295/70R18 vs 285/65R20
New overall diameter
34.59″
was 34.26″
True speed at an indicated 60
60.6 mph
speedometer reads low
Revolutions per mile
583
was 589 · −6
Switching from 295/70R18 to 285/65R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 295/70R18 | 285/65R20 | Change |
|---|---|---|---|
| Overall diameter | 34.26″ | 34.59″ | +0.33″ (+1.0%) |
| Sidewall height | 8.13″ (206.5 mm) | 7.29″ (185.3 mm) | −21.3 mm |
| Tread width | 295 mm | 285 mm | −10 mm |
| Circumference | 107.63″ | 108.66″ | +1.03″ |
| Revs per mile | 589 | 583 | −6 |
| Speedometer error | baseline | +1.0% | reads low |
Diameter overlay
295/70R18 285/65R20
Speedometer check
With 285/65R20 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
65.6 mph actual
75 mph shown
75.7 mph actual
Common questions
- Will 285/65R20 fit instead of 295/70R18?
- Good fit. Within the 3% rule — overall diameter changes only 1.0%. 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 34.26″ to 34.59″, a change of +1.0%. 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 285/65R20?
- At an indicated 60 mph you would actually be travelling about 60.6 mph — a +1.0% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 295/70R18 and 285/65R20?
- 295/70R18 turns roughly 589 revolutions per mile; 285/65R20 turns roughly 583. That is a change of −6 revolutions per mile, which is what shifts odometer and speedometer readings.