If your car has 20-inch diameter wheels, 245 45r20 and 275 40r20 are among the best tire options you might consider. Both of them are all-season tires and happen to be quite popular.

If your car came with 245 45r20 tires by default, you might be contemplating whether to switch to a set of 275 40r20 tires instead. This article will help you with your choice by giving you an in-depth understanding of how exactly the two tires differ from each other.

## 245 45r20 vs 275 40r20: Differences

Both the tires in question are good choices for sedans and other passenger cars. Their designs allow great on-road performance and efficiency.

For off-roading, however, neither of the two would be an ideal choice. This is simply because the tires are designed more for roads than rugged off-road terrains. However, they differ greatly in dimensions, which we’ll discuss soon.

Before we compare the dimensions of the two tires, let’s check out how to find the information in the first place. Well, in case you aren’t aware, the tire index number itself is a combination of different parameters of the tire’s design. Each part of the index number represents a particular specification. Here’s how it works:

- In the beginning, the three-digit number shows the width of the tire in millimeters.
- The following digit containing two numbers denotes the sidewall height ratio to the tire’s width, i.e., the aspect ratio.
- You’ll see a letter next (r, in this case). This letter represents the tire’s construction type.
- Ultimately, the last two digits of the tire’s index number represent its rim diameter in inches.

As you may notice from the index numbers of the two tires in question, they’re very different in dimensions. This makes it particularly important to choose the right one between the two as even a small difference in a tire’s specs can significantly impact the car’s ride quality and handling. Below is a detailed comparison of the two tires based on each of the four parameters in the index numbers.

### Width

Between 245 45r20 and 275 40r20, the latter is wider by 30 mm, which is a substantial difference. If you’re considering switching to a set of 275 40r20 tires from the default 245 45r20 ones, it’s likely because of the width. The width of a tire is directly proportional to the traction it offers, as wider tires come with larger surface areas.

With that said, 275 40r20 is the clear winner in this aspect as it offers more traction and increases grip while accelerating.

While this makes it a safer choice in general, you’ll get better traction from 245 45r20 tires when driving on snowy roads. This is because a narrower tire can dig into the snow more easily by concentrating the car’s weight into a smaller area.

### Aspect ratio

As shown by the index numbers, 245 45r20 and 275 40r20 have aspect ratios of 45% and 40% respectively. This means both tires have short sidewalls – less than half the overall width of the tires.

Although there’s a 5% difference in the aspect ratio, the two tires have almost the same sidewall length. To be precise, 245 45r20 has a sidewall height of 4.34 inches while 275 40r20 has a sidewall of 4.33 inches.

The short sidewall length and the low aspect ratio are two of the main reasons why these tires are unsuitable for driving on rugged off-road terrains.

Tires with a low aspect ratio result in a rather uncomfortable ride on such terrains. The bigger the sidewall length, the more shock the tire can absorb to provide additional suspension.

However, a low aspect ratio has its perks too. Tires with high side walls often make the ride feel a bit unstable. Thanks to their low aspect ratio, both 245 45r20 and 275 40r20 offer great lateral stability.

Between the two, 245 45r20 offers the perk of a more comfortable ride while 275 40r20 ensures better handling due to their respective aspect ratios.

### Construction type

The two tires are similar in construction, with both of them featuring the radial construction style. This is a good thing since radial tires are much more sophisticated and better than cross-ply tires.

The only drawback of radial construction as compared to cross-ply construction is durability. However, radial tires are a much better choice overall as they offer a more comfortable ride. As radial tires don’t feature an inner tube, they are more flexible and offer extra suspension.

### Rim diameter

The tire’s rim diameter is the last specification included in the index number. Both tires in question have index numbers ending with 20, which means they have a rim diameter of 20 inches.

A tire’s rim diameter is the most important factor determining whether it would fit your car’s wheels. As 245 45r20 and 275 40r20 share the same rim diameter, they fairly fit the same range of cars. As long as your vehicle has a wheel diameter of 20 inches, both these tires will fit.

The comparison above should give you a fairly good idea of how much these two tires differ and how switching from 245 45r20 to 275 40r20 might affect your vehicle’s performance. The table below should make your work even easier by comparing the parameters side-by-side:

Parameters |
245 45r20 | 275 40r20 | Difference |

Width | 245 mm | 275 mm | 30 mm (12.2%) |

Aspect ratio | 45% | 40% | -5% |

Rim diameter | 20 inches | 20 inches | 0 |

Circumference | 2288.65 mm | 2287.08 mm | -1.57 mm (-0.1%) |

Sidewall height | 110.25 mm | 110 mm | -0.25 mm (-0.2%) |

Comparing the specs, we can see that despite the differences in overall width and aspect ratio, the two tires have very similar sidewall height and circumference.

#### Conclusion: Which one to use and when?

Comparatively, 275 40r20 is a better choice in terms of both performance and aesthetics. The 245 45r20 tires that come by default often look rather skinny.

Switching to a set of 275 40r20 will be a good idea if you want to enhance your car’s appearance and grip. However, if you are unsure about the change in handling, you may choose to stick with 245 45r20 tires.