235/40R18 vs 235/45R18
New overall diameter
26.33″
was 25.40″
True speed at an indicated 60
62.2 mph
your speedo reads low
Revolutions per mile
766
was 794, −28
| Measurement | 235/40R18 | 235/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.40″ | 26.33″ | +0.93″ (+3.6%) |
| Sidewall height | 3.70″ (94.0 mm) | 4.16″ (105.8 mm) | +11.8 mm |
| Tread width | 235 mm | 235 mm | +0 mm |
| Circumference | 79.80″ | 82.71″ | +2.91″ |
| Revs per mile | 794 | 766 | −28 |
| Speedometer error | baseline | +3.6% | reads low |
Diameter overlay
235/40R18 235/45R18
Speedometer check
With 235/45R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.9 mph actual
35 mph shown
36.3 mph actual
45 mph shown
46.6 mph actual
55 mph shown
57.0 mph actual
65 mph shown
67.4 mph actual
75 mph shown
77.7 mph actual
Common questions
- Will 235/45R18 fit instead of 235/40R18?
- 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 low, so you are going faster than it says. Overall diameter goes from 25.40″ to 26.33″, 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 235/45R18?
- At an indicated 60 mph you are really doing about 62.2 mph, a +3.6% 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 235/45R18?
- 235/40R18 turns about 794 times per mile and 235/45R18 turns about 766, a difference of −28. Your speedometer and odometer both count rotations, so this number is what throws them off.