295/65R20 vs 285/70R18
New overall diameter
33.71″
was 35.10″
True speed at an indicated 60
57.6 mph
your speedo reads high
Revolutions per mile
598
was 575, +24
| Measurement | 295/65R20 | 285/70R18 | Change |
|---|---|---|---|
| Overall diameter | 35.10″ | 33.71″ | −1.39″ (−4.0%) |
| Sidewall height | 7.55″ (191.8 mm) | 7.85″ (199.5 mm) | +7.8 mm |
| Tread width | 295 mm | 285 mm | −10 mm |
| Circumference | 110.26″ | 105.90″ | −4.37″ |
| Revs per mile | 575 | 598 | +24 |
| Speedometer error | baseline | −4.0% | reads high |
Diameter overlay
295/65R20 285/70R18
Speedometer check
With 285/70R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.0 mph actual
35 mph shown
33.6 mph actual
45 mph shown
43.2 mph actual
55 mph shown
52.8 mph actual
65 mph shown
62.4 mph actual
75 mph shown
72.0 mph actual
Common questions
- Will 285/70R18 fit instead of 295/65R20?
- At 4.0% this lands between the 3% and 5% marks. Plenty of people run swaps like this, but you will see real speedometer error, and you want to check the wheel well at full lock and full compression before you buy. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 35.10″ to 33.71″, a change of −4.0%. 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 285/70R18?
- At an indicated 60 mph you are really doing about 57.6 mph, a −4.0% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 295/65R20 and 285/70R18?
- 295/65R20 turns about 575 times per mile and 285/70R18 turns about 598, a difference of +24. Your speedometer and odometer both count rotations, so this number is what throws them off.