205/55R16 vs 225/40R18
New overall diameter
25.09″
was 24.88″
True speed at an indicated 60
60.5 mph
speedometer reads low
Revolutions per mile
804
was 811 · −7
Switching from 205/55R16 to 225/40R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 205/55R16 | 225/40R18 | Change |
|---|---|---|---|
| Overall diameter | 24.88″ | 25.09″ | +0.21″ (+0.8%) |
| Sidewall height | 4.44″ (112.8 mm) | 3.54″ (90.0 mm) | −22.8 mm |
| Tread width | 205 mm | 225 mm | +20 mm |
| Circumference | 78.16″ | 78.81″ | +0.66″ |
| Revs per mile | 811 | 804 | −7 |
| Speedometer error | baseline | +0.8% | reads low |
Diameter overlay
205/55R16 225/40R18
Speedometer check
With 225/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.2 mph actual
35 mph shown
35.3 mph actual
45 mph shown
45.4 mph actual
55 mph shown
55.5 mph actual
65 mph shown
65.5 mph actual
75 mph shown
75.6 mph actual
Common questions
- Will 225/40R18 fit instead of 205/55R16?
- Good fit. Within the 3% rule — overall diameter changes only 0.8%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 24.88″ to 25.09″, a change of +0.8%. As a rule of thumb, under 3% is safe, 3–5% needs a clearance check, and over 5% is not recommended.
- How far off will my speedometer read on 225/40R18?
- At an indicated 60 mph you would actually be travelling about 60.5 mph — a +0.8% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 205/55R16 and 225/40R18?
- 205/55R16 turns roughly 811 revolutions per mile; 225/40R18 turns roughly 804. That is a change of −7 revolutions per mile, which is what shifts odometer and speedometer readings.