235/40r18 vs 235/45r18 Tire: What’s the Difference?

Switching tires is common because vehicles equipped with winter tires may need to switch to a different set of tires to respond to the changing climate. However, the problem is to find out the best combination for your existing Tire. 

For example, in our case, tires 235/40r18 and  235/45r18 are nearly similar except in the aspect ratio, which is 5% higher in the 235/45r18 tire. If you think you can interchange tires because the percentage change is just 5%, you are on the wrong track.

No worries, In this post, we’ll compare 235/40r18 vs. 235/45r18 Tire, find out how small changes in percentage affect other parameters, and finally tell you whether sizes are interchangeable or not. Continue reading to find out more!

235/40r18 vs. 235/45r18 Tire: Comparison Table

The table shows how 5% changes in the aspect ratio of the tire sizes between  235/40r18 and  235/45r18 change the other parameters.

235/40r18 TireType235/45r18 Tire
25.4 Inches or 645.2 mmDiameter difference 0.93 (23.5) 3.6% inches (mm)26.33 Inches or 668.7 mm
9.25 Inches or 235 mmWidth difference 0 (0) 0% inches (mm)9.25 Inches or 235 mm
79.8 Inches or 2026.96 mmCircumference difference 2.91 (73.83) 3.6% inches (mm)82.71 Inches or 2100.78 mm
3.7 Inches or 94 mmSidewall Height difference 0.46 (11.75) 12.5% inches (mm)4.16 Inches or 105.75 mm
793.97 RPM or 493.35 KMRevolutions per mile difference -27.9 (-17.34) -3.5% (km)766.07  RPM or 476.01 KM

As you can see the relative change in the diameter is more than 3%, which does not support the interchangeability of tires 235/40r18 and  235/45r18. 

Also, the percentage change in the circumference remains the same as that of diameter, which also does not support the interchangeability of these tires.

On the other hand, the Sidewall Height jumps by 12.5%, which is too high. According to experts’ opinion, you should not switch tires if the percentage change in diameter and sidewall height is more than 3%.

With such high changes in height percentage, it is risky to interchange the Tire. Also, due to the increased percentage, the 235/45r18 tire is slightly bigger than the 235/40r18 tire. As a result, 235/45r18 will make fewer revolutions per mile than the other.

How to read the tire sizes 235/40r18 and 235/45r18?

There are four parts to the tire size 235/40r18, and each part bears a certain meaning. Before you decide to interchange or replace the Tire, you must learn about them. Here is the explanation of each part of the tire size 235/40r18.

  • 235 indicates the width of the tires in millimeters
  • 40 indicates the percentage of height in relation to the width or the Aspect Ratio
  • R indicates Radial Construction
  • 18 indicates the Diameter of the Rim

Doing the Same for Tire Sizes 235/45r18

  • 235 indicates the width of the tires in millimeters
  • 45 indicates the percentage of height in relation to the width or the Aspect Ratio
  • R indicates Radial Construction
  • 18 indicates the Diameter of the Rim

What’s The Difference Between 235/40r18 and 235/45r18 Tire?

By now you’ve learned the basics of using two tires interchangeably, now let’s see implementing the theory in a real example. Below, we have compared 235/40r18 and 235/45r18 tires in terms of width, aspect ratio, construction materials, and rim diameter.


If you measure from one sidewall to the other sidewall of a tire, you will get the width. It is expressed in millimeters. 

You can instantly learn the Width of a tire just by looking at the tire size index.

The first three digits of a tire size index indicate the Width of that tire. As you can see, the first three digits of 235/40r18 are 235. It means the width of that tire is 235 millimeters. 

Likewise, the three-digit number of 235/45r18 is 235. It means the Width of that tire is 235 millimeters. So the difference in the Width is 235 – 235 = 0 millimeters.

Now, if you switch to the tire size which is 0 millimeters more or less in Width, it shouldn’t be a problem. 

But you cannot switch tire size depending on Width only. You must consider the Sidewall Height as well. The Sidewall Height depends on Width and Aspect Ratio.

Aspect Ratio

As you can see, the only difference between the two Tire sizes is the aspect ratio. The aspect ratio of 235/40r18 is 40, whereas the aspect ratio of 235/45r18 is 45. On the other hand, the width and the Rim diameter of the two Tire sizes are identical. 

So to check the interchangeability of these tires, our sole concern is to analyze how the differences in the aspect ratio impact the other measurements. Let’s start with the simple question first: what is the aspect ratio? And its relation with other measurements.

Aspect Ratio is the ratio of the sidewall from wheel to tread and the width. This measurement is usually represented in a percentage.

In our example, 235/40r18 Tire size has a width of 235 with an aspect ratio of 40%. So the sidewall height will be 235 x 0.40 or 3.7 inches or 94 mm. You can get the number by simply putting the value on the equation.

Equation: Height of a Sidewall = Aspect Ratio of Tire x Width

The Aspect ratio of Tire 235/45r18 is 5% more than Tire 235/40r18. This increase changes the value of the sidewall height, diameter, circumference, and a total number of revolutions per mile.

Construction Type

The tire size index informs you about the Construction Type of a tire. It is expressed by a letter in the tire size index. 

Both 235/40r18 and 235/45r18 have the same letter ‘R’. So, both the tires have Radial Construction in terms of construction type.

Such tires have a sophisticated construction with square footprints on the top. The Radial tires are ideal for driving on-road. 

They are resistant to heat and water. They are next-gen tires and can provide great fuel efficiency.  

