Compare two tire sizes
What changes when the size changes: diameter, width, sidewall, clearance, and the speedometer and odometer error the swap implies.
225/65R17 against 235/60R18. Overall diameter differs by 2.06 percent, which is inside the conventional 3 percent window. At an indicated 60 miles per hour the true speed is 61.2.
That window is a widely repeated trade convention, not a clause of any tire or vehicle standard, and it says nothing about whether the tire clears the vehicle.
| Dimension | 225/65R17 | 235/60R18 | Change |
|---|---|---|---|
| Overall diameter | 28.52 in | 29.1 in | +0.59 in (+2.06%) |
| Section width | 225 mm | 235 mm | +10.0 mm (+4.44%) |
| Sidewall height | 146.3 mm | 141 mm | -5.2 mm |
| Circumference | 89.58 in | 91.43 in | +1.84 in |
| Revolutions per mile | 707.3 | 693 | -14.3 |
| Axle height above ground | - | - | +0.29 in (+7.5 mm) |
Only half the diameter change lifts the axle; the other half goes up into the wheel arch, which is usually the clearance that runs out first.
Speedometer
| Speedometer reads | Actual speed | Speedometer reads | Actual speed |
|---|---|---|---|
| 30 mph | 30.6 mph (+0.6) | 50 km/h | 51 km/h (+1.0) |
| 50 mph | 51 mph (+1.0) | 80 km/h | 81.6 km/h (+1.6) |
| 60 mph | 61.2 mph (+1.2) | 100 km/h | 102.1 km/h (+2.1) |
| 70 mph | 71.4 mph (+1.4) | 120 km/h | 122.5 km/h (+2.5) |
| 80 mph | 81.6 mph (+1.6) | 130 km/h | 132.7 km/h (+2.7) |
The replacement is taller, so the vehicle travels further per revolution and the speedometer reads LOW: you are going faster than it says.
Odometer
Over a genuine 100 miles the odometer would record 97.98 miles. Over 1000 km it would record 979.8 km.
These figures assume the speedometer read correctly on the original size and that nothing was recalibrated. Many vehicles leave the factory reading a little high on purpose, so treat this as the CHANGE a size swap makes, not as an absolute claim about what the instrument now shows.
These are the nominal dimensions the size code implies. A real tire's measured dimensions depend on its mould, its construction and the rim width it is mounted on, so a manufacturer's specification sheet for a specific tire is the authority for that tire.
This is geometry, not fitment approval. It says what a size measures, never whether it fits a vehicle.
Sizes within a chosen window of 225/65R17
| Size | Rim | Overall diameter | Difference | At an indicated 60 mph |
|---|---|---|---|---|
| LT225/65R17 | 17 in | 28.52 in | 0.00% | 60 mph (0.0) |
| 345/35R19 | 19 in | 28.51 in | -0.03% | 60 mph (0.0) |
| 275/30R22 | 22 in | 28.5 in | -0.07% | 60 mph (0.0) |
| 295/45R18 | 18 in | 28.45 in | -0.22% | 59.9 mph (-0.1) |
| 275/35R21 | 21 in | 28.58 in | +0.22% | 60.1 mph (+0.1) |
| 315/30R21 | 21 in | 28.44 in | -0.26% | 59.8 mph (-0.2) |
| 335/25R22 | 22 in | 28.59 in | +0.28% | 60.2 mph (+0.2) |
| 245/55R18 | 18 in | 28.61 in | +0.33% | 60.2 mph (+0.2) |
| 305/35R20 | 20 in | 28.41 in | -0.39% | 59.8 mph (-0.2) |
| 225/70R16 | 16 in | 28.4 in | -0.40% | 59.8 mph (-0.2) |
| LT225/70R16 | 16 in | 28.4 in | -0.40% | 59.8 mph (-0.2) |
| 225/60R18 | 18 in | 28.63 in | +0.40% | 60.2 mph (+0.2) |
| LT225/60R18 | 18 in | 28.63 in | +0.40% | 60.2 mph (+0.2) |
| 265/45R19 | 19 in | 28.39 in | -0.44% | 59.7 mph (-0.3) |
| 245/50R19 | 19 in | 28.65 in | +0.46% | 60.3 mph (+0.3) |
| 155/85R18 | 18 in | 28.37 in | -0.50% | 59.7 mph (-0.3) |
| T155/85R18 | 18 in | 28.37 in | -0.50% | 59.7 mph (-0.3) |
| 275/40R20 | 20 in | 28.66 in | +0.51% | 60.3 mph (+0.3) |
| 325/30R21 | 21 in | 28.68 in | +0.57% | 60.3 mph (+0.3) |
| 245/45R20 | 20 in | 28.68 in | +0.58% | 60.3 mph (+0.3) |
| 315/35R20 | 20 in | 28.68 in | +0.58% | 60.3 mph (+0.3) |
| 265/40R20 | 20 in | 28.35 in | -0.59% | 59.6 mph (-0.4) |
| 215/75R16 | 16 in | 28.7 in | +0.64% | 60.4 mph (+0.4) |
| LT215/75R16 | 16 in | 28.7 in | +0.64% | 60.4 mph (+0.4) |
This ranks arithmetic. It does not know whether a size clears at full lock, whether a wheel exists in the offset the vehicle needs, or whether the size is made in the load index the vehicle placard requires.
