235/45R17 vs 235/40R18
New overall diameter
25.40″
was 25.33″
True speed at an indicated 60
60.2 mph
your speedo reads low
Revolutions per mile
794
was 796, −2
| Measurement | 235/45R17 | 235/40R18 | Change |
|---|---|---|---|
| Overall diameter | 25.33″ | 25.40″ | +0.07″ (+0.3%) |
| Sidewall height | 4.16″ (105.8 mm) | 3.70″ (94.0 mm) | −11.8 mm |
| Tread width | 235 mm | 235 mm | +0 mm |
| Circumference | 79.57″ | 79.80″ | +0.24″ |
| Revs per mile | 796 | 794 | −2 |
| Speedometer error | baseline | +0.3% | reads low |
Diameter overlay
235/45R17 235/40R18
Speedometer check
With 235/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.1 mph actual
55 mph shown
55.2 mph actual
65 mph shown
65.2 mph actual
75 mph shown
75.2 mph actual
Common questions
- Will 235/40R18 fit instead of 235/45R17?
- Overall diameter changes 0.3%; section width changes 0 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 25.33″ to 25.40″, a change of +0.3%. 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 60.2 mph, a +0.3% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 235/45R17 and 235/40R18?
- 235/45R17 turns about 796 times per mile and 235/40R18 turns about 794, a difference of −2. Your speedometer and odometer both count rotations, so this number is what throws them off.