Tire Size Calculator

Scenario 1 — Daily driver checking OEM spec

Inputs. 225/45R17 (factory spec on a 2022 sedan)

Output. Overall diameter 25.0 in / 634.3 mm. Circumference 78.5 in / 1,993 mm. Sidewall 101.25 mm. Revolutions per mile ≈ 808.

Interpretation. Confirms the factory tire dimension so the owner can buy a same-size replacement and avoid the speedometer drift, ABS recalibration and odometer error that come with an out-of-spec swap. Any candidate replacement should have an overall diameter within ±3% of 25.0 in.

Scenario 2 — Truck owner planning a plus-size lift

Inputs. Stock 265/70R17 vs. proposed 285/75R17

Output. Stock diameter 31.61 in. New diameter 33.83 in (+7.0%). Speedometer reads ~7% slow at all speeds. RPM-per-mile drops from ~638 to ~596.

Interpretation. The +7.0% diameter exceeds the ±5% acceptable speedometer error window. The driver should plan an ECU/speedometer recalibration via a programmer, or pick a smaller plus size such as 285/70R17 (~32.7 in, +3.4%) that stays inside the ±5% band.

Scenario 3 — EV owner comparing winter vs summer tires

Inputs. Summer 235/40R19 vs winter 225/55R17

Output. Summer diameter 26.40 in / 670.6 mm. Winter diameter 26.74 in / 679.3 mm. Δ diameter +1.3%. Speedometer error ~1.3%. RPM-per-mile delta ≈ 10.

Interpretation. Both sets sit comfortably inside the ±3% window, so the owner can swap seasonal sets freely without speedometer recalibration or ABS re-learn issues. The slightly taller winter tire also gives a marginally lower engine RPM at highway speeds, which is a small range bonus on an EV.

Scenario 4 — Cyclist verifying ETRTO marking

Inputs. ETRTO 37-622 (700×35C in legacy units)

Output. 37 mm casing on a 622 mm bead-seat rim. Outer diameter ≈ 696 mm. Circumference ≈ 2,186 mm — useful for setting wheel circumference in cycle computers.

Interpretation. The ETRTO numbers are the ground truth for tire-rim compatibility; the 700×35C marking is a marketing label on top of the same dimensions. Bicycle sizes use direct millimetre dimensions rather than the P-metric aspect-ratio system, so they fall outside the input range of this calculator (which targets the 100–355 mm width / 10–24 in rim window of automotive P-metric tires). For bicycle wheel circumferences use the formula directly: outer_diameter_mm = bead_seat_diameter_mm + 2 × tire_section_width_mm.

Drivers in North America expect inches and miles per hour while European drivers expect millimetres and kph. Always sanity-check which output column you are reading. A 5% diameter delta is the same number in either system, but the absolute numbers (25.0 vs 634 vs 1,993) live on totally different scales and it is easy to confuse circumference with diameter at a glance.

A larger replacement tire makes the speedometer read lower than the actual road speed because the tire turns fewer times per mile. Many DIY guides describe this as "negative speedometer error" or "+5% indicated", which causes confusion. Read the result as actual_speed = (new_diameter / old_diameter) × indicated_speed and you will never sign-flip.

Codes like 225/45ZR17 94W or 225/45R17 XL include construction (Z, ZR), reinforcement (RF, XL) and load/speed tokens (94W). The dimensional math ignores all of them — the parser strips everything except width, aspect and rim. If you type the full code into the paste field above the calculator picks out the three numbers automatically.

Light-truck flotation sizes like 33×12.50R15LT do not use a percent aspect ratio at all — the 12.50 is section width in inches and 33 is overall diameter in inches. The metric formula does not apply. Convert manually: 33 in × 25.4 = 838 mm overall diameter, then back-derive sidewall as (838 − 15 × 25.4) / 2 = 228.4 mm.

Bicycle, trailer and some commercial sizes use fractional rim diameters like 19.5" or 24.5". The calculator above accepts decimals — type 19.5 directly. Using an integer rounded value will under- or over-state diameter by roughly 12 mm per half-inch of rim error, which is bigger than the tolerance for any homologated replacement.

Codes commonly look like 225/45R17 94W — the 94W is load-index 94 (670 kg per tire) and speed rating W (270 km/h). When swapping tires, the new code must have an equal-or-greater load index and speed rating to remain road-legal in most jurisdictions. The dimensional math is unchanged either way, but the safety/legal envelope is not.

