Tire Diameter Calculator for P and LT Sizes
Turns a marking like 205/55R16 into overall diameter in mm: width x aspect/100 x 2, plus rim inches x 25.4. That size comes to 631.9 mm.
Diameter (mm)
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Tire diameter (LT/P designation)
A modern tire marking like 205/55R16 packs three numbers into a string. 205 is the section width in millimeters. 55 is the aspect ratio, the sidewall height as a percentage of the width. R flags radial construction, and 16 is the rim diameter in inches. From there the overall diameter is D = 2·(width·aspect/100) + rim·25.4 (in mm). LT (Light Truck) and P (Passenger) sizes use the very same rule; the prefix only sets the load and pressure spec, not the geometry.
Why it matters: changing tire diameter changes the effective rolling circumference, which makes the speedometer and odometer read incorrectly. Common designation prefixes are P (Passenger), LT (Light Truck), C (Commercial van) and ST (Special Trailer). Speed ratings sit at the end of the marking: Q = 160 km/h, V = 240, Y = 300, ZR > 240 km/h.
Real-world applications
Used when replacing tires (keep the overall diameter within roughly ±3% of the OEM size to preserve speedometer accuracy and ABS calibration), planning a rim upsize (lower aspect ratio to compensate for the larger rim, keeping total height), tuning, and off-road builds where a larger diameter buys ground clearance but forces speedometer recalibration and may change the gear ratio behavior.
FAQ
What is the "profile" or aspect ratio? It is the sidewall height expressed as a percentage of the section width. A 55-series tire has a sidewall equal to 55% of the width.
Can I mix LT and P on the same car? Not recommended. LT tires have stiffer sidewalls and higher load indices for trucks; P tires give a more comfortable ride and are calibrated for passenger cars.
How much diameter change is safe? A common rule of thumb is ±3%. Beyond that, the speedometer error grows, and the ABS/ESC may need recalibration.
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Bearing Mean Diameter
Calculate a bearing's mean (pitch) diameter, d_m = (D + d) ÷ 2, from the outer diameter D (mm, of the outer ring) and the inner diameter d (mm, of the bore, fitting the shaft). The mean diameter is the average of the bore diameter (seating on the shaft) and the outer diameter (seating in the housing), and roughly represents the diameter of the CIRCLE described by the rolling-element centers (the pitch diameter). It is a fundamental bearing geometric parameter, used in several calculations: in the SPEED FACTOR n·d_m (governing limit speed and heating), in estimating the rolling-element peripheral velocity, in the characteristic defect frequencies (used in vibration analysis for diagnosis — the ball-pass frequencies of inner/outer race, BPFI/BPFO, depend on d_m), and in the cage rotation speed. The mean diameter is the compact way to characterize a bearing's 'size' for these kinematic and dynamic calculations, without needing the internal details (number and diameter of rolling elements, contact angle). The outer D and inner d diameters are the basic catalog dimensions of any bearing (with the width), and d_m derives directly from them. Enter the outer and inner diameters.
Gear Base Diameter
Calculate the base circle diameter of an involute gear, d_b = d·cos(φ), from the pitch diameter d (mm) and the pressure angle φ (degrees). The base circle is the circle from which the INVOLUTE tooth profile is generated — the standard profile of modern gears. The involute is the curve traced by the tip of a string unwinding from a cylinder: that cylinder is exactly the base circle. The entire active tooth profile (the part that actually transmits force) is ABOVE the base circle; below it there is no involute profile. The base diameter is fundamental in gear geometry because it defines the involute profile and, with it, key properties: the LINE OF ACTION (the line tangent to both base circles of the mesh, along which tooth contact travels, always in the same direction — why involute gears transmit uniform motion), the base pitch and the contact ratio. The relation d_b = d·cos(φ) shows that the pressure angle is the angle between the line of action and the tangent to the pitch circles. It is an essential parameter in designing and manufacturing (generating) involute gears. Enter the pitch diameter and the pressure angle.
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Blank Diameter for Cup
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.