November 14, 2019

Changing tire sizes will directly impact any vehicle’s performance, handling, and appearance. In order to make the most informed tire choices, drivers need to be familiar with industry standard tire measurements, as well as the predictable results that come with changing a tire’s width, aspect ratio, overall height, and weight. 

Let’s dissect a common tire size, 225 50R15, from start to finish. The first number listed (225 in our case) represents the tire’s width from sidewall to sidewall. The second number listed is the tricky one, because it represents the tire’s aspect ratio (AKA sidewall AKA profile) as a percentage of the tire’s width. In our case, the 50 means that the sidewall is 50% as thick as the tire’s 225 width, meaning the sidewall will be around 112.5mm tall. The final measurement, R15 for us, simply means Radial tire and 15 inch wheel. 

There are many other potential letters and numbers that may prefix or suffix this standard measurement, all of them are important and must not be overlooked as they designate critical features of the tire. Load and speed ratings in particular are crucial to get right, as catastrophic failure may occur if overlooked, but our purpose here is to focus on tire sizes and size variations alone. 

To that end, when searching for more performance across different road surfaces, tire width becomes the first and most critical measurement. A driver looking for more grip on pavement should choose a slightly wider tire, as a larger contact patch on the road will instantly give the car more grip and stability. A driver looking for more grip on snow and ice should select a narrower tire, to get more pressure on a smaller contact patch, allowing them to cut through snow and slush to better grip as well as get the most out of the flexible tread blocks and sipes of modern winter tires. To boil this formula down: more road grip, more tire width… less road grip, less tire width. Keep in mind that any tire’s labeled width is measured from sidewall to sidewall and only suggests the width of the actual tread of the tire, your critical contact with the road. Several websites including list the actual tread width of each tire, a vital factor for predicting a tire’s actual characteristics for any application. 

For high performance driving, adding significantly wider tires may necessitate flared or wider fenders, different offset wheels, or even suspension geometry changes in extreme cases. The same is true for adding very narrow WRC-type winter tires, very narrow wheels with the correct offset become necessary to correct the handling of the car. Gravel rally conditions are found between these two extremes, and often a close-to-stock tire width will be appropriate for most applications (if grippy, go wider, if slippy, go narrower).

Once we have settled on a tire width appropriate for the vehicle and road conditions, the next factor that must be decided will be the aspect ratio, the thickness of the sidewall of the tire. For this measurement, one simple formula would be: The smoother the road is, the thinner a sidewall we would want, and the rougher the road, the thicker a sidewall we would want. This is because a low profile tire will give us more performance and better handling, but at the cost of ride quality, shock absorption, and durability. A taller sidewall will give us a more comfortable ride and damage resistance over bumps and potholes, with the tradeoff of feeling more “boaty” and less immediately responsive to our inputs. 

Once width and aspect ratio are sorted, we need to next consider the overall height of our tire. Mounting different height tires on a vehicle will change the overall gearing, as the diameter of the tires are the vehicle’s actual final drive ratio. A taller tire will give the effect of raising the final drive, meaning the vehicle will accelerate more poorly but each gear will have a higher top speed. The opposite is true of mounting smaller tires, acceleration will be improved and gears will feel shorter and closer together. For the street driver, it’s important to know that taller tires will also cause the speedometer and odometer to read lower than normal, and the opposite will be true with shorter tires. 

Manufacturers, race teams, and even normal street drivers will also sometimes mount different sized tires on the front and rear of a vehicle. This “staggered setup” is particularly prevalent in high performance two wheel drive vehicles where wider tires on the drive wheels will give additional grip and better acceleration. The non-driven wheels simply need less grip to do their job and are fitted with smaller tires to save weight and expense. This staggering may be done freely with two wheel drive cars, but great care must be taken with all wheel drive vehicles. Most production-based AWD systems MUST have the same diameter tires at all four corners, and any change to that can severely and permanently damage the differentials and AWD system. 

At this point, we should have our target width, aspect ratio, and overall height we’re looking for. More drastic changes may necessitate also changing to a different wheel size… At which point there are many other considerations, but we’ll leave that for another day. The last and final “ sanity check” with a wheel and tire package is to consider the total weight. Any change in this un-sprung weight will directly affect the vehicle’s acceleration, braking, handling, suspension feel, and longevity of parts. These effects are negligible on the street with minor tire size changes, but can be drastic when mounting very heavy-duty wheel and tire packages. The  trade-off between light weight and heavy duty is omnipresent as with all vehicle modifications, as is the cost associated with high quality lightweight parts that check all the boxes.

Thank you as always for visiting Team O’Neil. As you continue your education in advanced driving skills, please get in touch anytime to discuss what Team O’Neil can do for you. Have fun, be safe, and we’ll see you next time!