315/70R17 vs 37×12.50R17: What’s the Difference?

The main attraction of owning an SUV or Truck is the wheel size. It depends mainly upon the tire size. Upsizing the tire of the trucks is a common phenomenon as it enhances the vehicle’s aggressive look.

That is why the chassis and body of the SUV and pick-up trucks are made to accommodate bigger tires. But this does not mean you can swap the stock tire of these vehicles with a bigger version without any consequences.

Hence, before purchasing a 37×12.50R17 (318/80R17) tire to interchange with 315/70R17, it is important to know the compatibility and after-installation effect. 

Keep reading till the end of this article to understand the 315/70R17 vs 37×12.50R17 comparison and if these are interchangeable.

315/70R17 vs 37×12.50R17 Tire: Comparison Table

The comparison chart is made after converting the 37×12.50R17 tire size to a metric system. After conversion, it becomes 318/80R17. Now we can see that swapping these tires dramatically increases the aspect ratio by 10%.

315/70R17 TireType318/80R17 Tire(37×12.50R17)
34.36 inches or (872.8)mmDiameter difference: 2.67 (67.8) 7.8% inches (mm)37.03 inches or (940.6)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: 8.39 (213) 7.8%inches (mm)116.34 inches or (2954.98)mm
8.68inches or (220.5)mmSidewall Height difference 1.33 (33.9) 15.4% inches (mm)10.02 inches or (254.4)mm
586.93 RPM (364.7)kmRevolutions per mile difference -42.31 (-26.29) -7.2% km544.62 RPM (338.41)km

The standard accepted range of diameter is 3% higher or lower. From the comparison chart, you can see that. Interchanging the pre-selected tires will increase up to 7.8% of tire diameter.

This also impacts the elevation of the circumference by 7.8%. Both the diameter and circumference of the tire cause lower RPM values at high speeds. 

For this, your speedometer will show a lower speed than the vehicle’s actual speed. It may cause you trouble with the highway cops and even cost you a speeding ticket.

Your tire will also gain a sidewall height of 15.4%, which is massive for an SUV. However, it will be beneficial for you to go off-road riding. But the vehicle’s downforce will decrease, and thus stability also decreases.

How to read the tire sizes 315/70r17 and 318/80r17?

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 tires, 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 to the width or the Aspect Ratio
  • R indicates Radial Construction
  • 17 indicates the Diameter of the Rim

Doing the Same for Tire Sizes 318/80r17

  • 318 means the width of the tires in millimeters
  • 80 shows the percentage of height 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 318/80r17 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 318/80r17 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 318/80r17 is 318. It means the Width of that tire is 318 millimeters. So the difference in the Width is 318 – 315 = 3 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

The Aspect Ratio of 315/70r17 is 70, whereas the Aspect Ratio of 318/80r17 is 80.

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, 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 318/80r17 is 10.0 inches.

Hence, there’s a huge difference between the sidewall heights of the two tire sizes and they couldn’t 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 318/80r17 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 318/80r17 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 Vs 37×12.50R17 Tire

No. it is not recommended to interchange the tires as the size of the tire increases in all aspects crossing the limit of acceptance.

This bigger size of the tire will cause a lot more manipulation in the speedometer. Hence you may violate the speed limit while driving at high speed.

Speedometer Difference: Using the current Tire Size 315/70r17 Instead Of the New Tire Size 318/80r17

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 318/80r17.

Speedo ReadingActual Speed
20 mph (32.19 km/h)21.55 mph (34.69 km/h)
25 mph (40.23 km/h)26.94 mph (43.36 km/h)
30 mph (48.28 km/h)32.33 mph (52.03 km/h)
35 mph (56.33 km/h)37.72 mph (60.7 km/h)
40 mph (64.37 km/h)43.11 mph (69.37 km/h)
45 mph (72.42 km/h)48.5 mph (78.05 km/h)

Speedometer Difference: Using current Tire Size 318/80r17 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 318/80r17 instead of 315/70r17.

Speedo ReadingActual Speed
20 mph (32.19 km/h)18.56 mph
25 mph (40.23 km/h)23.2 mph (37.33 km/h)
30 mph (48.28 km/h)27.84 mph (44.8 km/h)
35 mph (56.33 km/h)32.48 mph (52.27 km/h)
40 mph (64.37 km/h)37.12 mph (59.73 km/h)
45 mph (72.42 km/h)41.76 mph (67.2 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 318/80r17 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 318/80r17

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 318/80r17 in Inches?

318/80r17 tires have a diameter of 37.0″, a section width of 12.5″, and a wheel diameter of 17″. 

The circumference is 116.2″ and they have 545 revolutions per mile. The Sidewall Height of this Tire is 10.0 inches.

Conclusion

All the facts and data have been presented to you regarding the comparison between 315/70r17 vs 37×12.50r17 Tire. It depends upon your preference and decision if you want to interchange the tires or not.

Either way, both options have some pros and cons. But if you are planning for the truck’s aesthetics and general driving, it is not advisable to interchange the tires.

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