the width measurement from the inner sidewall to the outer sidewall at the widest point. And in the case of the 235 40r18 and 235 45r18 tires, the width is the same.

This means that both these tires will fit your car the same and won’t arise any problems if you decide to interchange them.

Construction Type

Because they both are made for cars, both the 235 40r18 and 235 45r18 tires are of the same type and made the same way.

They are commonly known as radial tires and are made with steel belts around the circumference. The steel belts not only make them look expensive but also ensure flexibility, strength, and durability

You would never be disappointed with their quality.

Rim Size

Both the 235 40r18 and 235 45r18 tires have the same size rims (18 inches). This means that they will fit on your car indifferently. There wouldn’t be any difference in how these tires will fit on your vehicle.

235 40r18 vs 235 45r18 – Which Tire Size Is Right For You?

Choosing the right tire size for your vehicle is crucial to ensure optimal performance and safety. Here are some factors to consider when deciding between 235 40r18 and 235 45r18 tire sizes:

Vehicle Compatibility

Check your vehicle’s owner’s manual to see what tire sizes are recommended or allowed for your specific make and model.

It is important to choose a tire size that is compatible with your vehicle to ensure proper fit and performance.


The tire size you choose can impact your vehicle’s performance in areas such as acceleration, braking, and handling.

Consider which tire size provides the best balance of performance for your driving style and needs.

Speed Rating

The speed rating of a tire indicates the maximum speed at which the tire can safely operate. Make sure that the tire size you choose has a speed rating that is suitable for your vehicle and driving needs.


The tire size you choose can impact your vehicle’s traction in different weather conditions. Consider which tire size provides the best traction for your local climate and terrain.

Ride Comfort

The aspect ratio of a tire affects the height of the sidewall and can impact the ride comfort of your vehicle.

A higher aspect ratio tire (such as the 235 45r18) may provide a smoother ride, while a lower aspect ratio tire (such as the 235 40r18) may provide better handling and responsiveness.

Fuel Efficiency

The tire size you choose can impact your vehicle’s fuel efficiency. Consider which tire size provides the best balance of performance and fuel efficiency for your driving needs.

In general, the 235 45r18 tire size may provide a smoother ride and better traction in wet conditions, while the 235 40r18 tire size may provide better handling and responsiveness.

Ultimately, the right tire size for you will depend on your specific needs and driving style.

Also Read: 235 40r18 vs 225 40r18 Tire – The Main Comparison

Frequently Asked Questions

What is the correct pressure for 235 40r18 tires?

The air pressure of a tire depends on its size. For 235 40r18 tires, the air pressure should be 33 psi (2.3 bar) for Normal Load and 36 psi (2.5 bar) for Full Load. Remember this before inflating your 235 40r18 tires

Is it OK to fill my 235 45r18 tries to max PSI?

Contrary to popular belief, you should never fill your 235 45r18 or any other tires to maximum PSI. That is because, besides all other things, inflating your tires to the highest PSI can lead to structural failure and might most likely result in a prompt blowout.

Can I put different width tires on the same rim?

Yes, you can put different width tires on the same rim as long as the middle of the tires you are using is of the correct size. If they are not of the right size, there are chances that they won’t fit rightly.

Wrapping Up

There is no doubt that the 235 40r18 and 235 45r18 tires have their fair share of similarities and differences. And while discussing 235 40r18 vs 235 45r18, they become even more evident.

Even though we have discussed all the aspects in detail so that you know them in detail, feel free to ask anything about these tire sizes. We would definitely get back to you.

I am an engineer and the chief editor of tireslover.com, I am a passionate blogger as well. I am the person who can ensure a perfect, informative, honest, and helpful, guide with 10 years of work experience with different varieties of tires.

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