285/70R18 vs 255/70R18
New overall diameter
32.06″
was 33.71″
True speed at an indicated 60
57.1 mph
your speedo reads high
Revolutions per mile
629
was 598, +31
| Measurement | 285/70R18 | 255/70R18 | Change |
|---|---|---|---|
| Overall diameter | 33.71″ | 32.06″ | −1.65″ (−4.9%) |
| Sidewall height | 7.85″ (199.5 mm) | 7.03″ (178.5 mm) | −21.0 mm |
| Tread width | 285 mm | 255 mm | −30 mm |
| Circumference | 105.90″ | 100.70″ | −5.19″ |
| Revs per mile | 598 | 629 | +31 |
| Speedometer error | baseline | −4.9% | reads high |
Diameter overlay
285/70R18 255/70R18
Speedometer check
With 255/70R18 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
42.8 mph actual
55 mph shown
52.3 mph actual
65 mph shown
61.8 mph actual
75 mph shown
71.3 mph actual
Common questions
- Will 255/70R18 fit instead of 285/70R18?
- Overall diameter changes 4.9%; section width changes -30 mm. Diameter comparison alone cannot confirm fitment. Verify the vehicle maker's approved rim width, tire load and speed ratings, and clearance at full steering lock and suspension compression. Confirm any possible ABS, traction-control, speedometer, and drivetrain effects with the vehicle maker or a qualified tire professional. Overall diameter goes from 33.71″ to 32.06″, a change of −4.9%. The rule of thumb most shops use: under 3% is fine, 3 to 5% needs a clearance check, over 5% needs recalibration.
- How far off will my speedometer read on 255/70R18?
- At an indicated 60 mph you are really doing about 57.1 mph, a −4.9% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/70R18 and 255/70R18?
- 285/70R18 turns about 598 times per mile and 255/70R18 turns about 629, a difference of +31. Your speedometer and odometer both count rotations, so this number is what throws them off.