What tire size fits your rims depends on three numbers stamped on the rim itself

Your rim has a diameter, a width, and an offset — three measurements that determine which tires will fit safely. You do not need a calculator or a website tool. You need to read the numbers already on your rim, then match them to a tire's sidewall markings. The process takes five minutes and a flashlight.

Every rim is marked with its size in a format like "17x7.5 ET45" or "18x8 ET35". The first number is the diameter in inches. The second is the width in inches. The third (ET) is the offset — how far the rim sits from the wheel centerline. Your tire must fit that diameter and width. The offset does not change which tire fits; it changes how the wheel sits in the wheel well.

A tire's sidewall shows its size in a code like "225/45R17". The last number (17) is the rim diameter it fits. The first number (225) is the tire width in millimeters. The middle number (45) is the sidewall height as a percentage of width. You need the tire diameter and width to match your rim diameter and width — or to fall within the range your vehicle manufacturer allows.

Key Takeaways

  • Your rim is marked with three numbers: diameter (inches), width (inches), and offset (ET). Only diameter and width determine which tires fit.
  • A tire's sidewall code shows its diameter, width, and sidewall height; the diameter must match your rim diameter exactly.
  • Your vehicle's door jamb or owner's manual lists the tire sizes the manufacturer approves for your specific model and year.
  • Tire width can vary by one or two sizes from your rim width without safety issues, but diameter must match exactly.
  • An online tire size chart or your tire shop can show you all approved sizes once you know your rim measurements.

Finding the three numbers on your rim

Look at the back or inner edge of your rim — the side facing the brake. You will see a series of numbers stamped or cast into the metal. The format is usually "diameter x width ET offset". For example, "17x7.5 ET45" means a 17-inch diameter, 7.5-inch width, and 45-millimeter offset.

If the numbers are hard to read, take a photo with your phone's flashlight and zoom in. Write down all three numbers. If you cannot find them, your tire shop can measure the rim in minutes, usually at no charge.

Reading the tire size code on the sidewall

A tire's sidewall shows a code like "225/45R17 91H". Each part means something specific. The "225" is the tire width in millimeters. The "45" is the sidewall height as a percentage of that width — so 45% of 225 millimeters. The "R" means radial construction (standard for all modern tires). The "17" is the rim diameter in inches. The "91H" is the load rating and speed rating; you do not need to match these to your rim, only to your vehicle's needs.

To check if a tire fits your rim, match the last number (rim diameter) to your rim's diameter. A tire marked "R17" fits a 17-inch rim. A tire marked "R18" does not fit a 17-inch rim, no matter what the other numbers say.

Why tire width can vary, but diameter cannot

Your rim width is a starting point, not a hard limit. Tire manufacturers make the same diameter in several widths. A 17-inch rim that is 7.5 inches wide can safely fit tires that are 225 millimeters wide, 235 millimeters wide, or 245 millimeters wide — usually within one or two sizes of the rim width. Wider tires sit slightly higher and change your speedometer reading slightly; narrower tires sit slightly lower.

The rim diameter, by contrast, must match exactly. A 17-inch rim accepts only 17-inch tires. A 18-inch tire will not fit a 17-inch rim, and forcing it can damage both the tire and the rim.

Your vehicle's manufacturer has tested specific tire sizes and approved them for your model and year. That list is the safest guide. Staying within the approved range keeps your speedometer accurate, your brakes effective, and your suspension working as designed.

Where to find your vehicle's approved tire sizes

Open your driver's side door and look at the door jamb — the vertical edge where the door closes. You will see a sticker with your vehicle's specifications. It lists the tire size the vehicle came with and often lists one or two alternative sizes the manufacturer approves. This sticker is the most reliable source.

If the sticker is missing or unreadable, check your owner's manual. The tire section usually lists approved sizes by model year. You can also search online for your vehicle's year, make, and model plus "tire size specifications" — manufacturer websites and major tire retailers publish this information.

Using an online tire size chart

Once you know your rim diameter and width, you can enter those numbers into a tire size chart on a tire retailer's website. Tire shops like Tire Rack, Discount Tire, and Costco all have charts that show which tires fit a given rim size. You enter your rim diameter and width, and the chart returns a list of tires in all the widths that fit.

These charts are informational — they show you the range of tires that physically fit your rim. They do not tell you which sizes your vehicle manufacturer approves. Cross-check the results against your door jamb sticker or owner's manual to stay within the approved range.

Some tire shops also offer a "tire finder" tool where you enter your vehicle's year, make, and model instead of your rim size. This approach skips the rim measurements and goes straight to the manufacturer's approved list, which is faster if you do not have your rim measurements handy.

What offset means and why it matters less for tire fit

The offset (ET number) tells you how far the rim sits from the wheel centerline. A positive offset pushes the rim outward; a negative offset pulls it inward. Offset affects how the wheel sits in the wheel well and how your suspension handles, but it does not affect which tire fits the rim.

A tire does not "know" the offset. The tire only cares about the rim diameter and width. You can put a tire on a rim with any offset, and the tire will fit. However, changing the offset can make the wheel stick out beyond the fender or tuck too far inward, which affects handling and may violate local vehicle regulations. If you are replacing rims, keep the offset close to your vehicle's original specification.

Frequently Asked Questions

Can I put a wider tire on my rim than the rim width?

Yes, within limits. A tire that is one or two sizes wider than your rim width usually fits safely. A 7.5-inch rim can fit 225, 235, or 245-millimeter tires. Wider tires sit slightly taller and may rub the fender if your vehicle is lowered. Check your vehicle's approved tire sizes to stay within the manufacturer's tested range.

What happens if I put the wrong diameter tire on my rim?

The tire will not fit properly. A smaller-diameter tire will not seat on the rim bead. A larger-diameter tire may fit loosely and can slip or separate while driving, which is dangerous. Always match the tire diameter to the rim diameter exactly.

Do I need to match the load rating on the tire to my rim?

No. The load rating (the number before the speed rating on the sidewall, like "91" in "91H") is about the tire's strength, not the rim's. Your vehicle's weight determines the load rating you need. Check your door jamb sticker or owner's manual for the minimum load rating your vehicle requires.

Can I use a tire size calculator instead of reading the numbers myself?

A calculator is only as good as the numbers you enter. If you enter your rim measurements correctly, a calculator will show you which tires fit. But you still have to read your rim and verify the numbers are right. Reading the rim yourself takes five minutes and removes the risk of typing an error.

What if my rims are not marked with numbers?

Some aftermarket rims do not have clear markings. A tire shop can measure your rim diameter and width with calipers in a few minutes, usually at no charge. Once you have those two numbers, you can find your tire size using any tire retailer's chart.