Tire Size Calculator

That string of numbers and letters stamped on your sidewall tells you everything about a tire's dimensions. Take 225/65R17 as an example:

• 225 is the tire's section width in millimeters, measured from sidewall to sidewall.
• 65 is the aspect ratio. The sidewall height is 65% of the section width.
• R stands for radial construction, which is standard on virtually all passenger tires today.
• 17 is the wheel diameter in inches the tire is designed to fit.

Some tires also carry a load index and speed rating after the wheel diameter, like 225/65R17 102H. The 102 is the load index and H is the speed rating. Those matter for safety, but they don't factor into the dimensional math.

Light truck and SUV tires sometimes use a different format entirely. You might see LT265/70R17 or the older flotation style like 33x12.50R15. That flotation format uses inches directly and shows up a lot on off-road and older truck tires.

Once you know what each number means, the math is pretty simple. Here are the core formulas any tire size calculator uses:

Sidewall Height (inches)

Sidewall Height = (Section Width in mm × Aspect Ratio / 100) / 25.4

Tire Diameter (inches)

Tire Diameter = Wheel Diameter + (2 × Sidewall Height)

Tire Circumference (inches)

Circumference = π × Tire Diameter

Revolutions Per Mile

Revolutions Per Mile = 63,360 / Circumference

Working through a 225/65R17 as an example:

1. Sidewall Height = (225 × 65 / 100) / 25.4 = 146.25 / 25.4 ≈ 5.76 inches
2. Tire Diameter = 17 + (2 × 5.76) ≈ 28.52 inches
3. Circumference = π × 28.52 ≈ 89.59 inches
4. Revolutions Per Mile = 63,360 / 89.59 ≈ 707 rev/mile

These numbers feed directly into speedometer error calculations and let you compare two tire sizes on a level playing field.

Swapping tire sizes is probably the most common reason people reach for a calculator. Maybe you want more ground clearance, or a lower-profile tire for a sportier look. Either way, you need to know how much the dimensions actually change before you commit to anything.

A side-by-side comparison makes that easy. Here's what it looks like for 225/65R17 vs. 245/60R17:

In this case the 245/60R17 is noticeably wider but nearly the same diameter, so the speedometer impact is basically nothing. A comparison like this also tells you whether the new tire will clear your fenders and suspension, which matters just as much as the numbers on paper.

Tire diameter is the single most important dimension when you're changing sizes. It affects ground clearance, gearing, and speedometer accuracy all at once. The formula is simple:

Tire Diameter = Wheel Diameter (in) + 2 × Sidewall Height (in)

The wheel diameter stays fixed since you're usually keeping the same rim. What changes is the sidewall height, which depends on the tire's width and aspect ratio. A wider tire with the same aspect ratio will have a taller sidewall and a bigger overall diameter. Drop the aspect ratio and the sidewall shrinks, even if the width goes up.

Most vehicle manufacturers recommend staying within about 3% of the original tire diameter. Go beyond that and you start running into clearance problems, rubbing against wheel wells, and speedometer errors that are hard to ignore.

Circumference matters because your ABS, traction control, and speedometer all rely on counting wheel rotations to figure out speed and distance. Change the circumference and you change what those systems think is happening.

It's just the diameter multiplied by pi (3.14159). For a tire with a 28.52-inch diameter, that works out to about 89.59 inches per revolution, or roughly 7.47 feet.

Revolutions per mile flips that around. Divide 63,360 (inches in a mile) by the circumference. A slightly taller tire rotates fewer times per mile, which causes your speedometer to read low. A shorter tire spins more often and makes it read high.

That's why revolutions per mile is so useful when comparing sizes. A difference of 10 or more rev/mile usually means a noticeable speedometer error. Some vehicles let you recalibrate through the instrument cluster or an aftermarket tuner. Others don't, so it's worth checking before you buy.

When you change tire sizes, your speedometer doesn't automatically adjust. It keeps counting wheel rotations the same way it always has, so if your new tires have a different circumference, the speed reading will be off.

