275/40R20 vs 275/45R20
New overall diameter
29.74″
was 28.66″
True speed at an indicated 60
62.3 mph
your speedo reads low
Revolutions per mile
678
was 704, −26
| Measurement | 275/40R20 | 275/45R20 | Change |
|---|---|---|---|
| Overall diameter | 28.66″ | 29.74″ | +1.08″ (+3.8%) |
| Sidewall height | 4.33″ (110.0 mm) | 4.87″ (123.8 mm) | +13.8 mm |
| Tread width | 275 mm | 275 mm | +0 mm |
| Circumference | 90.04″ | 93.44″ | +3.40″ |
| Revs per mile | 704 | 678 | −26 |
| Speedometer error | baseline | +3.8% | reads low |
Diameter overlay
275/40R20 275/45R20
Speedometer check
With 275/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.9 mph actual
35 mph shown
36.3 mph actual
45 mph shown
46.7 mph actual
55 mph shown
57.1 mph actual
65 mph shown
67.5 mph actual
75 mph shown
77.8 mph actual
Common questions
- Will 275/45R20 fit instead of 275/40R20?
- Overall diameter changes 3.8%; section width changes 0 mm. Diameter comparison alone cannot confirm fitment. Verify the vehicle maker's approved rim width, tire load and speed ratings, and clearance at full steering lock and suspension compression. Confirm any possible ABS, traction-control, speedometer, and drivetrain effects with the vehicle maker or a qualified tire professional. Overall diameter goes from 28.66″ to 29.74″, a change of +3.8%. 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 62.3 mph, a +3.8% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 275/40R20 and 275/45R20?
- 275/40R20 turns about 704 times per mile and 275/45R20 turns about 678, a difference of −26. Your speedometer and odometer both count rotations, so this number is what throws them off.