295/70R18 vs 295/65R20
New overall diameter
35.10″
was 34.26″
True speed at an indicated 60
61.5 mph
your speedo reads low
Revolutions per mile
575
was 589, −14
| Measurement | 295/70R18 | 295/65R20 | Change |
|---|---|---|---|
| Overall diameter | 34.26″ | 35.10″ | +0.84″ (+2.4%) |
| Sidewall height | 8.13″ (206.5 mm) | 7.55″ (191.8 mm) | −14.8 mm |
| Tread width | 295 mm | 295 mm | +0 mm |
| Circumference | 107.63″ | 110.26″ | +2.63″ |
| Revs per mile | 589 | 575 | −14 |
| Speedometer error | baseline | +2.4% | reads low |
Diameter overlay
295/70R18 295/65R20
Speedometer check
With 295/65R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.6 mph actual
35 mph shown
35.9 mph actual
45 mph shown
46.1 mph actual
55 mph shown
56.3 mph actual
65 mph shown
66.6 mph actual
75 mph shown
76.8 mph actual
Common questions
- Will 295/65R20 fit instead of 295/70R18?
- Diameter moves 2.4%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer will read low, so you are going faster than it says. Overall diameter goes from 34.26″ to 35.10″, a change of +2.4%. 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 295/65R20?
- At an indicated 60 mph you are really doing about 61.5 mph, a +2.4% 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 295/65R20?
- 295/70R18 turns about 589 times per mile and 295/65R20 turns about 575, a difference of −14. Your speedometer and odometer both count rotations, so this number is what throws them off.