If you accidentally enter 0 for any of the three numbers the calculator surfaces an inline error rather than dividing by zero. Section width must be positive, aspect ratio must be positive, and rim diameter must be positive — and all three must be in the realistic ranges (100–355 mm width, 25–95 aspect, 10–24 in rim) for the calculator to accept the inputs.

Wheel offset and backspacing

Even when overall tire diameter matches OEM, a different wheel offset (ET, in millimetres) can push the tire into the inner suspension or out past the fender. ET +35 means the mounting face sits 35 mm outboard of wheel centreline. Plus-size builds usually combine a wider tire with a slightly lower offset; verify clearance with a wheel-specific fitment guide before ordering.

Load index and speed rating

The numbers and letters after the size code (94W in 225/45R17 94W) define maximum load per tire and sustained speed. Load index 94 = 670 kg; speed rating W = 270 km/h. Replacement tires must equal or exceed both OEM values; downgrading either is a common cause of failed safety inspections in EU and AU markets.

Rolling circumference vs geometric circumference

Manufacturers publish a "revolutions per kilometre" or rolling-circumference figure that is 1–3% smaller than the geometric value the calculator computes, because tires deflect under vehicle load. For homologation work use the manufacturer figure; for plus-size planning the geometric value is more than accurate enough.

TPMS and ABS recalibration

Many vehicles need their TPMS sensors re-paired after a new wheel set is installed, and some require an ABS/ECU re-learn after a meaningful tire-diameter change so wheel-speed signals stay calibrated. Modern direct-TPMS sensors auto-relearn within ~10 minutes of driving; older systems need a magnet wand or OBD-II tool.

The number 225 is the section width in millimetres — the widest point of the tire when mounted. The number 45 is the aspect ratio: sidewall height equals 45% of section width, so this tire has a 101.25 mm sidewall. The letter R indicates radial construction (the dominant passenger-car standard since the 1970s) and 17 is the rim diameter in inches. The size code is defined by the European Tyre and Rim Technical Organisation (ETRTO) and the U.S. Tire and Rim Association.

Compute the overall diameter for each tire and compare. Two sizes are practically equivalent when the diameter difference is within ±3% — that keeps the speedometer accurate, the ABS happy and the suspension clear. Anything beyond ±5% will produce a noticeable speedometer error and may cause clearance, gearing or driveline issues. Manufacturers usually homologate replacement sizes within ±2%.

Yes. Vehicle speedometers are calibrated to a specific tire diameter, so a larger replacement tire makes the dash read lower than the actual speed (you are going faster than the speedometer shows by roughly the same percentage that the diameter increased). A +3% diameter swap means the dash reads 60 mph while you are actually doing 61.8 mph. Most factory speedometers themselves are calibrated to read up to about 2.5% high, so a small over-tire change can simply cancel that bias.

Multiply the section width by the aspect ratio (as a decimal). For 265/70R17 the sidewall height is 265 × 0.70 = 185.5 mm. The overall diameter is then rim_diameter_inches × 25.4 + 2 × sidewall_mm = 17 × 25.4 + 371 = 802.8 mm, which is 31.61 inches. The same arithmetic underlies every tire dimension chart from the Inch Calculator and TireSize.com.

Plus-sizing means moving up one or more rim diameters (a "Plus 1" or "Plus 2") while reducing the aspect ratio so that overall tire diameter stays roughly constant. The goal is sharper handling and a more aggressive look while preserving speedometer accuracy and ABS calibration. The swap is safe when the new diameter is within ±3% of stock, the load index is equal to or greater than original equipment, and the speed rating is at least equal to the OEM specification.

Use the formula 63,360 ÷ (overall_diameter_inches × π). A 225/45R17 with a 24.97-inch diameter turns about 808 revolutions per mile; a 33.83-inch 285/75R17 turns about 596. Manufacturers publish a slightly lower "rolling circumference" or "revs per kilometre" figure because tires deflect under load — typically 1–3% smaller than the geometric value computed here.

Slightly. A larger overall diameter lowers engine RPM at any given speed, which can improve highway fuel economy by a fraction of a percent. However, larger tires also weigh more, present a bigger contact patch and often have more aggressive tread, so real-world economy usually drops by 1–3% after a meaningful plus-size or tall off-road swap. Aerodynamic drag from wider, taller tires further offsets the gear-ratio advantage.

Section width is the widest point of the tire including the sidewall bulge, measured on a specified rim — that is the number printed in the size code. Tread width is the narrower, flat band that actually contacts the road, typically 75–90% of section width. Manufacturer datasheets list both. When fitting wheels, section width matters for clearance and tread width matters for grip and tire-wear behaviour.