245/45R18 vs 235/40R18
New overall diameter
25.40″
was 26.68″
True speed at an indicated 60
57.1 mph
your speedo reads high
Revolutions per mile
794
was 756, +38
| Measurement | 245/45R18 | 235/40R18 | Change |
|---|---|---|---|
| Overall diameter | 26.68″ | 25.40″ | −1.28″ (−4.8%) |
| Sidewall height | 4.34″ (110.3 mm) | 3.70″ (94.0 mm) | −16.3 mm |
| Tread width | 245 mm | 235 mm | −10 mm |
| Circumference | 83.82″ | 79.80″ | −4.02″ |
| Revs per mile | 756 | 794 | +38 |
| Speedometer error | baseline | −4.8% | reads high |
Diameter overlay
245/45R18 235/40R18
Speedometer check
With 235/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.8 mph actual
35 mph shown
33.3 mph actual
45 mph shown
42.8 mph actual
55 mph shown
52.4 mph actual
65 mph shown
61.9 mph actual
75 mph shown
71.4 mph actual
Common questions
- Will 235/40R18 fit instead of 245/45R18?
- Overall diameter changes 4.8%; 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 26.68″ to 25.40″, a change of −4.8%. 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 235/40R18?
- At an indicated 60 mph you are really doing about 57.1 mph, a −4.8% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 245/45R18 and 235/40R18?
- 245/45R18 turns about 756 times per mile and 235/40R18 turns about 794, a difference of +38. Your speedometer and odometer both count rotations, so this number is what throws them off.