245/65R18 vs 235/65R18
New overall diameter
30.03″
was 30.54″
True speed at an indicated 60
59.0 mph
speedometer reads high
Revolutions per mile
672
was 660 · +11
Switching from 245/65R18 to 235/65R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/65R18 | 235/65R18 | Change |
|---|---|---|---|
| Overall diameter | 30.54″ | 30.03″ | −0.51″ (−1.7%) |
| Sidewall height | 6.27″ (159.3 mm) | 6.01″ (152.8 mm) | −6.5 mm |
| Tread width | 245 mm | 235 mm | −10 mm |
| Circumference | 95.94″ | 94.33″ | −1.61″ |
| Revs per mile | 660 | 672 | +11 |
| Speedometer error | baseline | −1.7% | reads high |
Diameter overlay
245/65R18 235/65R18
Speedometer check
With 235/65R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.6 mph actual
35 mph shown
34.4 mph actual
45 mph shown
44.2 mph actual
55 mph shown
54.1 mph actual
65 mph shown
63.9 mph actual
75 mph shown
73.7 mph actual
Common questions
- Will 235/65R18 fit instead of 245/65R18?
- Good fit. Within the 3% rule — overall diameter changes only 1.7%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read higher than your true speed. Overall diameter goes from 30.54″ to 30.03″, a change of −1.7%. 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 235/65R18?
- At an indicated 60 mph you would actually be travelling about 59.0 mph — a −1.7% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 245/65R18 and 235/65R18?
- 245/65R18 turns roughly 660 revolutions per mile; 235/65R18 turns roughly 672. That is a change of +11 revolutions per mile, which is what shifts odometer and speedometer readings.