How To Select The Right Tire Size For Your Aftermarket Wheel

Selecting an aftermarket wheel can be challenging. Once you settle on a style, a color, and a finish, you get to consider how large of a wheel you want, how wide you want to go, and things only become more complicated when offset, and backspacing come into the question.

After I had decided on getting the Rohana RC22 wheels in matte graphite for my 2014 Ford Mustang, I spent months agonizing over the sizing and whether I should run 19’s-, or 20-inch wheels. I didn’t want to lose all my ride quality or performance, so I opted for a set of 19-inch wheels.

I was so eager to get the new wheels on the car I recklessly slapped on a set of cheap tires without much thought and was disappointed when it looked like I was riding around on rubber bands. I also encountered another issue: I had no idea how to determine the correct tire pressure to run for my aftermarket wheels.

OE wheels and tires: 215/65R17

I am all about spirited driving, so I know I wanted something sticky and performance oriented but remained clueless on the proper tire size. A new set of proper tires will set me back $600-$1000, which is a bit of an investment and makes me even more concerned about running the correct tire pressure. I would hate to destroy them right away because of something small and easy to check.

To best determine the size needed, I reached out to Cameron Parsons, the Competition & Specialty Tires Product Manager at Toyo Tires to assist.

Like many other enthusiasts, my goal is to put the widest tire possible on the wheel. Unsurprisingly, my first question was, “how wide can I go?”

Cameron Parsons: Toyo Tires offers a powerful “Find a Tire” tool on www.toyotires.com that lists the specifications of each of its tires, including the approved rim widths for each size tire.

If I get a tire slightly wider than the wheel, will it bulge out and protect the wheel from curb rash?

Parsons: When selecting a tire size, the focus should be on optimizing the tire’s fitment for the wheel and performance for the vehicle. Every tire size will feature a maximum and minimum rim width range, so it is essential to select a tire and wheel combination that is compatible with each other. A tire that bulges out the sides enough to protect from curb rash would likely alter its performance and be considered a misapplication. Depending on what tires you’re shopping for, Toyo Tires offers multiple products with rim protector features.

Does wheel offset have any effect on the tire size / selection?

Parsons: The offset of the wheel does not affect how the tire size fits to the wheel, but it does affect how the wheel and tire package fits to the car. Many car owners like bringing the outside edge of the wheel further out to improve the car’s looks, but taking it too far can cause the tires to rub the inside of the fenders while turning and/or during suspension compression.

Wheel offsets that differ from the factory configuration can also make small changes to how the vehicle’s suspension acts and how its overall weight distribution.  

Rohana RC22 wheels

What are the benefits to plus sizing?

Parsons: For many car owners, the main goal of plus sizing is to improve the overall looks of their vehicle. However, plus sizing also has the potential to enhance steering response as a result of a shorter and thus stiffer sidewall. On the downside, taking away sidewall height also takes away some of the tire’s effectiveness as part of the vehicle’s suspension, resulting in a firmer ride.

While you can shop for plus sizing that increases wheel diameter and retains overall tire diameter, you can also find “plus zero” options. This is when you keep the same wheel size but increase the tire section width and reduce the aspect ratio. If done with a reasonable section width increase, this increases the tire contact patch to improve vehicle handling and traction.

What is the “overall diameter” and why does it matter?

Parsons: Overall diameter is the measured outside diameter of an inflated tire. Most speedometers determine speed from a sensor at the transmission to track its rotational speed. This information is sent to the speedometer which is calibrated to display transmission rotation speed relative to tire circumference, producing distance traveled over time, or miles per hour.

Changing the overall diameter affects the tire circumference, which means the tire experience fewer or more revolutions per mile than the original factory size. This causes the speedometer to display slower or faster speeds than what the vehicle is actually traveling at.

In AWD applications, overall diameter becomes even more critical. Since the front and rear wheels work together to drive the car, there is a center differential that manages the power delivery at both ends. If the front and rear tires are mismatched in diameter and thus spinning at different rates, this can cause significant wear and damage to that center differential.

Another view of the Mustang with Rohana RC22 wheels

Will I feel a difference in the ride between a 35 series and a 40 series sidewall?

Parsons: This largely depends on the vehicle, tire size, inflation pressure, and who is driving it. If, for example, you compare a 255/35R19 and 255/40R19, users more in tune with their vehicles will likely notice a difference in comfort and performance, especially those who drive their cars in autocross and track events.

What is the load rating and speed rating? Why does it matter if it is different from the OE tire?

