235/40R19 vs 255/40R18
New overall diameter
26.03″
was 26.40″
True speed at an indicated 60
59.2 mph
your speedo reads high
Revolutions per mile
775
was 764, +11
| Measurement | 235/40R19 | 255/40R18 | Change |
|---|---|---|---|
| Overall diameter | 26.40″ | 26.03″ | −0.37″ (−1.4%) |
| Sidewall height | 3.70″ (94.0 mm) | 4.02″ (102.0 mm) | +8.0 mm |
| Tread width | 235 mm | 255 mm | +20 mm |
| Circumference | 82.94″ | 81.78″ | −1.16″ |
| Revs per mile | 764 | 775 | +11 |
| Speedometer error | baseline | −1.4% | reads high |
Diameter overlay
235/40R19 255/40R18
Speedometer check
With 255/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.6 mph actual
35 mph shown
34.5 mph actual
45 mph shown
44.4 mph actual
55 mph shown
54.2 mph actual
65 mph shown
64.1 mph actual
75 mph shown
73.9 mph actual
Common questions
- Will 255/40R18 fit instead of 235/40R19?
- Overall diameter changes 1.4%; section width changes 20 mm. Diameter comparison alone cannot confirm fitment. Verify the vehicle maker's approved rim width, tire load and speed ratings, and clearance at full steering lock and suspension compression. Confirm any possible ABS, traction-control, speedometer, and drivetrain effects with the vehicle maker or a qualified tire professional. Overall diameter goes from 26.40″ to 26.03″, a change of −1.4%. 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 255/40R18?
- At an indicated 60 mph you are really doing about 59.2 mph, a −1.4% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 235/40R19 and 255/40R18?
- 235/40R19 turns about 764 times per mile and 255/40R18 turns about 775, a difference of +11. Your speedometer and odometer both count rotations, so this number is what throws them off.