The formula is straightforward:

Actual Speed = Indicated Speed × (New Tire Diameter / Original Tire Diameter)

Say your original tires were 28.52 inches in diameter and your new ones are 30.0 inches. At an indicated 60 mph, your actual speed would be:

60 × (30.0 / 28.52) ≈ 63.1 mph

That's a 3.1 mph gap at highway speed. It adds up over a long trip and can throw off your fuel economy calculations and odometer readings too. You can flip the formula to find the indicated speed when you know the actual speed:

Indicated Speed = Actual Speed × (Original Diameter / New Diameter)

Most states put the legal limit on speedometer error at around 5%, so there's a little room. Still, it's better to know the exact number before you hit the road.

These three measurements are connected. Understanding how they interact saves you from some unpleasant surprises when switching sizes.

Section Width is the widest point of the tire from sidewall to sidewall, in millimeters. A 245-width tire is 20mm wider than a 225. Wider tires can improve grip on dry pavement but sometimes struggle more in snow or slush.

Aspect Ratio is a percentage. A 65 aspect ratio means the sidewall height is 65% of the section width. Drop that from 65 to 55 on a 245-width tire and the sidewall shrinks from about 6.24 inches down to about 5.31 inches. That's a meaningful change.

Sidewall Height has a real-world feel to it. Taller sidewalls cushion the ride and protect your rims on rough roads. Lower-profile tires feel stiffer and are more vulnerable to pothole damage, though they usually look better on modern wheels and can sharpen steering response.

The relationship between these three values is what makes tire sizing feel like a balancing act. Go wider and you often need to drop the aspect ratio to keep the diameter close to stock. That's the basic logic behind most plus-sizing setups.

Most passenger car and crossover tires use the metric format, like 225/65R17. Off-road, truck, and older tires often use the inch-based flotation format, like 33x12.50R15. Both describe the same physical dimensions, just in different ways.

To convert from metric to an approximate inch equivalent, use the tire diameter formula and express the width in inches. A 225/65R17 has a diameter of about 28.5 inches and a width of about 8.9 inches, which is close to a 28.5x8.9R17 in flotation terms (though that exact size isn't something you'd find on a shelf).

When shopping for off-road tires you'll often see both formats listed for the same tire. Worth knowing how to read both so you can cross-reference sizes without getting confused.

The chart below shows common metric tire sizes alongside their approximate overall diameter, sidewall height, and inch-format equivalents. These are rounded values meant for quick reference, not precise specs.

Keep in mind these are calculated values. Actual measured dimensions can vary a bit between manufacturers depending on construction and the specific rim width the tire ends up mounted on.

Not every tire size is available in every brand or model. When your exact OEM size is out of stock, discontinued, or just priced too high, an alternate size can work just as well if you pick it carefully.

The goal is to match the original tire's overall diameter as closely as possible, ideally within 1 to 2 percent. A good calculator will show you which sizes fall within that range for your wheel diameter. From there it's a matter of checking width and aspect ratio to find something that fits your rims and wheel wells.

A few common alternate size pairings worth knowing:

• 215/70R15 and 205/75R15: nearly identical diameters, common on older sedans and minivans
• 235/75R15 and 31x9.50R15: popular crossover options for light trucks
• 265/70R17 and 255/75R17: both measure around 31.6 inches and work on many half-ton trucks

Always check the load index and speed rating when substituting a size. A tire that fits dimensionally but has a lower load rating than the original isn't a safe swap, full stop.

Plus sizing means going to a larger wheel diameter while reducing the aspect ratio to keep the overall diameter roughly the same. It's popular for performance and appearance upgrades. "Plus one" means you've gone up one inch in wheel size, say from 17 to 18 inches. "Plus two" means two inches, and so on.

