315/70r17 vs 35×12.50r17 Tire: What’s the Difference?

Commonly, the best type of tire depends on the type of vehicle you are driving. The same can be said for 315/70r17 vs 35×12.50r17 tires.

For the most part, the difference between the two is the width and sidewall height of the tire. The 315/70r17 tires are wider and offer more stability, making them more popular for trucks and SUVs. 

The 35×12.50r17 (320/70r17) tires are narrower, which means they are more appropriate for smaller vehicles.

But are these two types of tires interchangeable? In this article, we will discover the reasons behind if they are interchangeable or not as well as a thorough comparison between both tires. Let’s begin.

315/70r17 vs 320/70r17 Tire: Comparison Table

Look at the table below about the comparison between 315/70r17 and 320/70r17 tires.

315/70r17 TireType35×12.5r17 Tire
34.36 Inches or 872.8 mmDiameter difference 0.67 (16.92) 1.9% inches (mm)35.03 inches or 889.72 mm
12.4  inches or 315 mmWidth difference 0.12 (3) 1% inches (mm)12.52  inches or 318 mm
107.95 inches or 2741.98 mmCircumference difference 2.09 (53.16) 1.9% inches (mm)110.04 Inches or 2795.14 mm
8.68 inches or 220.5 mmSidewall Height difference 0.33 (8.46) 3.8% inches (mm)9.01 inches or 228.96 mm
586.93 RPM or 364.7 KMRevolutions Per Mile difference -11.16 (-6.94) -1.9% (km)575.77 RPM or 357.76 KM

Although both tires are identical to each other, the only glaring difference is their sidewall height. 

The sidewall height difference between these two tires is 3.8% (0.33 inches or 8.46 mm), where 315/70-17 is 8.68 inches or 220.5 mm and 35/12.5-17 is 9.01 inches or 228.96 mm. 

Besides, despite the revolutions per mile being negligible, 315/70-17 still possesses a faster speedometer score than its counterpart with a better performance in the rotation per minute category. 

Finally, the 35/12.5-17 tire has higher load-carrying abilities due to a larger sidewall height, whereas 315/70-17 has a superior ride quality because it has less load range.

How to read the tire sizes 315/70r17 and 320/70r17?

There are four parts to the tire size 315/70r17, 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 315/70r17.

  • 315 indicates the width of the tires in millimeters
  • 70 shows the percentage of height in relation to the width or the Aspect Ratio
  • R indicates Radial Construction
  • 17 indicates the Diameter of the Rim

Doing the Same for Tire Sizes 320/70r17

  • 320 means the width of the tires in millimeters
  • 70 shows the percentage of height in relation to the width or the Aspect Ratio
  • R indicates Radial Construction
  • 17 indicates the Diameter of the Rim

What’s The Difference Between 315/70r17 and 320/70r17 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 315/70r17 and 320/70r17 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 315/70r17 are 315. It means the width of that tire is 315 millimeters. 

Likewise, the three-digit number of 320/70r17 tire size is 320. It means the Width of that tire is 320 millimeters. So the difference in the Width is 320 – 315 = 5 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, there is no difference between the Aspect Ratio of the two tire sizes. Both tires have an Aspect Ratio of 70%. 

But it doesn’t mean that the sidewall height of the two tires will be similar. Due to the difference in Width, the sidewall heights will be dissimilar.

However, if the sidewall heights are close, they could be interchangeable. 

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, 315/70r17 Tire size has a width of 315 with an aspect ratio of 70%. So the sidewall height will be 315 x 0.70 or 8.7 inches or 220.5 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 320/70r17 is 9.0 inches. Hence, there’s a minor gap between the sidewall heights of the two tires and they could be interchangeable.

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 315/70r17 and 320/70r17 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 315/70r17 is 17 inches. Similarly, the Rim Diameter of 320/70r17 is 17 inches.

It means that if your vehicle has a 17-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 315/70r17 and 320/70r17?

Yes, you can easily interchange between 315/70r17 and 320/70r17 tires.

As we know, a tire is interchangeable if and only if both tires’ diameter difference is less than or equal to 3%.

So, as 315/70r17 and 320/70r17 tires have a diameter discrepancy of only 1.9%, both tires are highly interchangeable. 

Also, as the aspect ratio of both tires is almost neck and neck ~1.4 inches, the driver might feel some steering changes while driving.

Speedometer Difference: Using the current Tire Size 315/70r17 Instead Of the New Tire Size 320/70r17

Look at the Table. It shows differences in speedometer reading and the actual speed at any given time when the current Tire Size is 315/70r17 instead of 320/70r17. 

Speedo ReadingActual Speed
20 mph (32.19 km/h)20.39 mph (32.81 km/h)
25 mph (40.23 km/h)25.48 mph (41.01 km/h)
30 mph (48.28 km/h)30.58 mph (49.22 km/h)
35 mph (56.33 km/h)35.68 mph (57.42 km/h)
40 mph (64.37 km/h)40.78 mph (65.62 km/h)
45 mph (72.42 km/h)45.87 mph (73.82 km/h)

Speedometer Difference: Using current Tire Size 320/70r17 Instead Of New Tire Size 315/70r17

The table shows the differences in speedometer reading and the actual speed at any given time when the current Tire Size is 320/70r17 instead of 315/70r17.

Speedo ReadingActual Speed
20 mph (32.19 km/h)19.62 mph (31.57 km/h)
25 mph (40.23 km/h)24.52 mph (39.47 km/h)
30 mph (48.28 km/h)29.43 mph (47.36 km/h)
35 mph (56.33 km/h)34.33 mph (55.26 km/h)
40 mph (64.37 km/h)39.24 mph (63.15 km/h)
45 mph (72.42 km/h)44.14 mph (71.04 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 315/70r17 tire with the 320/70r17 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 315/70r17 and 320/70r17

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: 315/70r17 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 315/70r17 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 315/70r17 in Inches?

315/70r17 tires have a 34.4″ diameter, 12.4″ section width, and 17″ wheel diameter.

The circumference is 107.9″ and they make 587 revolutions per mile. The Sidewall Height of this Tire is 8.7 inches.

What Size Tires Is 320/70r17 in Inches?

320/70r17 tires have a diameter of 35.0″, a section width of 12.5″, and a wheel diameter of 17″. 

The circumference is 109.9″ and they have 577 revolutions per mile. The Sidewall Height of this Tire is 9.0 inches.

Conclusion 

In conclusion, the 315/70r17 and 35×12.50r17 (320/70r17) tires are interchangeable. The reason for this is that both of these tires have a diameter that is approximate ~34.3 inches, which makes them equivalent to a 34.5″ tire, which is the actual rim diameter for standard 35″ tires. 

Most vehicles will accept either 315/70r17 or 320/70r17 tires with little trouble, although it’s vital to check with the manufacturer if you’re unsure how to best handle the exchange.

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