245/40R20 vs 235/55R18
New overall diameter
28.18″
was 27.72″
True speed at an indicated 60
61.0 mph
your speedo reads low
Revolutions per mile
716
was 728, −12
| Measurement | 245/40R20 | 235/55R18 | Change |
|---|---|---|---|
| Overall diameter | 27.72″ | 28.18″ | +0.46″ (+1.7%) |
| Sidewall height | 3.86″ (98.0 mm) | 5.09″ (129.3 mm) | +31.3 mm |
| Tread width | 245 mm | 235 mm | −10 mm |
| Circumference | 87.07″ | 88.52″ | +1.45″ |
| Revs per mile | 728 | 716 | −12 |
| Speedometer error | baseline | +1.7% | reads low |
Diameter overlay
245/40R20 235/55R18
Speedometer check
With 235/55R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.4 mph actual
35 mph shown
35.6 mph actual
45 mph shown
45.7 mph actual
55 mph shown
55.9 mph actual
65 mph shown
66.1 mph actual
75 mph shown
76.2 mph actual
Common questions
- Will 235/55R18 fit instead of 245/40R20?
- Overall diameter changes 1.7%; section width changes -10 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 27.72″ to 28.18″, a change of +1.7%. 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 235/55R18?
- At an indicated 60 mph you are really doing about 61.0 mph, a +1.7% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 245/40R20 and 235/55R18?
- 245/40R20 turns about 728 times per mile and 235/55R18 turns about 716, a difference of −12. Your speedometer and odometer both count rotations, so this number is what throws them off.