275/45R20 vs 245/60R18
New overall diameter
29.57″
was 29.74″
True speed at an indicated 60
59.7 mph
your speedo reads high
Revolutions per mile
682
was 678, +4
| Measurement | 275/45R20 | 245/60R18 | Change |
|---|---|---|---|
| Overall diameter | 29.74″ | 29.57″ | −0.17″ (−0.6%) |
| Sidewall height | 4.87″ (123.8 mm) | 5.79″ (147.0 mm) | +23.3 mm |
| Tread width | 275 mm | 245 mm | −30 mm |
| Circumference | 93.44″ | 92.91″ | −0.53″ |
| Revs per mile | 678 | 682 | +4 |
| Speedometer error | baseline | −0.6% | reads high |
Diameter overlay
275/45R20 245/60R18
Speedometer check
With 245/60R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.9 mph actual
35 mph shown
34.8 mph actual
45 mph shown
44.7 mph actual
55 mph shown
54.7 mph actual
65 mph shown
64.6 mph actual
75 mph shown
74.6 mph actual
Common questions
- Will 245/60R18 fit instead of 275/45R20?
- Diameter moves 0.6%, 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 high, so you are going slower than it says. Overall diameter goes from 29.74″ to 29.57″, a change of −0.6%. 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 245/60R18?
- At an indicated 60 mph you are really doing about 59.7 mph, a −0.6% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 275/45R20 and 245/60R18?
- 275/45R20 turns about 678 times per mile and 245/60R18 turns about 682, a difference of +4. Your speedometer and odometer both count rotations, so this number is what throws them off.