255/55R20 vs 255/50R20 Tire: Are They Interchangeable?

Swapping the stock tire of your car may look easy, but in reality, it is very tricky and confusing. Many variations are available in all sorts of tires for all types of vehicles.

Everything matters when interchanging tires from the tire type, geometry, and brand. Though different tire readings may look very similar to you, it impacts the overall performance of the vehicle in the long run.

The 255/55R20 and 255/50R20 are mostly used tires for high-performance cars. But it may leave you wondering about the comparison between 255/55R20 vs 255/50R20 tires. Read this article completely to find out if these tires are interchangeable.

255/55R20 vs 255/50R20 Tire: Comparison Table

In the comparison chart, we can see that a decrease in aspect ratio by 5% changes a lot of factors which also changes the performance.

255/55R20 TireType255/50R20 Tire
31.04 inches (788.5)mmDiameter difference: -1 (-25.5) -3.2% inches (mm)30.04 inches (763)mm
10.04 inches (255)mmWidth difference 0 (0) 0% inches (mm)10.04 inches (255)mm
97.53 inches (2477.15)mmCircumference difference -3.15 (-80.11) -3.2% inches (mm)94.37 inches (2397.04)mm
5.52 inches (140.25)mmSidewall Height difference -0.5 (-12.75) -9.1% inches (mm)5.02 inches (127.5)mm
649.68RPM or (403.69)KmRevolutions per mile difference 21.71 (13.49) 3.3% (km)671.39 RPM or (417.18)Km

In the chart, we can see that diameter of the tire changes by 3.2%. The difference in the tire’s diameter also leads to changing the circumference to 3.2% lower.

The accepted range for interchanging tires while downsizing is 3%. Moreover, the lesser diameter of the tire shifts the reading in the speedometer and the odometer of the vehicle.

Downsizing the tire will result in displaying less speed in the speedometer than the vehicle’s actual speed. But due to no change in the width of the tire, it will experience more due to reduced surface area.

Again, the tire’s sidewall gets decreased by 9.1%, which is way too much. Cornering sharp banks at high speed can be dangerous.

How to read the tire sizes 255/55r20 and 255/50r20?

There are four parts to the tire size 255/55r20, 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 255/55r20.

  • 255 indicates the width of the tires in millimeters
  • 55 shows the percentage of height to the width or the Aspect Ratio
  • R indicates Radial Construction
  • 20 indicates the Diameter of the Rim

Doing the Same for Tire Sizes 255/50r20

  • 255 means the width of the tires in millimeters
  • 50 shows the percentage of height to the width or the Aspect Ratio
  • R indicates Radial Construction
  • 20 indicates the Diameter of the Rim

What’s The Difference Between 255/55r20 and 255/50r20 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 255/55r20 and 255/50r20 tires in terms of width, aspect ratio, construction materials, and rim diameter.

Width

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 255/55r20 are 255. It means the width of that tire is 255 millimeters. 

Likewise, the three-digit number of 255/50r20 is 255. It means the Width of that tire is 255 millimeters. So the difference in the Width is 255 – 255 = 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 255/55r20 is 55, whereas the Aspect Ratio of 255/50r20 is 50. 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, 255/55r20 Tire size has a width of 255 with an aspect ratio of 55%. So the sidewall height will be 255 x 0.55 or 5.5 inches or 140.5 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.

On the other hand, the Sidewall Height of 255/50r20 is 5.0 inches.

Though the Width of the two tire sizes is identical, there’s a gap between the sidewall heights because of the Aspect Ratio.

This change affects the value of diameter, circumference, and the 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 255/55r20 and 255/50r20 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 255/55r20 is 20 inches. Similarly, the Rim Diameter of 255/50r20 is 20 inches.

It means that if your vehicle has a 20-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 255/55R20 vs 255/50R20 Tire

No. The parameters make the tires not interchangeable. We can see that the difference in the radius and circumference gets reduced below the accepted range with no change in the width. This may cause issues while driving the vehicle on highly curved roads.

Interchanging these tires will cause downsizing, leading to lower actual speed and more RPM to cover a unit distance. Thus your time and fuel both will be wasted on long journeys.

Speedometer Difference: Using the current Tire Size 255/55r20 Instead Of the New Tire Size 255/50r20

Look at the Table. It shows differences in speedometer reading and the actual speed at any given time when the current Tire Size is 255/55r20 instead of 255/50r20. 

Speedo ReadingActual Speed
20 mph (32.19 km/h)19.35 mph (31.15 km/h)
25 mph (40.23 km/h)24.19 mph (38.93 km/h)
30 mph (48.28 km/h)29.03 mph (46.72 km/h)
35 mph (56.33 km/h)33.87 mph (54.51 km/h)
40 mph (64.37 km/h)38.71 mph (62.29 km/h)
45 mph (72.42 km/h)43.54 mph (70.08 km/h)

Speedometer Difference: Using current Tire Size 255/50r20 Instead Of New Tire Size 255/55r20

The table shows the differences in speedometer reading and the actual speed at any given time when the current Tire Size is 255/50r20 instead of 255/55r20.

Speedo ReadingActual Speed
20 mph (32.19 km/h)20.67 mph (33.26 km/h)
25 mph (40.23 km/h)25.84 mph (41.58 km/h)
30 mph (48.28 km/h)31 mph (49.89 km/h)
35 mph (56.33 km/h)36.17 mph (58.21 km/h)
40 mph (64.37 km/h)41.34 mph (66.53 km/h)
45 mph (72.42 km/h)46.5 mph (74.84 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 255/55r20 tire with the 255/50r20 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 various 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 255/55r20 and 255/50r20

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. 

Here is an example of how the index information is usually written on the sidewall: 255/55r20 90A. The 90A stands for load and speed index, with 90 indicating a maximum load-bearing capacity of 600 kg, whereas ‘A’ denotes an allowed speed of up to 170 km/h.

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 exchange the tires properly. The large number on the sidewall shows the tire size.

For example, the marking 255/55r20 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 255/55r20 in Inches?

255/55r20 tires have a 31.0″ diameter, 10.0″ section width, and 20″ wheel diameter.

The circumference is 97.5″ and they make 650 revolutions per mile. The Sidewall Height of this Tire is 5.5 inches.

What Size Tires Is 255/50r20 in Inches?

255/50r20 tires have a diameter of 30.0″, a section width of 10.0″, and a wheel diameter of 20″. 

The circumference is 94.3″ and they have 672 revolutions per mile. The Sidewall Height of this Tire is 5.0 inches.

Conclusion

Tire swapping is an important upgrade for any vehicle. Improper interchanging of the tires can cause harm to you and your vehicle.

Going from 255/55R20 to 255/50R20 is not sustainable. Lots of changes are seen with a simple decrement of a 5% aspect ratio. Staying with the 255/55R20 is thus better in terms of vehicle performance and budget compared to the 255/50R20.

It is better to check the 255/55R20 Vs. 255/50R20 tire size interchange chart for clarity and better judgment.

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