205/65R16 vs 235/45R18
New overall diameter
26.33″
was 26.49″
True speed at an indicated 60
59.6 mph
your speedo reads high
Revolutions per mile
766
was 761, +5
| Measurement | 205/65R16 | 235/45R18 | Change |
|---|---|---|---|
| Overall diameter | 26.49″ | 26.33″ | −0.17″ (−0.6%) |
| Sidewall height | 5.25″ (133.3 mm) | 4.16″ (105.8 mm) | −27.5 mm |
| Tread width | 205 mm | 235 mm | +30 mm |
| Circumference | 83.23″ | 82.71″ | −0.52″ |
| Revs per mile | 761 | 766 | +5 |
| Speedometer error | baseline | −0.6% | reads high |
Diameter overlay
205/65R16 235/45R18
Speedometer check
With 235/45R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.8 mph actual
35 mph shown
34.8 mph actual
45 mph shown
44.7 mph actual
55 mph shown
54.7 mph actual
65 mph shown
64.6 mph actual
75 mph shown
74.5 mph actual
Common questions
- Will 235/45R18 fit instead of 205/65R16?
- Diameter moves 0.6%, 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 high, so you are going slower than it says. Overall diameter goes from 26.49″ to 26.33″, a change of −0.6%. 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 59.6 mph, a −0.6% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 205/65R16 and 235/45R18?
- 205/65R16 turns about 761 times per mile and 235/45R18 turns about 766, a difference of +5. Your speedometer and odometer both count rotations, so this number is what throws them off.