245/40R18 vs 255/45R18
New overall diameter
27.04″
was 25.72″
True speed at an indicated 60
63.1 mph
your speedo reads low
Revolutions per mile
746
was 784, −38
| Measurement | 245/40R18 | 255/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.72″ | 27.04″ | +1.32″ (+5.1%) |
| Sidewall height | 3.86″ (98.0 mm) | 4.52″ (114.8 mm) | +16.8 mm |
| Tread width | 245 mm | 255 mm | +10 mm |
| Circumference | 80.79″ | 84.93″ | +4.14″ |
| Revs per mile | 784 | 746 | −38 |
| Speedometer error | baseline | +5.1% | reads low |
Diameter overlay
245/40R18 255/45R18
Speedometer check
With 255/45R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.3 mph actual
35 mph shown
36.8 mph actual
45 mph shown
47.3 mph actual
55 mph shown
57.8 mph actual
65 mph shown
68.3 mph actual
75 mph shown
78.8 mph actual
Common questions
- Will 255/45R18 fit instead of 245/40R18?
- 5.1% is past the 5% mark, which is where this stops being a tire choice and starts being a project. Expect rubbing, meaningful speedometer error, and extra load on ABS, stability control and the drivetrain. Recalibration is the usual fix. Your speedometer will read low, so you are going faster than it says. Overall diameter goes from 25.72″ to 27.04″, a change of +5.1%. 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 255/45R18?
- At an indicated 60 mph you are really doing about 63.1 mph, a +5.1% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 245/40R18 and 255/45R18?
- 245/40R18 turns about 784 times per mile and 255/45R18 turns about 746, a difference of −38. Your speedometer and odometer both count rotations, so this number is what throws them off.