The arithmetic behind these numbers
- Diameter difference
(new overall diameter / old overall diameter - 1) x 100- Speedometer
true speed = indicated speed x (new overall diameter / old overall diameter)- Odometer
distance recorded = distance travelled / (new overall diameter / old overall diameter)- Axle height change
diameter change / 2
Reading the speedometer figure
These figures assume the speedometer read correctly on the original size and that nothing was recalibrated. Many vehicles leave the factory reading a little high on purpose, so treat this as the CHANGE a size swap makes, not as an absolute claim about what the instrument now shows.
- True speed
true speed = indicated speed x (new overall diameter / old overall diameter)- Distance the odometer records
distance recorded = distance travelled / (new overall diameter / old overall diameter)
About the 3 percent window
A replacement within 3 percent of the original overall diameter is conventionally treated as a like-for-like swap. That figure is a trade convention that is widely repeated and rarely attributed. It is not a clause of a tire or vehicle standard, and this site does not present it as one: the actual percentage is always shown next to the verdict so the convention is never doing the work on its own.
Diameter is also only one constraint. Load index, speed symbol, wheel width, offset and physical clearance all have to be satisfied, and none of them is a percentage.
Common questions
- How much can tire size change before the speedometer is wrong?
- Any change in overall diameter changes the reading immediately and in proportion: a one percent taller tire makes the speedometer read one percent low. The conventional three percent window is about how much error is usually accepted, not about where the error begins.
- Where does the 3 percent rule come from?
- It is a widely repeated trade convention rather than a clause of any tire or vehicle standard. This site prints it because people expect to see it, labels it as a convention, and shows the actual percentage so the convention is never doing the work on its own.
- Do bigger tires make the speedometer read high or low?
- Low. A taller tire covers more ground per revolution, so the vehicle is going faster than the instrument reports. A shorter tire does the opposite. The odometer moves the same way: taller tires under-count distance.
- How much lift do bigger tires give?
- Half the diameter change, at the axle. A one inch larger overall diameter raises the axle half an inch. The other half goes up into the wheel arch, which is normally the clearance that runs out first, and neither figure says whether the tire clears at full lock or at full suspension travel.
- Is a size within 3 percent safe to fit?
- This site cannot answer that. Diameter is one of several constraints, and load index, speed rating, wheel width, offset, clearance and the vehicle placard all have to be satisfied too. Confirm any change with a qualified fitter and the vehicle manufacturer's specification.
Start from a size page instead
Every size page already lists the alternatives nearest to it on the same rim and one size up or down, with the speedometer effect worked out.
Every number on this page, and where it comes from
Computed on this page, not looked up. Each figure and the arithmetic behind it:
- Diameter difference
(new overall diameter / old overall diameter - 1) x 100 - True speed
true speed = indicated speed x (new overall diameter / old overall diameter) - Odometer reading
distance recorded = distance travelled / (new overall diameter / old overall diameter) - Axle height change
diameter change / 2
Before you fit anything
This is geometry, not fitment approval. It says what a size measures, never whether it fits a vehicle.
SidewallRef calculates the dimensions a size code implies and reproduces the published load index and speed symbol tables. It does not know your vehicle.
Load index and speed rating must meet or exceed what the vehicle placard specifies. The placard is in the driver's door jamb on most vehicles and is the authority for your vehicle, not this site.
Clearance at full lock and full suspension travel, wheel width and offset, brake and suspension component clearance, axle load capacity, inflation pressure and any legal requirement where you drive are the owner's and the fitter's responsibility.
Nothing here approves a fitment. Confirm any change with a qualified tire fitter and with the vehicle manufacturer's own specification before driving on it.