Parsons: These ratings specify the tire’s maximum load-carrying and speed capabilities when inflated to its proper pressure. Tires rely on inflation pressure to be effective, as this pressure directly determines their load-carrying capacity. There is no harm in selecting a tire with a higher load index or speed rating, as long as you can confirm the inflation pressure necessary to meet the OE tire’s capabilities.

Tire speed ratings, which are noted by a letter designation on the sidewall of a tire, are based on laboratory tests and do not imply that a vehicle can be safely driven at the speed for which the tire is rated.  When replacing tires on your car, you should use replacement tires with a speed rating equal to or greater than the OE tire if the speed capability and handling characteristics of the vehicle are to be maintained.

What tires would you recommend for my wheels?

Parsons: Your OE tires have a 28-inch overall diameter, we have a couple of options that could fit the wheels you mentioned with our new Proxes Sport tire that will be just a few tenths of an inch shy of that. This is our Max Summer Performance that manages to offer excellent wet and dry handling with a decent tread life and quiet ride.  

You can find the specifications on our website, as you’ll also want to check the overall width of these sizes that they will fit with your new wheel offsets to not rub the inside fenders of your car.

Proxes Sport Tire | Toyo Tires photo

Since I made the move to an aftermarket wheel, can I still use the recommended tire pressure info on the tire placard?

Parsons: If you retain the same size tire and load rating or load index as the OE offering, you can still use the recommended tire pressure found on the vehicle information placard. If you change the tire size, including going from a P-metric to a hard metric or vice versa, your dealer can recommend the new inflation pressure, or you can contact our customer service team.  

Our website, toyotires.com, also has information on tire inflation when you use our “Find a Tire” search tool, and under Tires 101.

Every tire carries a Load Index Code on the sidewall, right behind the tire size. You can use these to compare the load-carrying capacity of different size tires. Courtesy Toyo Tire

Is there a significant difference between 35 and 36 psi?

Parsons: Load-carrying capacity is directly related to inflation pressure. Between 35 and 36 PSI? It depends on the tire and application but usually would make little difference. An average commuter car with original tires might carry a manufacturer recommendation of only 35 PSI. If it is driven through different altitudes or temperatures, this pressure will likely fluctuate but remain within a safe range.

In a vehicle being put to more substantial use or on the track, differences in inflation pressure could be more critical because of its impact on load capacity and performance. Depending on the tire, if that extra one PSI increases a tire’s load capacity by 15 pounds, multiply that by four, and the overall load capacity could increase by up to 60 pounds.

What would happen if I drove around with different pressure in each wheel?

Parsons: You’ll find mismatched wear between the tires and, depending on how much they vary from factory recommendations, decreased performance, and potential damage.

When would a tire be considered too low and what would happen if I drove with  under inflated tires?

Parsons: The pressure is too low on an OE tire when its cold pressure (hasn’t been driven for a few hours) is below the recommended inflation pressure listed on the information placard. Driving with underinflated tires will cause abnormal wear and adversely affect vehicle handling and, in worse case scenarios, permanent damage or tire failure due to excess load from underinflation.

Tires often list a max fill, but I don’t see a minimum. Is that information harder to find on the tire, or is it not included?

Parsons: There is no minimum listed because that depends on the application that the tire is being used on. Refer to your vehicle’s tire information placard, commonly found along the driver’s side door sill or in the owner’s manual, for the recommended inflation pressure. If you change from the original tire size, then your tire dealer will advise you of the new recommended inflation pressure. It is important to note that the maximum air pressure indicated on the sidewall is NOT the recommended inflation pressure.

Aside from a slow leak, what are some other reasons tire pressure could be too high or too low?

Parsons: Your tire’s inflation pressure can be too high or too low depending on the load on the tires. Temperature conditions and altitude will commonly affect inflation pressures, but generally not enough to worry about.  Some vehicles come with different recommended inflation pressure on the front and rear tires so if you rotate your tires, make sure to re-check and adjust inflation pressures as necessary.

Inflation pressures can decrease by one psi for every 10° F temperature drop.  In general, the effect on a typical tire’s performance is negligible, but it is advised to monitor the inflation pressure drop to prevent an underinflation scenario. Also, some tires, like many of Toyos’ competition tires, should not be used in severe cold conditions as it can lead to potential cracking.

For more information on tire inflation, or to use the “Find a Tire” search tool, visit Toyotires.com.

About the author

Nicole Ellan James

As an automotive journalist and avid car enthusiast, Nicole Ellan James has a passion for automotive that is reflected in every aspect of her lifestyle. Follow Nicole on Instagram and Facebook - @nicoleeellan
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