185/55R16 vs 225/40R18
New overall diameter
25.09″
was 24.01″
True speed at an indicated 60
62.7 mph
your speedo reads low
Revolutions per mile
804
was 840, −36
| Measurement | 185/55R16 | 225/40R18 | Change |
|---|---|---|---|
| Overall diameter | 24.01″ | 25.09″ | +1.07″ (+4.5%) |
| Sidewall height | 4.01″ (101.8 mm) | 3.54″ (90.0 mm) | −11.8 mm |
| Tread width | 185 mm | 225 mm | +40 mm |
| Circumference | 75.44″ | 78.81″ | +3.38″ |
| Revs per mile | 840 | 804 | −36 |
| Speedometer error | baseline | +4.5% | reads low |
Diameter overlay
185/55R16 225/40R18
Speedometer check
With 225/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.1 mph actual
35 mph shown
36.6 mph actual
45 mph shown
47.0 mph actual
55 mph shown
57.5 mph actual
65 mph shown
67.9 mph actual
75 mph shown
78.4 mph actual
Common questions
- Will 225/40R18 fit instead of 185/55R16?
- Overall diameter changes 4.5%; section width changes 40 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 24.01″ to 25.09″, a change of +4.5%. 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 225/40R18?
- At an indicated 60 mph you are really doing about 62.7 mph, a +4.5% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 185/55R16 and 225/40R18?
- 185/55R16 turns about 840 times per mile and 225/40R18 turns about 804, a difference of −36. Your speedometer and odometer both count rotations, so this number is what throws them off.