215/70R16 vs 235/50R18
New overall diameter
27.25″
was 27.85″
True speed at an indicated 60
58.7 mph
your speedo reads high
Revolutions per mile
740
was 724, +16
| Measurement | 215/70R16 | 235/50R18 | Change |
|---|---|---|---|
| Overall diameter | 27.85″ | 27.25″ | −0.60″ (−2.1%) |
| Sidewall height | 5.93″ (150.5 mm) | 4.63″ (117.5 mm) | −33.0 mm |
| Tread width | 215 mm | 235 mm | +20 mm |
| Circumference | 87.49″ | 85.61″ | −1.88″ |
| Revs per mile | 724 | 740 | +16 |
| Speedometer error | baseline | −2.1% | reads high |
Diameter overlay
215/70R16 235/50R18
Speedometer check
With 235/50R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.5 mph actual
35 mph shown
34.2 mph actual
45 mph shown
44.0 mph actual
55 mph shown
53.8 mph actual
65 mph shown
63.6 mph actual
75 mph shown
73.4 mph actual
Common questions
- Will 235/50R18 fit instead of 215/70R16?
- Diameter moves 2.1%, 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 27.85″ to 27.25″, a change of −2.1%. 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 235/50R18?
- At an indicated 60 mph you are really doing about 58.7 mph, a −2.1% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 215/70R16 and 235/50R18?
- 215/70R16 turns about 724 times per mile and 235/50R18 turns about 740, a difference of +16. Your speedometer and odometer both count rotations, so this number is what throws them off.