255/45R18 vs 235/55R18
New overall diameter
28.18″
was 27.04″
True speed at an indicated 60
62.5 mph
your speedo reads low
Revolutions per mile
716
was 746, −30
| Measurement | 255/45R18 | 235/55R18 | Change |
|---|---|---|---|
| Overall diameter | 27.04″ | 28.18″ | +1.14″ (+4.2%) |
| Sidewall height | 4.52″ (114.8 mm) | 5.09″ (129.3 mm) | +14.5 mm |
| Tread width | 255 mm | 235 mm | −20 mm |
| Circumference | 84.93″ | 88.52″ | +3.59″ |
| Revs per mile | 746 | 716 | −30 |
| Speedometer error | baseline | +4.2% | reads low |
Diameter overlay
255/45R18 235/55R18
Speedometer check
With 235/55R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.1 mph actual
35 mph shown
36.5 mph actual
45 mph shown
46.9 mph actual
55 mph shown
57.3 mph actual
65 mph shown
67.7 mph actual
75 mph shown
78.2 mph actual
Common questions
- Will 235/55R18 fit instead of 255/45R18?
- At 4.2% this lands between the 3% and 5% marks. Plenty of people run swaps like this, but you will see real speedometer error, and you want to check the wheel well at full lock and full compression before you buy. Your speedometer will read low, so you are going faster than it says. Overall diameter goes from 27.04″ to 28.18″, a change of +4.2%. 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 62.5 mph, a +4.2% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 255/45R18 and 235/55R18?
- 255/45R18 turns about 746 times per mile and 235/55R18 turns about 716, a difference of −30. Your speedometer and odometer both count rotations, so this number is what throws them off.