Tire Size Calculator
Compare any two tire sizes from their codes. Enter the width, aspect ratio, and rim size (e.g., 225/45R17) for your current and proposed tires to get a full dimension comparison, the diameter difference, and how far off your speedometer will read.
Read a code like 225/45R17 as: 225 mm section width / sidewall is 45% of width / R = radial / 17-inch rim. Load index and speed rating follow the size.
How tire dimensions come from the code
The three numbers tell you everything geometric. Sidewall height = width × aspect ratio (225 × 45% = 101.3 mm). Overall diameter = rim + two sidewalls. Circumference = π × diameter, which sets how many times the tire turns per mile — and that’s what your speedometer and odometer actually count.
Fit a taller tire and each revolution covers more road than the car assumes: the speedometer reads low and the odometer under-counts miles. A shorter tire does the opposite. The common industry guideline when up- or down-sizing is to stay within about ±3% of the original diameter so gearing, ABS, and stability-control calibrations stay happy — the classic “plus-size” trick pairs a bigger rim with a shorter sidewall to hold diameter nearly constant.
How it’s calculated
Sidewall = width × aspect ÷ 100 (mm). Diameter (in) = rim + 2 × sidewall ÷ 25.4. Circumference = πD; revolutions per mile = 63,360 ÷ circumference (in). Speedometer: actual speed = indicated × Dnew ÷ Dold.
Nominal geometry from the size code — real mounted diameter varies about 1% with brand, load, pressure, and tread wear.
Worked example
Upsizing from 225/45R17 to 245/40R18: diameter grows from 24.97″ to 25.72″ (+3.0%), sidewall shrinks from 101.3 mm to 98.0 mm, and revolutions drop from 808 to 784 per mile. At a true 60 mph your speedometer would read about 58.3 — equivalently, when it shows 60 you’re really doing 61.8 mph. That sits right at the edge of the ±3% guideline.
Common mistakes
- Comparing only widths — a 245/45R17 is both wider and 4% taller than a 225/45R17.
- Forgetting the aspect ratio is a percentage of width, not a height in millimeters.
- Ignoring speedometer/odometer drift after a size change (it also skews measured MPG).
- Skipping load index and speed rating — match or exceed the factory values when you change sizes.
Where it is used
- Checking whether a plus-size wheel package keeps overall diameter in spec.
- Explaining speedometer error after fitting taller all-terrains.
- Comparing winter set sizes that share the stock diameter.
- Estimating gearing/effective final-drive changes for track or towing use.
Frequently asked questions
How do I read a tire code like 225/45R17 91W?
225 = section width in mm; 45 = sidewall height as a % of width; R = radial construction; 17 = rim diameter in inches. The trailing 91W is the load index (91 ≈ 1,356 lb per tire) and speed rating (W = 168 mph).
How much size difference is safe?
The widely used guideline is to keep the new overall diameter within about ±3% of stock so ABS, traction control, and gearing stay calibrated. Beyond that, expect speedo error, possible rubbing, and in some vehicles warning lights.
Will bigger tires make my speedometer read fast or slow?
Slow. A taller tire covers more ground per revolution, so the car counts fewer revolutions than reality — the speedometer under-reads and the odometer logs fewer miles than you drive (which also makes computed MPG look worse).
Do wider tires change the diameter?
Only through the sidewall: height = width × aspect %. Going 225→245 at the same 45 aspect adds 9 mm of sidewall each side (+0.71″ diameter). To widen without height change, drop the aspect ratio — 245/40 nearly matches 225/45.
Does this work for LT and flotation sizes?
Metric LT sizes (e.g., LT265/70R17) use the same math. Flotation sizes like 33×12.50R15 state the diameter directly — 33″ — so no conversion is needed for the height.