235/40R18 vs 245/45R18
New overall diameter
26.68″
was 25.40″
True speed at an indicated 60
63.0 mph
your speedo reads low
Revolutions per mile
756
was 794, −38
| Measurement | 235/40R18 | 245/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.40″ | 26.68″ | +1.28″ (+5.0%) |
| Sidewall height | 3.70″ (94.0 mm) | 4.34″ (110.3 mm) | +16.3 mm |
| Tread width | 235 mm | 245 mm | +10 mm |
| Circumference | 79.80″ | 83.82″ | +4.02″ |
| Revs per mile | 794 | 756 | −38 |
| Speedometer error | baseline | +5.0% | reads low |
Diameter overlay
235/40R18 245/45R18
Speedometer check
With 245/45R18 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 245/45R18 fit instead of 235/40R18?
- 5.0% 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 25.40″ to 26.68″, a change of +5.0%. 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 245/45R18?
- At an indicated 60 mph you are really doing about 63.0 mph, a +5.0% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 235/40R18 and 245/45R18?
- 235/40R18 turns about 794 times per mile and 245/45R18 turns about 756, a difference of −38. Your speedometer and odometer both count rotations, so this number is what throws them off.