How is overall tire diameter calculated from a P-metric size?

The three-number code works like this: 225 is the section width in millimetres, 45 is the aspect ratio as a percentage of the section width, and 17 is the rim diameter in inches. Sidewall height = 225 × 0.45 = 101.25 mm. There are two sidewalls, one on each side of the rim. Overall diameter (mm) = rim diameter in mm + 2 × sidewall = (17 × 25.4) + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm (about 24.97 inches).

What is plus sizing and does it affect speedometer accuracy?

Plus sizing means fitting a wider, lower-profile tire on a larger-diameter rim while keeping the overall diameter nearly the same. For example, going from 225/45R17 to 235/40R18 adds 1 inch of rim but reduces sidewall height so the total diameter barely changes. When the overall diameter stays the same, your speedometer stays accurate. If the new diameter is larger, your true speed is higher than the dash shows; if smaller, your true speed is lower.

How does changing tire diameter affect the speedometer?

Your vehicle speed sensor (VSS) or ABS ring counts wheel revolutions and assumes the factory-calibrated tire circumference. A larger tire travels a greater distance per revolution, so the real speed is higher than the indicated speed by the ratio of diameters. True speed = indicated × (original diameter / alternate diameter).

What is revolutions per mile and why does it matter?

Revolutions per mile (RPM) is how many times a tire rotates in one mile: 63360 inches ÷ tire circumference in inches. A lower RPM means the tire covers more distance per revolution (larger tire). This metric affects odometer accuracy, ABS calibration, and traction-control thresholds — so fitting tires with a significantly different RPM can trip warning lights or read mileage incorrectly.

How much diameter difference is safe when fitting alternate tires?

Most tyre manufacturers and vehicle engineers recommend staying within ±3% of the original overall diameter. Beyond ±3% you risk clearance issues in the wheel arch, inaccurate speedometer and odometer readings, ABS/stability-control errors, and altered gearing that changes acceleration and fuel economy. The calculator flags differences over 20 mm as large.

Does the calculator work for metric (LT, ST) and non-standard sizes?

Yes. Any size in the standard P-metric or LT-metric format — section width in mm, aspect ratio as a percentage, and rim in inches — works directly. Older alpha-numeric sizes (like 195/65R15) use the same calculation. Numeric sizes (6.00-16) used on classic vehicles use a different conversion and are not directly supported by this tool.

Tire Diameter Calculator

The tire diameter calculator decodes a P-metric tire code into six real dimensions — overall diameter, sidewall height, circumference, and revolutions per mile/km — and lets you compare two fitments side by side so you can see at a glance whether a plus-size swap or a budget-brand substitute keeps you within the safe ±3 % diameter window.

How the P-metric formula works

A tire code like 225/45R17 packs three measurements into one string:

Part	Meaning	Value
225	Section width	225 mm
45	Aspect ratio (% of width)	45%
R17	Rim diameter	17 inches

Sidewall height is simply width × (aspect ÷ 100):

sidewall = 225 × 0.45 = 101.25 mm

Overall diameter adds two sidewalls to the rim converted to millimetres (1 inch = 25.4 mm):

diameter = (17 × 25.4) + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm (24.97”)

Circumference follows from pi times diameter:

C = π × 634.3 = 1,991.9 mm (78.43”)

Revolutions per mile uses the fact that one mile = 63,360 inches:

rev/mile = 63,360 ÷ 78.43 = 807.8

Worked example — plus sizing from 225/45R17 to 235/40R18

Dimension	225/45R17	235/40R18	Difference
Sidewall	101.3 mm	94.0 mm	−7.3 mm
Diameter	634.3 mm	645.2 mm	+10.9 mm (+1.7%)
Circumference	1992.0 mm	2027.0 mm	+35 mm
Revs/mile	807.6	794.0	−13.6
Revs/km	501.7	493.4	−8.3

A 1.7% diameter increase is well within the safe ±3% window. At an indicated 100 km/h, your true speed is 98.3 km/h — the speedo reads slightly high because the larger tire covers more ground per revolution.

Speedometer correction

Your vehicle was factory-calibrated using the number of revolutions per unit distance for the OEM tire. When you fit a larger tire, each revolution covers more ground, so the revolution counter underestimates speed:

true speed = indicated speed × (original diameter ÷ alternate diameter)

A 5% larger-diameter tire makes you travel 5% faster than the dash shows. At a motorway limit of 70 mph, that 5% gap is 3.5 mph — enough to affect legality in a speed camera dispute, so always verify with a GPS app after fitting non-OEM sizes.

Formula note

All calculations follow the SAE J2564 P-metric standard. Section width is measured at maximum inflation (not mounted width, which is typically 6–8 mm narrower). Actual rolling radius under load is 1–2% less than the static radius used here, which is why tire manufacturers publish a “loaded radius” separately for precise tachograph/odometer work.