Enter your original (OEM) tire size, then your new tire size. Hit Calculate to see exact error %, real speed at every indicated speed, and odometer drift.
| Per 1,000 miles driven | — |
| Per 10,000 miles driven | — |
| Per 30,000 miles driven (2–3 yr average) | — |
| Per 100,000 miles driven | — |
Your speedometer counts wheel revolutions per second using a sensor on the ABS ring or transmission output shaft. It multiplies that count by a fixed circumference value programmed at the factory — the rolling circumference of your OEM tire.
When you fit a different tire size, the actual rolling circumference changes but the ECU’s programmed value stays the same. The result is a permanent, systematic error on every speed reading and every odometer increment.
A larger tire has a greater circumference. It covers more distance per revolution. The ECU undercounts distance covered, so the speedometer reads lower than actual speed — you are going faster than the dashboard shows. A smaller tire has the opposite effect: it causes the speedometer to read higher than actual speed.
For the full technical explanation, see our guide: Speedometer Error After Tire Change — Complete Guide.
| Tire Change | Speedo Reads | Actual Speed |
|---|---|---|
|
Larger diameter |
LOW (under) |
FASTER than shown |
|
Smaller diameter |
HIGH (over) |
SLOWER than shown |
|
Same diameter |
Accurate |
Same as shown |
Error % = ((New circumference − OEM circumference) ÷ OEM circumference) × 100
OEM circumference: π × 24.88 in × 25.4 mm = 1,983 mm
The three tire dimensions that drive error are section width, aspect ratio, and rim diameter. Increasing any of them increases overall diameter and circumference, reducing speedometer reading relative to actual speed.
| Change | Error % | Real speed at 60 mph |
|---|---|---|
|
205/55R16 → 215/55R16 |
+0.8% |
60.5 mph |
|
205/55R16 → 225/55R16 |
+1.6% |
60.9 mph |
|
205/55R16 → 225/50R17 |
+4.4% |
62.6 mph |
|
205/55R16 → 235/40R18 |
+1.8% |
61.1 mph |
|
205/55R16 → 265/70R17 |
+26% |
75.7 mph |
|
225/45R17 → 215/45R17 |
−0.4% |
59.8 mph |
|
225/45R17 → 235/40R18 |
+0.1% |
60.1 mph |
Tire engineers design OEM tire sizes so the vehicle's speedometer reads within ±1–2% of actual speed. A ±3% tolerance is the common threshold used by most automotive standards and legal regulations worldwide.
The US has no federal law specifying maximum speedometer error for private vehicles. However, all new vehicles must meet FMVSS 101 — speedometers must show speeds up to 85 mph. Dealers and states may impose different standards for modified vehicles.
ECE Regulation 39 (adopted by EU and UK) requires that speedometers must never read lower than actual speed, and the positive error must not exceed V1 = 1.1 × V2 + 6.25 mph (where V2 is actual speed). At 60 mph actual, the speedometer may show up to 72.25 mph but never below 60 mph.
Your ABS and ESC systems use the same wheel speed sensors as the speedometer. If tire size changes alter the sensor's revolution count by more than 3%, these safety systems may trigger incorrectly or fail to trigger when needed — a genuine safety risk beyond just speedo accuracy.
| Speedo Error | At 30 mph | At 50 mph | At 60 mph | At 70 mph | At 80 mph |
|---|---|---|---|---|---|
|
+5% (large upsizing) |
31.5 mph |
52.5 mph |
63.0 mph |
73.5 mph |
84.0 mph |
|
+4% (plus 2 sizing) |
31.2 mph |
52.0 mph |
62.4 mph |
72.8 mph |
83.2 mph |
|
+3% (±limit) |
30.9 mph |
51.5 mph |
61.8 mph |
72.1 mph |
82.4 mph |
|
+2% (plus 1 sizing) |
30.6 mph |
51.0 mph |
61.2 mph |
71.4 mph |
81.6 mph |
|
+1% (minor change) |
30.3 mph |
50.5 mph |
60.6 mph |
70.7 mph |
80.8 mph |
|
0% (OEM size) |
30.0 mph |
50.0 mph |
60.0 mph |
70.0 mph |
80.0 mph |
|
−1% (slightly smaller) |
29.7 mph |
49.5 mph |
59.4 mph |
69.3 mph |
79.2 mph |
|
−2% (smaller tire) |
29.4 mph |
49.0 mph |
58.8 mph |
68.6 mph |
78.4 mph |
|
−3% (±limit) |
29.1 mph |
48.5 mph |
58.2 mph |
67.9 mph |
77.6 mph |
Most automotive engineers consider ±3% speedo error to be acceptable for road use. Most automotive engineers and SAE International recommend keeping your tire diameter within 3% of OEM spec to avoid affecting ABS, traction control, and odometer accuracy., and a maximum of +10% + 4 km/h over is permitted by law. Beyond 3%, your ABS, traction control, and odometer may also be affected. Use the speedo error calculator above to check your specific size change.
Wheel offset (ET from German Einpress Tiefe) is the distance in mm from the wheel’s mounting face to its geometric centre line. A higher ET pushes the wheel further into the arch (more inset, less poke). A lower ET pulls the wheel outward (more poke, less inset). Changing ET affects both inner and outer clearances simultaneously. Our wheel offset calculator shows the impact in mm.
Yes, within limits. As a general rule, you can go 10–20mm wider than the OEM tire width on the same rim. Going too wide causes “tyre stretch” the sidewall becomes more upright or even angled inward, reducing stability and risking bead unseating. Use our ideal rim range metric in the results to check compatibility. NHTSA recommends only fitting tires matching your OEM spec or a manufacturer-approved size NHTSA tire buying guide
Plus sizing means fitting a larger rim diameter with a lower-profile tire, keeping the overall tire diameter the same as OEM. For example, upgrading from 205/55R16 to 225/45R17 is “plus one” sizing. The benefits include sportier appearance, improved lateral stiffness, and better handling. The trade-off is a harsher ride due to less sidewall cushioning.
205 = section width in mm (sidewall to sidewall). 55 = aspect ratio — the sidewall height is 55% of 205mm = 112.75mm. R = radial construction. 16 = the rim diameter in inches the tire is designed to fit. You may also see a load index and speed rating after (e.g. 91V). Enter the three main numbers into our tire size calculator to get full dimensional data.
Yes. Wider and taller tires have higher rolling resistance, which slightly reduces fuel economy. A 3% increase in rolling radius typically causes a 1–2% reduction in mpg. However, the effect depends heavily on tire compound, inflation pressure, and driving style. The bigger practical concern for most drivers is speedo error and fitment clearance. Learn more at FuelEconomy.gov.
Yes — the calculator works for any vehicle. For trucks and SUVs, pay special attention to ride height change and speedo error, as larger off-road tires (e.g. 265/70R17 to 285/75R17) cause significant diameter changes. A 3-inch diameter increase causes roughly a 5% speedo error — you will need a speedo recalibration tool or ECU tune to correct it.
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