How to calculate it
Divide the new overall diameter by the original, subtract 1 and multiply by 100. 265/70R17 is 802.8 mm (31.61 in) tall and 285/70R17 is 830.8 mm (32.71 in): 830.8 ÷ 802.8 = 1.0349, or +3.49%. That popular truck upgrade lands just over the line.
| From | To | Change | Within 3%? |
|---|---|---|---|
| 205/55R16 | 225/45R17 | +0.38% | Yes |
| 205/55R16 | 215/45R18 | +2.98% | Yes, barely |
| 175/70R14 | 185/65R15 | +3.48% | No |
| 265/70R17 | 285/70R17 | +3.49% | No |
| 245/75R17 | 33x12.50R17 | +4.87% | No |
What the 3% protects
- Speedometer and odometer: at +3%, an indicated 60 mph is really 61.8 mph.
- ABS, traction and stability control (ESC), which rely on wheel speed sensors and on the expected tire size.
- Automatic transmission shift points and effective gearing.
- Clearance to the fender, frame and suspension, which shrinks as the tire grows.
- On 4WD and AWD vehicles, the drivetrain: all four tires must be the same size, whatever the size.
Is it a law?
We found no federal rule in the United States that limits tire diameter changes; the 3% figure comes from the tire and wheel industry. Some states have rules on lift height, bumper height or tires that extend past the fenders, and a large change can matter for your warranty or insurance. Check your state's rules before a big change.
Other countries are stricter. Brazil, for example, writes a ±3% limit on overall diameter into its national traffic regulations.
Within 3% is a starting point, not an approval. The load index and speed rating should be equal to or higher than the original, the tire must clear everything at full lock and full suspension compression, and the owner's manual and a tire professional have the final word.
When people go past 3%
Off-road builds routinely go well past 3%, for example 33 in tires on trucks and Jeeps that came with 31 in. They make up for it with a lift, a speedometer recalibration and sometimes new axle gears. That is a planned modification, not a simple tire swap.
Questions
Is the 3% rule bigger or smaller?
Both: it is usually given as plus or minus 3%. Some shops prefer a tighter window on the smaller side, because a shorter tire also lowers ground clearance and often load capacity.
Does the 3% rule apply to width?
No, it refers to overall diameter. Width is limited by the rim, the fender and the suspension.
Can a 3% change set off the ABS light?
Usually not when all four tires are the same size. Warnings typically come from large changes or from different sizes between axles. If a light comes on, have it checked.