The trade-off is a stiffer ride since there's less sidewall to absorb bumps. On the upside, lower-profile tires tend to respond more crisply to steering inputs, and larger wheels often look better with modern vehicle styling. Some people plus-size specifically to fit bigger brake rotors and calipers, too.

Minus sizing goes the other direction: smaller wheel, taller sidewall. This is common with dedicated winter tire sets. A narrower, taller tire cuts through snow better and protects your low-profile summer tires from pothole damage over the winter. Running a minus-one or minus-two winter setup is a genuinely smart move if you live somewhere cold.

In either direction, the key rule is to keep the overall tire diameter within about 3% of stock. Stray further than that and you're looking at clearance problems, gear ratio changes, and inaccurate ABS readings.

A tire doesn't just need to fit the vehicle. It needs to fit the wheel. Every tire has a recommended rim width range, and mounting one on a rim that's too narrow or too wide affects its shape, handling, and how evenly it wears.

As a general rule, the recommended rim width for a given tire is usually about 70 to 90 percent of the tire's section width converted to inches. A 225mm-wide tire is about 8.86 inches wide, so it typically fits best on rims between 6.5 and 8 inches wide. Most tire manufacturers publish exact fitment charts, and it's worth looking those up before you order anything.

Mounting a tire on a rim that's too wide stretches the sidewall and can make the tire feel unstable in corners. Too narrow and the tire bulges outward, which hurts handling and can wear the shoulders faster. Neither is catastrophic in a mild case, but staying within spec is obviously better.

Bolt pattern and offset are separate from tire sizing but equally important when you're swapping wheels. Wrong offset and the tire rubs the inner fender liner, or sticks out past the fender. Neither situation is one you want to discover after you've already bolted everything on.

A size change that looks fine on paper can still cause real problems on the road. Before you finalize anything, run through this checklist:

• Clearance: Does the tire fit without rubbing the fender, suspension components, or steering linkage at full lock and full compression? A wider or taller tire needs more room on all sides.
• Load capacity: Your new tire's load index must meet or exceed the vehicle manufacturer's minimum. This one is non-negotiable.
• Speed rating: Match or exceed the original. Dropping the speed rating on a highway vehicle is a risk that simply isn't worth taking.
• Speedometer accuracy: Even a modest diameter change shifts your speedometer reading. Check whether your vehicle allows recalibration and factor in the error if it doesn't.
• TPMS sensors: Your tire pressure monitoring system may need reprogramming if you're changing wheel sizes, and the sensors from your old wheels may or may not transfer over.
• Fuel economy: Heavier and taller tires add rolling resistance and rotational mass, which can nudge mpg down. Usually a small effect, but worth knowing.
• Warranty: Some vehicle warranties include language about modifications, tire size changes included. Quick check if you're still covered.

None of this should scare you away from a tire upgrade. It just needs to be part of the decision, not something you think about after the fact.

A few real-world examples help tie all of this together.

Example 1: Stock to Performance Upgrade

A driver wants to go from 215/55R17 to 225/50R17 for a sportier look. Here's the comparison:

The new tire is about 1.7% smaller in diameter. The speedometer will read slightly high at highway speed, and the ride will feel a bit firmer with the shorter sidewall.

Example 2: Truck Lift with Larger Tires

A truck owner running stock 265/70R17 tires wants to fit 285/75R17 after a 2-inch lift. The diameter goes from 31.61 inches to 33.83 inches, a difference of about 2.22 inches, or roughly 7%. That's well past the 3% guideline, so a speedometer correction is definitely needed and fender clearance should be checked carefully.

Example 3: Winter Tire Minus Sizing

A driver with 245/45R19 summer tires wants a winter set on 17-inch wheels. A good minus-two choice would be 225/55R17, which has a diameter of about 27.76 inches compared to 27.68 inches on the original. That's less than 0.3% different, so the speedometer stays accurate and the taller sidewall actually helps in snow.

These examples show why running the numbers before buying beats guessing every time. A few minutes with a calculator can save a lot of headaches at the tire shop.