255/45R18 vs 235/50R18
New overall diameter
27.25″
was 27.04″
True speed at an indicated 60
60.5 mph
your speedo reads low
Revolutions per mile
740
was 746, −6
| Measurement | 255/45R18 | 235/50R18 | Change |
|---|---|---|---|
| Overall diameter | 27.04″ | 27.25″ | +0.22″ (+0.8%) |
| Sidewall height | 4.52″ (114.8 mm) | 4.63″ (117.5 mm) | +2.8 mm |
| Tread width | 255 mm | 235 mm | −20 mm |
| Circumference | 84.93″ | 85.61″ | +0.68″ |
| Revs per mile | 746 | 740 | −6 |
| Speedometer error | baseline | +0.8% | reads low |
Diameter overlay
255/45R18 235/50R18
Speedometer check
With 235/50R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.2 mph actual
35 mph shown
35.3 mph actual
45 mph shown
45.4 mph actual
55 mph shown
55.4 mph actual
65 mph shown
65.5 mph actual
75 mph shown
75.6 mph actual
Common questions
- Will 235/50R18 fit instead of 255/45R18?
- Diameter moves 0.8%, 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 27.04″ to 27.25″, a change of +0.8%. 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 60.5 mph, a +0.8% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 255/45R18 and 235/50R18?
- 255/45R18 turns about 746 times per mile and 235/50R18 turns about 740, a difference of −6. Your speedometer and odometer both count rotations, so this number is what throws them off.