235/55R18 vs 225/70R16
New overall diameter
28.40″
was 28.18″
True speed at an indicated 60
60.5 mph
speedometer reads low
Revolutions per mile
710
was 716 · −6
Switching from 235/55R18 to 225/70R16? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 235/55R18 | 225/70R16 | Change |
|---|---|---|---|
| Overall diameter | 28.18″ | 28.40″ | +0.22″ (+0.8%) |
| Sidewall height | 5.09″ (129.3 mm) | 6.20″ (157.5 mm) | +28.3 mm |
| Tread width | 235 mm | 225 mm | −10 mm |
| Circumference | 88.52″ | 89.23″ | +0.71″ |
| Revs per mile | 716 | 710 | −6 |
| Speedometer error | baseline | +0.8% | reads low |
Diameter overlay
235/55R18 225/70R16
Speedometer check
With 225/70R16 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
55.4 mph actual
65 mph shown
65.5 mph actual
75 mph shown
75.6 mph actual
Common questions
- Will 225/70R16 fit instead of 235/55R18?
- 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 28.18″ to 28.40″, 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 225/70R16?
- 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 235/55R18 and 225/70R16?
- 235/55R18 turns roughly 716 revolutions per mile; 225/70R16 turns roughly 710. That is a change of −6 revolutions per mile, which is what shifts odometer and speedometer readings.