A tire size conversion chart is useful when you are comparing the metric size on a passenger vehicle tire with a flotation or inch-based size often used on light trucks, SUVs, and off-road tires. It can show whether two sizes are close in nominal overall diameter and width, but it cannot confirm that either tire is safe or appropriate for your vehicle. Before changing sizes, compare the tire placard and owner’s manual requirements with the proposed tire’s load index, speed rating, construction, approved wheel-width range, inflation requirements, and clearance at full steering lock and suspension travel.
Most tire size conversion charts translate between three common formats: metric, flotation, and older inch-based passenger-car sizing. The goal is to compare the three basic dimensions that influence fitment: section width, sidewall height, and overall diameter.
For example, a metric tire marked 265/70R17 and a flotation tire marked 32x10.50R17 may appear in the same general size range. That does not mean they are interchangeable. The 265/70R17 designation provides a nominal width and aspect ratio, while 32x10.50R17 provides approximate diameter and width. The actual specifications must be checked in the tire manufacturer’s data for the exact models under consideration.
| Size format | Example | What the numbers communicate | Where you commonly see it |
|---|---|---|---|
| Metric | 225/65R17 | 225 mm nominal width, 65% sidewall ratio, radial construction, 17-inch wheel | Cars, crossovers, minivans, SUVs, and many trucks |
| Flotation | 31x10.50R15 | About 31-inch overall diameter, 10.50-inch nominal width, radial construction, 15-inch wheel | Light trucks, Jeeps, off-road and all-terrain applications |
| Older inch-based | P235/75R15 | Metric width and aspect ratio with a wheel diameter in inches; the P indicates passenger-car type | Older vehicles and some current replacement listings |
The R in these examples means radial construction. It does not identify load capacity, tire type, tread pattern, winter capability, or whether the tire is approved for your wheel. Those details appear elsewhere on the sidewall or in the manufacturer’s specifications.
A metric tire size contains enough information to estimate its nominal dimensions. Take 225/65R17:
To estimate the sidewall height, multiply the width by the aspect ratio. For a 225/65R17 tire, 225 mm multiplied by 0.65 equals 146.25 mm. Divide that by 25.4 to convert millimeters to inches, giving a nominal sidewall height of about 5.76 inches.
Overall diameter is estimated by adding two sidewalls to the wheel diameter:
Overall diameter = wheel diameter + (2 × section width × aspect ratio ÷ 25.4)
Using the same example, the estimate is 17 + (2 × 225 × 0.65 ÷ 25.4), or roughly 28.5 inches. This is an estimate, not a guaranteed mounted dimension. Tire makers publish a tire’s measured overall diameter and section width, typically tied to a stated measuring rim width.
Flotation sizing is more direct to read but still approximate. A tire marked 33x12.50R17 is generally described as approximately 33 inches tall, 12.50 inches wide, and intended for a 17-inch wheel. Unlike a metric designation, it does not state the sidewall ratio.
You can estimate its sidewall height with this calculation:
Sidewall height = (overall diameter − wheel diameter) ÷ 2
For a 33x12.50R17 tire, the estimated sidewall is (33 − 17) ÷ 2, or 8 inches. That thicker sidewall can be attractive for rough-road use, but it also changes handling response, weight, clearance needs, and sometimes braking and acceleration behavior compared with a lower-profile street tire.
Flotation sizes are often associated with truck and off-road tires, yet they should not be treated as automatic upgrades. A wider or taller tire may contact the fender liner, suspension, brake components, or bodywork. It may also exceed the wheel’s approved width range or reduce clearance around the spare-tire location.
The chart below offers approximate cross-references for research. It is best used to narrow a tire search, not to select a final replacement size. Always compare the exact manufacturer-listed diameter, section width, revolutions per mile, load index, and approved rim-width range before ordering.
| Approximate flotation size | Common metric sizes in a similar range | Approximate diameter range | Wheel diameter | Fitment caution |
|---|---|---|---|---|
| 27x8.50R14 | 205/75R14, 215/70R14 | About 26 to 27 inches | 14 inches | Widths and actual diameters can differ noticeably. |
| 28x8.50R15 | 215/75R15, 225/70R15 | About 27 to 28 inches | 15 inches | Check wheel width and suspension clearance. |
| 30x9.50R15 | 235/75R15, 245/70R15 | About 29 to 30 inches | 15 inches | May alter speedometer readings if replacing a smaller original tire. |
| 31x10.50R15 | 265/70R15, 275/60R15 | About 30 to 31 inches | 15 inches | Compare actual tread and section width, not the nominal 10.50-inch label alone. |
| 32x11.50R17 | 265/70R17, 275/65R17 | About 31 to 32 inches | 17 inches | Steering and fender clearance deserve close attention. |
| 33x12.50R17 | 285/70R17, 315/70R17 | About 32 to 34 inches | 17 inches | Nominal matches can vary substantially by tire family and wheel width. |
| 35x12.50R17 | 315/70R17, 325/70R17 | About 34 to 35 inches | 17 inches | Often requires a vehicle-specific clearance and fitment assessment. |
A conversion chart may show more than one metric option beside a flotation size because there is rarely a single exact equivalent. For instance, two metric tires can have similar overall diameter while differing in section width and sidewall shape. The wheel width used for measurement can further change the mounted profile.
