Tire Size 235/40r18 vs 235/45r18

The main difference between 235/40r18 and 235/45r18 tire sizes is the aspect ratio, which, in turn, affects the tire’s height, diameter, and other physical attributes. These differences, though they might appear small, can significantly impact the performance and look of the tires on your vehicle.

235/40r18 vs 235/45r18 Table

This comparison table will help you see the main differences between the two sizes at a glance.

Feature 235/45r18 235/40r18 Difference
Diameter inches 26.33 (668.7) 25.4 (645.2) -0.93 (-23.5)
Width inches 9.25 (235) 9.25 (235) 0 (0) 0%
Circum. inches 82.71 (2100.78) 79.8 (2026.96) -2.91 (-73.83)
Sidewall Height 4.16 (105.75) 3.7 (94) -0.46 (-11.75)
Revolutions 766.07 (476.01) 793.97 (493.35) 27.9 (17.34)

Overall Diameter
The diameter of the 235/45r18 tire is larger by 0.93 inches (23.5 mm) than the 235/40r18. The increase in diameter can provide more ground clearance, which is beneficial for off-road driving or navigating rough terrain. 

However, this also results in a lower speedometer reading as the tire covers more ground per rotation. It’s important to note that tires with a diameter exceeding a 3% difference from the factory-recommended tires are generally not recommended.

Section Width

Both 235/40r18 and 235/45r18 tires have identical section widths of 9.25 inches (235 mm). A wider tread width generally improves traction due to a larger contact patch, increasing the tire’s grip, particularly during acceleration, braking, and cornering.

Sidewall Height
The 235/45r18 has a taller sidewall height of 0.46 inches (11.75 mm). The taller sidewall acts as a buffer, absorbing more bumps and road irregularities for a smoother ride.

Additionally, taller sidewalls can enhance traction on snowy or icy roads due to the increased surface area. In contrast, the 235/40r18’s shorter sidewall could lead to a harsher ride due to less cushioning. However, the smaller sidewalls could offer a sportier, more aggressive look.

Wheel Size
The wheel size for both the 235/40r18 and 235/45r18 is 18 inches. Therefore, both tires will fit the same wheel. Larger rims are often associated with a sportier look.

While smaller rims tend to provide a more classic or conservative appearance. In this case, both tires will offer the same aesthetic due to the identical wheel size.

The circumference of the 235/45r18 is larger by 2.91 inches (73.83 mm). This larger circumference results in a lower speedometer reading as it covers more ground in each rotation.

On the other hand, the 235/40r18’s smaller circumference will result in a higher speedometer reading due to lesser ground coverage per rotation.

Revolutions Per Mile
The 235/40r18 makes 27.9 more revolutions per mile than the 235/45r18. A higher RPM often leads to more wear, as the tire experiences increased friction with the road surface.

This could lead to a shorter lifespan for the 235/40r18. Conversely, with fewer revolutions per mile, the 235/45r18 covers more distance in each rotation, potentially extending its lifespan due to reduced wear and tear.

Difference Between 235/40r18 And 235/45r18?

The main differences between 235/40r18 and 235/45r18 tire sizes lie in their aspect ratios, which directly influence the tire’s sidewall height, overall diameter, and revolutions per mile.

The aspect ratio of a tire indicates the height of the tire’s sidewall as a percentage of its width. In 235/40r18, the aspect ratio is 40, while in 235/45r18, it is 45.

Can i use 235/40r18 instead of 235/45r18 ?

The diameter of a 235/45r18 tire is 26.33 inches. The diameter of a 235/40r18 tire is 25.4 inches. The difference in diameters is 0.93 inches, or approximately 3.5%.

While the 235/40r18 can physically fit on the same rim as the 235/45r18, it is not recommended as a replacement due to the diameter difference exceeding the 3% guideline.

Our Observation
Choosing between the 235/40r18 and 235/45r18 tire sizes largely depends on your driving conditions, style, and preference. If you desire a smoother ride with more ground clearance and longer tire life, the 235/45r18 may be the better choice.

Conversely, if you prefer a sportier look and feel with higher speedometer readings, the 235/40r18 is the tire for you. Regardless, ensure the chosen tire’s diameter is within 3% of the factory recommended tires to avoid potential issues. Always remember safety and comfort should be primary considerations when choosing a tire size for your vehicle.

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