Another famous construction type is Bias Ply. They are also known as Cross-ply. It is expressed by the letter ‘B’ in a tire size index. This type is great for driving off-road.

Rim Diameter

To know which tire size fits your vehicle, you must know the wheel rim size of your vehicle as well as the Rim Diameter of a tire. 

For instance, if your vehicle has a 16-inch wheel rim, you can use the tire which has a 16-inch Rim Diameter.

By looking at a tire size index, you can learn instantly the Rim Diameter of a tire. The last two digits of a tire size index indicate the Rim Diameter. It is measured in inches. 

As you can see, the Rim Diameter of 235/40r18 is 18 inches. Similarly, the Rim Diameter of 235/45r18 is 18 inches.

It means that if your vehicle has an 18-inch wheel rim, you can use both tire sizes. But make sure to use the same size tires for all the wheels.

Can you interchange Between 235/40r18 and 235/45r18 Tire?

From the relative changes in the measurements, it is clear that replacing tire  235/40r18 with 235/45r18 might not be a good idea. Particularly the significant 12.5% changes in height.

With such a massive change in the height, you might have an inaccurate speedometer, changes in the top speed, and other issues. 

A Good Rule of Thumb: Always aim to keep the replacement tire and wheel combination within 3% of your existing tire.

Speedometer Difference: Using the current Tire Size 235/40r18 Instead Of the New Tire Size 235/45r18

Look at the Table. It shows differences in speedometer reading and the actual speed at any given time when the current Tire Size is 235/40r18 Instead Of 235/45r18. 

Speedo ReadingActual Speed
20 mph (32.19 km/h)20.73 mph (33.36 km/h)
25 mph (40.23 km/h)25.91 mph (41.7 km/h)
30 mph (48.28 km/h)31.09 mph (50.04 km/h)
35 mph (56.33 km/h)36.27 mph (58.38 km/h)
40 mph (64.37 km/h)41.46 mph (66.72 km/h)
45 mph (72.42 km/h)46.64 mph (75.06 km/h)

Speedometer Difference: Using current Tire Size 235/45r18 Instead Of New Tire Size 235/40r18

The table shows the differences in speedometer reading and the actual speed at any given time when the current Tire Size is 235/45r18 Instead Of 235/40r18.

Speedo ReadingActual Speed
20 mph (32.19 km/h)19.3 mph (31.06 km/h)
25 mph (40.23 km/h)24.12 mph (38.82 km/h)
30 mph (48.28 km/h)28.95 mph (46.58 km/h)
35 mph (56.33 km/h)33.77 mph (54.35 km/h)
40 mph (64.37 km/h)38.59 mph (62.11 km/h)
45 mph (72.42 km/h)43.42 mph (69.88 km/h)

What you must consider Before Interchange or Replace Tire? 

There are important things to consider Before replacing or swiping the Tires. You cannot simply replace the 235/40r18 tire with the 235/45r18 tire that is in a different shape.

For Example, if the aspect ratio of the existing tire is 40%, replacing it with a tire that has an aspect ratio of  45% might not be a good option.

Doing so will have a variety of consequences on the car’s overall performance, such as variations in top speed and speedometer.

However, to a certain extent, such as minor changes in the diameter or the sidewall, you can switch Tires. However, this can only be done in compliance with the manufacturers’ recommendations.

Anyway, When replacing the Tire, the simple guideline to follow is that the current tire’s size, load index, and speed rating should all match with the replacement Tire.

Load Index and Speed Index of a Tire 235/40r18 and 235/45r18

The load index refers to the maximum load-bearing capacity, whereas the speed index refers to the Tire’s maximum allowable speed at a full load. These are very crucial stats to know before you decide to interchange tires.

Index information can be found on the sidewall right after the Tire size number. It is a combination of numbers and letters that bear a particular meaning. 

Check The load and speed index. They must be as close as possible to fit precisely. These values change depending on the vehicle’s weight and wheel size. Here is a table that shows various load and speed indexes, so you get the idea.

Load IndexKgSpeed IndexSpeed Limit
65290QUp to 160 Km/h
66300RUp to 170 Km/h
67307SUp to 180 Km/h
68315TUp to 190 Km/h
69325HUp to 210 Km/h

Size of the Tire 

The size of the Tire is another vital piece of information to know to exchange the tires properly. The large number on the sidewall shows the tire size.

For Example, the marking 235/40r18 on the sidewall represents the size of the Tire. You can also find it in the user manual, on the sidewall, and in other places such as inside the fuel flap or glove compartment.

What Size Tires Is 235/40r18 in Inches?

235/40r18 tires have a 25.4″ diameter, 9.3″ section width, and 18″ wheel diameter.

The circumference is 79.8,” and they make 794 revolutions per mile. The Sidewall Height of this Tire is 3.7 inches.

What Size Tires Is 235/45r18 in Inches?

235/45r18 tires have a diameter of 26.33″, a section width of 9.3″, and a wheel diameter of 18″. 

The circumference is 82.7,” and they have 766.0 revolutions per mile. The Sidewall Height of this Tire is 4.2 inches.


All in all, swiping or replacing tires is a common thing. With some basic professional skills and knowing how to read tire size, you can replace your current tire with suitable tire size.

We believe the above comparison of the 235/40r18 vs 235/45r18 tire will help you get the idea of how a slight difference in measurements influences the interchangeability of tires.

Anyway, when replacing tires, put a great emphasis on the size of the tire, the load index, and the speed index.

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