265/40R21 vs 275/45R20
New overall diameter
29.74″
was 29.35″
True speed at an indicated 60
60.8 mph
your speedo reads low
Revolutions per mile
678
was 687, −9
| Measurement | 265/40R21 | 275/45R20 | Change |
|---|---|---|---|
| Overall diameter | 29.35″ | 29.74″ | +0.40″ (+1.4%) |
| Sidewall height | 4.17″ (106.0 mm) | 4.87″ (123.8 mm) | +17.8 mm |
| Tread width | 265 mm | 275 mm | +10 mm |
| Circumference | 92.19″ | 93.44″ | +1.25″ |
| Revs per mile | 687 | 678 | −9 |
| Speedometer error | baseline | +1.4% | reads low |
Diameter overlay
265/40R21 275/45R20
Speedometer check
With 275/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.3 mph actual
35 mph shown
35.5 mph actual
45 mph shown
45.6 mph actual
55 mph shown
55.7 mph actual
65 mph shown
65.9 mph actual
75 mph shown
76.0 mph actual
Common questions
- Will 275/45R20 fit instead of 265/40R21?
- Diameter moves 1.4%, 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 29.35″ to 29.74″, a change of +1.4%. 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 275/45R20?
- At an indicated 60 mph you are really doing about 60.8 mph, a +1.4% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 265/40R21 and 275/45R20?
- 265/40R21 turns about 687 times per mile and 275/45R20 turns about 678, a difference of −9. Your speedometer and odometer both count rotations, so this number is what throws them off.