Tire Size Change & Speedometer Error Calculator
Compare an old and new tire size to see the change in overall diameter and exactly how far off your speedometer and odometer will read.
How much will a new tire size change my speedometer?
Your speedometer error equals the percentage change in overall tire diameter. Going from a 225/65R17 (about 28.5 in) to a 245/60R18 (about 29.6 in) is a ~3.7% increase, so the speedometer reads low by that much — at an indicated 60 mph you're really doing about 62.2 mph, and your odometer under-counts by the same percentage.
Your numbers
Speedometer error
3.7%
reads slow (you're going faster)
New overall diameter
29.57
inches
Current overall diameter
28.52
inches
Insight — Keep a tire-size change within about 3% of the original diameter to stay inside most speedometer, ABS and traction-control tolerances. A taller tire makes the speedometer under-read — the actual speed above is higher than the dash shows — and your odometer records fewer miles than you drive.
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For context: going from a 225/65R17 to a 245/60R18 raises overall diameter from about 28.5 to 29.6 inches — roughly a 3.7% increase, so at an indicated 60 mph you're actually doing about 62.2 mph. Enter your own sizes above.
How this calculator works
The calculator converts each tire's width, aspect ratio and wheel diameter into a single overall diameter in inches, then compares them. Because a factory speedometer is calibrated to the original tire's circumference, any difference in diameter shows up directly as speedometer and odometer error. It reports the error percentage, both diameters and your true speed at an indicated 60 mph.
Formula
Overall diameter(in) = wheel + 2 × (width × aspect ÷ 100) ÷ 25.4. Speedometer error % = (new − old) ÷ old × 100. True speed = indicated × new ÷ old.
Worked example
225/65R17: 17 + 2 × (225 × 0.65) ÷ 25.4 ≈ 28.5 in. 245/60R18: 18 + 2 × (245 × 0.60) ÷ 25.4 ≈ 29.6 in. Error = (29.6 − 28.5) ÷ 28.5 × 100 ≈ 3.7%. True speed at 60 indicated ≈ 62.2 mph.
What affects your result
- →Tread width (mm)
- →Aspect ratio (sidewall height %)
- →Wheel diameter (in)
- →The resulting overall diameter difference
- →Your speedometer's factory calibration
What this estimate includes
- ✓Old and new overall diameter
- ✓Speedometer and odometer error percentage
- ✓True speed at an indicated 60 mph
- ✓The raw diameter change
What it does not include
- ×Load-rating and fitment/clearance checks
- ×Real rolling radius under load and wear
- ×ABS / stability-control recalibration
- ×Rim width and offset effects
Good to know
Clear, practical answers about the tire size change calculator.
How do I calculate speedometer error from a tire change?＋
Compute each tire's overall diameter, then take the percentage difference. A larger new diameter makes the speedometer read low by that same percentage, so your true speed is higher than indicated. The same percentage applies to your odometer.
How do I read a tire size like 245/60R18?＋
245 is the tread width in millimetres, 60 is the aspect ratio (sidewall height as a percentage of width), and 18 is the wheel diameter in inches. Overall diameter = wheel + two sidewalls, converted to a single figure the calculator handles for you.
How much tire size change is safe?＋
A common guideline is to stay within about 3% of the original overall diameter. Beyond that, speedometer and odometer error grow, and larger changes can affect ABS, stability control, gearing and clearance. Always confirm load rating and fitment for your vehicle.
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