265/45R21 vs 265/50R20
New overall diameter
30.43″
was 30.39″
True speed at an indicated 60
60.1 mph
speedometer reads low
Revolutions per mile
663
was 664 · −1
Switching from 265/45R21 to 265/50R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 265/45R21 | 265/50R20 | Change |
|---|---|---|---|
| Overall diameter | 30.39″ | 30.43″ | +0.04″ (+0.1%) |
| Sidewall height | 4.69″ (119.3 mm) | 5.22″ (132.5 mm) | +13.3 mm |
| Tread width | 265 mm | 265 mm | +0 mm |
| Circumference | 95.47″ | 95.61″ | +0.14″ |
| Revs per mile | 664 | 663 | −1 |
| Speedometer error | baseline | +0.1% | reads low |
Diameter overlay
265/45R21 265/50R20
Speedometer check
With 265/50R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.0 mph actual
35 mph shown
35.0 mph actual
45 mph shown
45.1 mph actual
55 mph shown
55.1 mph actual
65 mph shown
65.1 mph actual
75 mph shown
75.1 mph actual
Common questions
- Will 265/50R20 fit instead of 265/45R21?
- Good fit. Within the 3% rule — overall diameter changes only 0.1%. 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 30.39″ to 30.43″, a change of +0.1%. 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 265/50R20?
- At an indicated 60 mph you would actually be travelling about 60.1 mph — a +0.1% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 265/45R21 and 265/50R20?
- 265/45R21 turns roughly 664 revolutions per mile; 265/50R20 turns roughly 663. That is a change of −1 revolutions per mile, which is what shifts odometer and speedometer readings.