TireGeoCalcHow we calculate
Our calculator uses the dimensions written in the tire size code. It calculates nominal geometry, not a measurement of a particular tire or a vehicle fitment decision.
Metric sizes
For 225/65R17, width is 225 mm, aspect ratio is 65%, and rim diameter is 17 inches. One inch is exactly 25.4 mm. One mile is exactly 1,609,344 mm.
- Sidewall height = width × aspect ratio ÷ 100.
- Diameter = rim diameter × 25.4 + 2 × sidewall height.
- Circumference = π × diameter.
- Revolutions per mile = 1,609,344 ÷ circumference in mm.
- Revolutions per km = 1,000,000 ÷ circumference in mm.
Flotation sizes and comparisons
For a flotation code such as 35x12.50R17, diameter and width are already in inches. Sidewall height is (overall diameter − rim diameter) ÷ 2. The same circumference formula then applies.
Size change (%) = (replacement diameter ÷ original diameter − 1) × 100. With the original speedometer calibration retained, calculated actual speed = indicated speed × replacement diameter ÷ original diameter. This is a geometric estimate; it does not measure the instrument.
Rounding
The engine keeps full precision until display. Standard display helpers round mm to 1 decimal, inches to 2, percentages to 2, revolutions to 1 and speed to 1. Halfway values round away from zero. Some comparison pages show speed to 2 decimals for readability. Manufacturer figures keep their published precision; adding a trailing zero does not add measurement precision.
Reference tables
The load index chart and speed rating chart reproduce public manufacturer reference values from Toyo load data and Toyo speed data, checked . Pounds are kg ÷ 0.45359237, rounded to a whole pound. Mph is km/h ÷ 1.609344, rounded to a whole mph. These conversions do not create new load or speed ratings.
How we check manufacturer figures
We compare 50 nominal metric dimensions with published diameter specifications from two manufacturers per dimension. Each observation records the model, product code, original designation, service description, source link and check date. This is a selected reference sample, not a ranking of sales or a list of every available model. A Canadian manufacturer catalog may supply a reference model; check local product availability separately.
Difference (%) = (published value ÷ calculated value − 1) × 100. A positive difference means the published value is larger. Diameter differences above 2% are marked in the comparison table. The calculated width, sidewall, diameter, circumference and revolutions all use the same existing geometry engine.
Why published figures can differ
The size code gives nominal dimensions. Tread design, construction, approved measuring rim width and manufacturing tolerances affect actual diameter and width. Wear reduces tread height. Inflation pressure, load and rim width affect the tire's shape. Rolling revolutions per mile are measured under load and are not simply the inverse of unloaded geometric circumference.
Do not treat a small diameter difference as approval for a vehicle. Check the vehicle manual, tire placard, exact tire specifications and wheel requirements with a qualified installer. Read the disclaimer.