255/70R18 vs 245/65R18
New overall diameter
30.54″
was 32.06″
True speed at an indicated 60
57.2 mph
speedometer reads high
Revolutions per mile
660
was 629 · +31
Switching from 255/70R18 to 245/65R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 255/70R18 | 245/65R18 | Change |
|---|---|---|---|
| Overall diameter | 32.06″ | 30.54″ | −1.52″ (−4.7%) |
| Sidewall height | 7.03″ (178.5 mm) | 6.27″ (159.3 mm) | −19.3 mm |
| Tread width | 255 mm | 245 mm | −10 mm |
| Circumference | 100.70″ | 95.94″ | −4.76″ |
| Revs per mile | 629 | 660 | +31 |
| Speedometer error | baseline | −4.7% | reads high |
Diameter overlay
255/70R18 245/65R18
Speedometer check
With 245/65R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.8 mph actual
35 mph shown
33.3 mph actual
45 mph shown
42.9 mph actual
55 mph shown
52.4 mph actual
65 mph shown
61.9 mph actual
75 mph shown
71.5 mph actual
Common questions
- Will 245/65R18 fit instead of 255/70R18?
- Marginal fit — proceed with care. The 4.7% diameter change is between 3% and 5%. It can work, but expect noticeable speedometer error and check wheel-well clearance, scrub radius and gearing before committing. Your speedometer will read higher than your true speed. Overall diameter goes from 32.06″ to 30.54″, a change of −4.7%. 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 245/65R18?
- At an indicated 60 mph you would actually be travelling about 57.2 mph — a −4.7% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 255/70R18 and 245/65R18?
- 255/70R18 turns roughly 629 revolutions per mile; 245/65R18 turns roughly 660. That is a change of +31 revolutions per mile, which is what shifts odometer and speedometer readings.