What tire size do I need?
Your vehicle leaves the factory with wheels and tires that are a certain size, and they’re that size for a lot of different reasons. Engineers take tire size into account when they’re designing for ride quality, fuel economy, braking performance, handling, steering response, and a number of other factors in the overall driving experience.
That stock tire size is optimal and recommended by the factory, but that doesn’t mean that you can’t enhance your car by making subtle tweaks in wheel or tire size, also known as ‘plus sizing’. It could be for aesthetics, changing to a wheel and tire combo that’s more stylish. Or it could be for function, boosting your traction. Or perhaps you’ve just got an oddball tire size that’s hard to find and you’d like to go to something a little more common. Whatever the reason for making the modification, it all starts with understanding the tire markings.
How to read tire size
A typical tire size marking looks like this, P225/65R17. Here’s a graphic to help break that code down:
We should note that P-Metric is the industry’s most common classification, where the P originally stood for Passenger, but now P tires are common for cars, minivans, some light trucks, crossovers (CUV’s), and SUVs. Another common classification is the LT-Metric, required on ¾ ton and 1 ton trucks, such as a Ford F250 or Ford F350.
You’ll also find coded information on the tire’s speed rating and load rating on the sidewall.
Now, if you’re considering changing wheel and tire sizes, here are some things to bear in mind.
- Wider tires will give you better grip and cornering ability on dry pavement, but if you go too wide you add to rolling resistance and your fuel economy can suffer.
- A wider contact patch of a wide tire means you can boost cornering ability, but only past a certain point, making steering feel heavier and clumsier.
- For a high-horsepower vehicle with a lot of torque, wider tire sizes can better grip the rear wheels to the pavement for better acceleration and less wheelspin.
- When you go with tires that are too wide, they can end up rubbing the inside of the wheel wells, wearing out prematurely.
- In winter weather, narrower tires are advised for traction in snow, which seems counterintuitive when you consider that a tire’s contact patch is what actually delivers traction. However, going to a tire that’s too narrow for snow can create a safety risk of its own.
- Taller wheels and tires are pretty popular for 4WD owners who want to boost ground clearance.
- Taller wheels can affect overall handling, increasing the likelihood of excessive body roll during a quick emergency maneuver.
- Too-tall wheels and tires can result in rubbing on the inside of the wheel well.
Determining the right tire size
Here’s a general rule of thumb for tire sizes: going up one size in tire width and down one size in aspect ratio (the height of the sidewall) will still equate to the same as your original tire size. In other words, let’s say your original tire size is 195/60R15, with a load index of 87. If you go to a 205/55R15, which is wider by 10mm but shorter in aspect ratio, you can still keep that same load index of 87. Online tire size calculators are available to take some of the guesswork out of these decisions.
Unsprung weight is another factor that engineers take a pretty hard look at during the design process. Like the term suggests, “unsprung weight” is the weight of anything that isn’t supported by the suspension. That includes not just wheels and tires but also the differential, axles, brakes, spindles, and hubs and wheel bearings.
Almost everything that’s unsprung weight in a vehicle will also be rotating mass, and the more rotating mass there is to get moving, the more it can affect acceleration and response. The flip side of that, of course, is that excessive unsprung weight can affect braking performance as the brakes have more momentum to slow. Not surprisingly, bigger wheels and tires will typically weigh more and add to unsprung weight.
Oversize wheels and tires
Everyone has seen 4WD trucks rolling on huge mud tires, or ‘70s or ‘80s-era sedans on 24” or even 26” wheels and low-profile tires. Well, there are a few things to consider when up-sizing drastically like that, starting with the unsprung weight issues just mentioned:
- For larger-circumference wheels the bigger the wheels, the lower your speedometer readings will be. If you don’t want to find yourself getting a ticket because your speedometer was reading 12 mph slower than you were actually going, you’ll need to have the speedometer software reset to compensate for bigger wheels.
- On a 4WD truck, oversize tires will also require a lift kit. Past a certain point, a lifted truck essentially means a re-engineering of the suspension and drivetrain, with angles and geometry completely altered. If it’s not done right, a lift job can mean vibration from the driveshafts, premature wear on drivetrain components and atrocious road manners on the highway.
- Oversize wheels can throw off the software for the vehicle’s ABS, traction control, and vehicle stability systems. In extreme cases, it can affect their ability to respond and work like they’re supposed to.
- Taller wheels and low profile tires can also mean ride quality that’s absolutely punishing, since there’s less rubber to absorb and cushion against bumps and shocks from pavement irregularities.
- Expect changes in braking and cornering performance.
- An oversize set of wheels will spin more slowly to cover the same amount of ground, which effectively lowers your vehicle’s effective gear ratio. In extreme cases, oversize wheels can cause wear on hubs and spindles and can even result in transmission problems.
None of this is to say that you shouldn’t upsize wheels and tires, but just be aware of what you’re getting into with it. If you plan on a drastic change in wheel/tire size, it’s a good idea to talk to a professional and get some expert opinions first.
Keeping your original size
If you want to stay with your car’s factory tire size or just do plus-sizing by changing width and aspect ratio by one increment each, you can find your tire size on a placard on the driver’s door frame (some older models may have the sticker on the passenger door jamb, under the center console, in the owner’s manual or even on the inside of the fuel door). If you can’t find the owner’s manual (often MIA on an older used car), you can typically go online and download it in PDF form - isn’t technology great?
No doubt there’s kind of a lot here to take in, but the good news is that SimpleTire’s tools and resources make it easy to find the right size tires for your vehicle, including specific tire recommendations that will fit your preferences, your driving style and your budget.
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