Decode the sidewall code, compare two sizes, see the speedometer truth
A tire code like 225/65R17 is three measurements in a trench coat: width in millimetres, sidewall height as a percentage of that width, and rim diameter in inches. This calculator decodes any metric code (or an imperial flotation size like 31×10.50R15) into sidewall height, overall diameter, width and circumference, then compares a second size against it: diameter difference in millimetres and percent, ground clearance change (half the diameter difference, the number that matters under the vehicle), revolutions per mile, and the speedometer correction, what your true speed is when the dial says 60 on the new tires. All nominal, from the code, with the real-world caveats stated in the limitations.
Current tire
Comparison tire (optional)
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Dimensions are nominal from the size code; actual tires vary by manufacturer and model within standards tolerances, and fitment depends on rim width, clearance and load rating, which a tire professional must confirm.
What this calculator does
For a metric size it computes sidewall height as width x aspect ratio, overall diameter as rim + two sidewalls, and circumference as pi x diameter, in millimetres and inches, with revolutions per mile and per kilometre. For an imperial flotation size it starts from the stated overall diameter and derives the sidewall from the rim. The comparison then differences the two overall diameters: the percent change is the gearing and speedometer change in one number, half the millimetre change is the clearance change under the vehicle, and multiplying an indicated speed by the diameter ratio gives the true speed on the new tires, the figure with legal consequences that most size swaps never compute.
How to use it
- Read the code off the sidewall, all of it. The format is width/aspect, a letter (R for radial), rim: 225/65R17. Load index and speed rating follow (like 102H) and matter for fitment, though not for dimensions; this calculator uses the dimensional part only and says so.
- The 3 percent rule of thumb. Common fitment guidance keeps overall diameter within about 3 percent of original to protect speedometer accuracy, gearing, gear-shift logic and driver aids calibrated to wheel speed. The comparison prints the percent so the rule can be applied rather than guessed.
- Clearance is half the difference. A tire 50 mm taller lifts the vehicle 25 mm and closes gaps to the arch and suspension by roughly that figure at the top; width gains close the side gaps instead, which is why both diameter and width changes print.
- Plus sizing keeps diameter, trades sidewall. Going up a rim size while dropping aspect ratio (plus one, plus two) preserves diameter and the speedometer; the calculator shows the trade directly when you compare the stock code against the plus-size code.
How it is calculated
- Metric: sidewall (mm) = width x aspect / 100; diameter (mm) = rim (in) x 25.4 + 2 x sidewall; circumference = pi x diameter; revolutions per mile = 1,609,344 / circumference (mm). Imperial: diameter (mm) = stated diameter (in) x 25.4; sidewall = (diameter – rim) / 2.
- Comparison: diameter change % = (new / old – 1) x 100; clearance change = (new diameter – old diameter) / 2; actual speed = indicated speed x new diameter / old diameter.
Worked examples
Decoding 225/65R17. Sidewall = 225 x 0.65 = 146.25 mm; rim 17 in = 431.8 mm; diameter = 431.8 + 292.5 = 724.30 mm (28.52 in); circumference 2,275.46 mm; revolutions per mile 707.26. Those five numbers are the tire for engineering purposes: gearing, speedometer and clearance all derive from them.
The common upsize. 245/70R17 against it: sidewall 171.50 mm, diameter 774.80 mm, which is +50.50 mm, +6.97 percent, lifting clearance by 25.25 mm. At an indicated 60 mph the vehicle is actually travelling 60 x 774.8 / 724.3 = 64.18 mph: an illegal surprise in strict enforcement territory and a warranty conversation with the driveline, delivered by a size swap many owners consider mild, which is why this comparison exists.
The flotation check. A 31×10.50R15 decodes straight from its stated diameter: 787.40 mm overall, sidewall (787.4 – 381) / 2 = 203.20 mm, circumference-based revolutions per mile 650.58, the low figure that gives tall off-road tires their relaxed gearing feel and their speedometer error in the other direction when replacing smaller stock rubber.
Reading your results
Read the percent first: inside about 3 percent, the swap is dimensionally conservative and the remaining questions are width, load rating and rim fit; outside it, gearing, shift points, speedometer and odometer (which over- or under-records mileage by the same ratio) all move, and the decision should be made knowingly. Then clearance, then the true-speed line, which is worth memorising for the size you actually fit: “at an indicated 60 I am doing 64” is the single most useful sentence a tire calculator can give a driver.
Limitations
- Nominal, not measured. Standards allow real tires to vary from nominal dimensions by model and manufacturer, and wear removes up to several millimetres of radius across a tire’s life; a tape measure on the actual tire beats the code for precision work.
- Loaded radius differs from free diameter. Under load the tire squats, so effective rolling circumference is a few percent below the pi-times-diameter figure; speedometer maths here uses the standard nominal convention every comparison tool uses, and GPS remains the arbiter.
- Fitment is more than diameter. Rim width range, offset, load index, speed rating, brake clearance and steering lock all gate a swap independently; this page dimensions the tire, it does not clear the fit.
- Drivetrain aids notice size changes. ABS, stability control, automatic transmissions and ADAS are calibrated around wheel speed and diameter ratios; large deviations can fault systems the arithmetic does not model.
Frequently asked questions
What do the numbers in 225/65R17 mean?
Width 225 mm across the tread, sidewall height equal to 65 percent of that width (146.25 mm, as the decoder shows), R for radial construction, and a 17-inch rim. The code is a hybrid, millimetres for the tire and inches for the wheel, a historical accident every driver inherits. From those three numbers the whole geometry follows: diameter 724.3 mm in the worked example, and with it circumference, revolutions per mile, and every comparison figure on this page.
How much can I change tire size before the speedometer is wrong?
The speedometer is wrong immediately, in exact proportion to the diameter change; the question is by how much you care about. The worked upsize, +6.97 percent diameter, turns an indicated 60 mph into a real 64.18 mph and makes the odometer under-record distance by the same 6.97 percent, which is why fitment guidance clusters around keeping changes within roughly 3 percent: at that level the error at 60 mph is under 2 mph, inside most drivers’ and regulators’ tolerance. The comparison prints your exact percent so the decision uses your numbers, not the rule’s comfort.
What is plus sizing?
Replacing a wheel and tire package with a larger rim and a lower-aspect tire of the same overall diameter: from 225/65R17, a plus-one package might be 245/60R18, trading sidewall height for rim while holding diameter (and therefore gearing and speedometer) nearly constant. The handling and appearance change; the dimensions should not, and comparing the two codes here verifies the match in millimetres rather than by the seller’s assurance.
Why do off-road tires use sizes like 31×10.50R15?
Flotation sizing states the two numbers off-roaders think in: overall diameter (31 inches) and width (10.50 inches), with the rim last. It skips the aspect-ratio indirection entirely, since sidewall falls out of diameter minus rim. The formats describe the same geometry, and this calculator accepts both so a metric stock size and an imperial upgrade can be compared directly, which is the comparison trail and truck owners actually run.
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Method: nominal geometry from the size code as printed in the method section; speedometer change as the diameter ratio. Last reviewed: 14 October 2026.