How to read a tire size
Every tire has its size molded into the sidewall, in a format like 225/45R17. The same notation is used around the world:
- 225 is the section width in millimeters, measured at the widest point of the sidewall.
- 45 is the aspect ratio: the sidewall height as a percentage of the width. Here the sidewall is 45% of 225 mm, or about 101 mm.
- R means radial construction, which almost every modern car tire uses.
- 17 is the wheel (rim) diameter in inches.
You'll often see more characters after the size, such as 94W. The number is the load index (how much weight each tire can carry) and the letter is the speed rating. A replacement tire should match or exceed both.
The tire diameter formula
Sidewall = width × aspect ratio ÷ 100
Overall diameter = rim diameter + 2 × sidewall
Because the width is in millimeters and the rim is in inches, convert one of them first. One inch is 25.4 mm.
Worked example: going from 225/45R17 to 235/40R18. The current sidewall is 225 × 0.45 = 101.25 mm (3.99 in), so the diameter is 17 + 2 × 3.99 = 24.97 in. The new sidewall is 235 × 0.40 = 94 mm (3.70 in), so the diameter is 18 + 2 × 3.70 = 25.40 in. The new tire is 1.72% larger, so when the speedometer reads 60 mph you'll actually be doing about 61 mph.
How much can tire size change?
The common guideline is to keep the new overall diameter within 3% of the original. Inside that range, speedometer and odometer error stay small, and the car's gearing, ABS and stability control behave as designed. Larger changes are possible, especially on trucks, but may need a lift, trimming or speedometer recalibration.
| Original size | Popular alternative | Diameter change |
|---|---|---|
| 205/55R16 (24.88 in) | 225/45R17 (24.97 in) | +0.4% |
| 225/45R17 (24.97 in) | 225/40R18 (25.09 in) | +0.5% |
| 225/45R17 (24.97 in) | 235/40R18 (25.40 in) | +1.7% |
| 265/70R17 (31.61 in) | 275/70R17 (32.16 in) | +1.7% |
| 265/70R17 (31.61 in) | 285/70R17 (32.71 in) | +3.5% |
Plus sizing: bigger wheels, same diameter
"Plus one" or "plus two" sizing means fitting a wheel one or two inches larger while lowering the aspect ratio so the overall diameter stays nearly the same. You get a sharper look and slightly crisper steering, but a lower sidewall also means a firmer ride, more road noise, and wheels that are easier to damage on potholes. Tires with low aspect ratios also tend to cost more.
Before you buy
- Check clearance. Wider or taller tires can rub on the fender liner or suspension at full lock or when the car is loaded.
- Match the load index and speed rating on your door-jamb placard, or exceed them.
- Replace in pairs or sets. On all-wheel-drive cars, mismatched diameters can strain the drivetrain; many makers require all four tires to match.
- Remember the odometer. A larger tire makes the odometer under-count miles, which can affect warranty and lease mileage records.
Frequently asked questions
Will bigger tires make my speedometer read wrong?
Yes. A taller tire covers more ground per turn, so the speedometer reads lower than your true speed. With a tire that's 3% larger, the speedometer shows 60 mph when you're doing about 61.8 mph. A smaller tire has the opposite effect.
Can I put 235 tires instead of 225?
Usually, yes. A 10 mm wider tire generally fits the same rim, as long as the rim width is within the tire maker's approved range and there's enough clearance. If you keep the same aspect ratio, the diameter also grows slightly, so check the result above.
What does the 45 mean in 225/45R17?
It's the aspect ratio: the sidewall height is 45% of the tire's width. A 225 mm wide tire with a 45 aspect ratio has a sidewall about 101 mm (4 in) tall.
Is a 3% tire size difference OK?
Within 3% of the original overall diameter is the widely used safe range. Beyond that, speedometer error grows and clearance, gearing and electronic safety systems may be affected.
Results are calculated from the nominal size. Actual tire dimensions vary slightly by brand and model; check the maker's specifications and your vehicle's placard before buying.