245/45R18 vs 245/40R18
New overall diameter
25.72″
was 26.68″
True speed at an indicated 60
57.8 mph
your speedo reads high
Revolutions per mile
784
was 756, +28
| Measurement | 245/45R18 | 245/40R18 | Change |
|---|---|---|---|
| Overall diameter | 26.68″ | 25.72″ | −0.96″ (−3.6%) |
| Sidewall height | 4.34″ (110.3 mm) | 3.86″ (98.0 mm) | −12.3 mm |
| Tread width | 245 mm | 245 mm | +0 mm |
| Circumference | 83.82″ | 80.79″ | −3.03″ |
| Revs per mile | 756 | 784 | +28 |
| Speedometer error | baseline | −3.6% | reads high |
Diameter overlay
245/45R18 245/40R18
Speedometer check
With 245/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.1 mph actual
35 mph shown
33.7 mph actual
45 mph shown
43.4 mph actual
55 mph shown
53.0 mph actual
65 mph shown
62.7 mph actual
75 mph shown
72.3 mph actual
Common questions
- Will 245/40R18 fit instead of 245/45R18?
- At 3.6% this lands between the 3% and 5% marks. Plenty of people run swaps like this, but you will see real speedometer error, and you want to check the wheel well at full lock and full compression before you buy. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 26.68″ to 25.72″, a change of −3.6%. 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 57.8 mph, a −3.6% 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 245/40R18?
- 245/45R18 turns about 756 times per mile and 245/40R18 turns about 784, a difference of +28. Your speedometer and odometer both count rotations, so this number is what throws them off.