Thinking about changing tire size or wheel diameter? Even small changes to overall tire height affect how your speedometer (and odometer) read. Here’s how it works, how to stay within safe limits, and easy ways to keep your readings accurate.
Why it happens: Most vehicles calculate speed from wheel rotations. A taller tire covers more distance per revolution, so the car thinks you’re moving slower; a shorter tire covers less, so it thinks you’re moving faster.
What else changes when you change overall tire height: - Odometer and trip readings - ABS, traction, and stability control calibrations - Transmission shift points (on many automatics) - Ground and fender clearance (potential rubbing if too large; harsher ride if too small)
Plus sizing swaps to a different tire size (and often a different wheel diameter) while keeping the same overall tire height. Done correctly, it keeps your speedometer accurate.
Common “plus” moves: - Plus 1: go up 1 inch in wheel diameter and lower the tire’s aspect ratio to maintain overall height. - Plus 2: go up 2 inches in wheel diameter and reduce aspect ratio further to match height.
Always confirm: - Overall diameter stays within your vehicle’s acceptable range - Load index and speed rating meet or exceed OE specs - Wheel width and offset are compatible - Brake and fender clearance remain safe at full lock and over bumps
Enter your vehicle to see sizes that fit without guesswork. Check compatible sizes.
Aim to keep overall diameter within about ±3% of your original equipment size. Beyond that, plan on recalibration and check fitment carefully.
Quick rule of thumb: Actual speed = Indicated speed × (New overall diameter ÷ Original overall diameter)
How to get overall diameter from a tire size (example: 235/40R18):
1) Section height (mm) = width × aspect ratio
235 × 0.40 = 94 mm
2) Convert to inches: 94 ÷ 25.4 = 3.70 in
3) Overall diameter (in) = wheel diameter + 2 × section height
18 + 2 × 3.70 = 25.4 in
Worked example:
- Original: 225/45R17 ≈ 25.0 in
- New: 235/40R18 ≈ 25.4 in
At an indicated 60 mph: 60 × (25.4 ÷ 25.0) ≈ 61.0 mph actual.
Want a fast, accurate comparison? Use our tire size and speedometer calculator.
We recommend recalibrating any time you change tire size so you can drive safely and avoid unintended speeding.