235/45R17 vs 245/40R18
New overall diameter
25.72″
was 25.33″
True speed at an indicated 60
60.9 mph
your speedo reads low
Revolutions per mile
784
was 796, −12
| Measurement | 235/45R17 | 245/40R18 | Change |
|---|---|---|---|
| Overall diameter | 25.33″ | 25.72″ | +0.39″ (+1.5%) |
| Sidewall height | 4.16″ (105.8 mm) | 3.86″ (98.0 mm) | −7.8 mm |
| Tread width | 235 mm | 245 mm | +10 mm |
| Circumference | 79.57″ | 80.79″ | +1.22″ |
| Revs per mile | 796 | 784 | −12 |
| Speedometer error | baseline | +1.5% | reads low |
Diameter overlay
235/45R17 245/40R18
Speedometer check
With 245/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.4 mph actual
35 mph shown
35.5 mph actual
45 mph shown
45.7 mph actual
55 mph shown
55.8 mph actual
65 mph shown
66.0 mph actual
75 mph shown
76.2 mph actual
Common questions
- Will 245/40R18 fit instead of 235/45R17?
- Diameter moves 1.5%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer will read low, so you are going faster than it says. Overall diameter goes from 25.33″ to 25.72″, a change of +1.5%. 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 245/40R18?
- At an indicated 60 mph you are really doing about 60.9 mph, a +1.5% 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 245/40R18?
- 235/45R17 turns about 796 times per mile and 245/40R18 turns about 784, a difference of −12. Your speedometer and odometer both count rotations, so this number is what throws them off.