Overall diameter is often the first number drivers compare because it affects ride height, gearing, and speedometer accuracy. It matters, but it is only one part of fitment. A tire can be close in diameter and still be too wide for the wheel, too wide for the steering clearance, or unable to meet the vehicle’s required load capacity.
Replacement tires must meet the vehicle manufacturer’s required load capacity. A physically larger tire does not automatically carry more weight. Load index, load range where applicable, and inflation pressure determine the tire’s rated capacity. Passenger-type and light-truck-type tires may also be evaluated differently in vehicle applications, so compare the load information directly rather than assuming a size conversion provides an equivalent rating.
Speed rating should meet the vehicle requirement and the operating conditions stated in the owner’s manual. A size conversion chart cannot tell you whether a tire is suitable for your driving needs, such as frequent highway travel, towing, winter use, or off-road service. Tread design and tire category matter as much as the size printed on the sidewall.
Every tire model has an approved wheel-width range. Mounting a tire on a wheel outside that range can distort the tread and sidewall, affect handling, and create service concerns. Do not choose a wheel simply because its diameter matches the final number in the tire size. A 17-inch tire needs a 17-inch wheel, but the wheel’s width, offset, condition, and bead-seat design still matter.
A tire may clear while parked and still rub during turns, bumps, braking, cargo loading, or suspension compression. On four-wheel-drive and off-road vehicles, clearance can change significantly with steering angle and axle articulation. Check the full range of movement, including clearance to brake hoses, suspension parts, fender liners, and the spare-tire carrier where relevant.
When comparing an alternative with the original tire, calculate the percentage difference in nominal overall diameter:
Percentage difference = (new diameter − original diameter) ÷ original diameter × 100
This calculation helps show how much the rolling diameter may change, but it is not a pass-or-fail rule. A small diameter change can still create a clearance problem on a tight-fitting vehicle, and a larger change may have implications for vehicle systems, gearing, spare-tire fit, and required calibration. Use actual manufacturer specifications whenever possible instead of dimensions estimated from the sidewall code.
Also compare tire revolutions per mile if the manufacturer provides them. This figure reflects the tire’s rolling behavior under a defined test condition and can be useful when evaluating speedometer or drivetrain differences. It should still be treated as a product-specific specification rather than a universal number for every tire bearing the same nominal size.
Metric sizing is usually the simplest choice for drivers replacing original-equipment tires on cars, crossovers, and stock trucks. It aligns closely with most vehicle placards and makes it easier to retain the intended diameter, load rating, and wheel fitment. Choose the original metric size when dependable everyday operation and straightforward replacement are the priority.
Flotation sizing can make sense for drivers researching certain light-truck, all-terrain, mud-terrain, or off-road tire options. Its stated diameter and width can be convenient for comparing larger tire packages, but the nominal format may hide meaningful differences between models. Choose it only after verifying the exact tire’s specifications and vehicle clearance.
| Priority | Usually the better starting point | Main advantage | What to verify |
|---|---|---|---|
| Stock replacement | Original placard-listed metric size | Most direct path to intended fitment | Load index, speed rating, tire type, and current tire condition |
| Maintaining similar diameter on a different wheel | Metric size calculated for the new wheel diameter | Allows sidewall and wheel changes while targeting a similar overall diameter | Approved rim width, offset, brake clearance, and actual tire diameter |
| Truck or off-road fitment research | Flotation size comparison | Quick comparison of nominal height and width | Clearance through suspension travel, load capacity, wheel suitability, and driveline effects |
| Towing or heavy payload use | Vehicle-specified size and load configuration | Helps preserve required load capability | Placard requirements, tire load rating, inflation guidance, and axle limits |
Possibly, but only if the proposed tire is appropriate for the wheel and vehicle. Compare its actual overall diameter, section width, approved wheel-width range, load index, speed rating, and clearance rather than relying on a nominal conversion alone. A qualified tire professional should inspect unusual or upsized fitments.
A tire size conversion chart is accurate enough for broad comparisons, especially when identifying similar diameter ranges. It is not precise enough to predict the installed dimensions of every tire model. Construction, tread design, load range, and the wheel used for measurement can all change the final dimensions.
Often, but the correct wheel width depends on the tire manufacturer’s approved rim-width range for that exact tire. A wider tire mounted on an unsuitable wheel can change sidewall shape and handling characteristics. Check the tire data sheet and your wheel specifications before mounting.
A larger rolling diameter generally means the vehicle travels farther per wheel revolution, so the speedometer and odometer may read lower than the actual speed and distance. A smaller rolling diameter generally has the opposite effect. The amount depends on the real difference in rolling circumference, not just the sidewall label.
Only use different front and rear sizes when the vehicle manufacturer specifies a staggered arrangement or approves the combination. On many all-wheel-drive and four-wheel-drive vehicles, tire circumference differences can be especially problematic. Follow the owner’s manual and vehicle-specific service guidance.
For most vehicles, matching tires at all four positions is the simplest way to maintain predictable handling and consistent rolling circumference. Some vehicles permit other arrangements, but tire size, type, and tread-depth requirements can be stricter for all-wheel-drive systems. Check the vehicle manufacturer’s instructions before mixing tires.
A tire size conversion chart can quickly identify metric, flotation, and inch sizes that are in a similar dimensional range. Use it to build a shortlist, then verify the exact tire specifications against the vehicle placard, owner’s manual, wheel requirements, and real clearance needs. If a proposed size changes diameter, width, wheel size, or tire type, get a vehicle-specific fitment check before buying or mounting the tires.