Tire size comparison
255/45R18 vs 255/40R19
Overall diameter, sidewall, revs per mile, and speedometer error — 255/45R18 is the taller tire, with a 0% speedo delta when swapping.
255/45R18
27.04in
687 mm overall diameter
255/40R19
27.03in
687 mm overall diameter
Diameter Δ
0 in
Speedo error
0%
255/45R18 revs/mi
746
255/40R19 revs/mi
746
Within the ±3% overall-diameter band usually considered safe for plus-sizing — speedometer and ABS calibration should stay close to factory.
Full spec
Side by side
| Metric | 255/45R18 | 255/40R19 |
|---|---|---|
| Section width | 255 mm | 255 mm |
| Aspect ratio | 45 | 40 |
| Wheel diameter | 18″ | 19″ |
| Sidewall height | 115 mm | 102 mm |
| Overall diameter | 27.04 in (687 mm) | 27.03 in (687 mm) |
| Circumference | 84.93 in | 84.92 in |
| Revolutions / mile | 746 | 746 |
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FAQ
Frequently asked
- What is the diameter difference between 255/45R18 and 255/40R19?
- 255/45R18 has an overall diameter of 27.04 inches (687 mm). 255/40R19 is 27.03 inches (687 mm). That's a 0 inch (0 mm) difference — 255/40R19 is shorter.
- Will 255/40R19 cause speedometer error on a car fitted with 255/45R18?
- Swapping 255/45R18 for 255/40R19 produces a speedometer error of about 0%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 60 mph. Anything under 3% is usually acceptable without recalibration.
- How many revolutions per mile does each size do?
- 255/45R18 turns 746 revs per mile. 255/40R19 turns 746 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
- What's the sidewall height difference?
- 255/45R18 has a sidewall of 115 mm (4.52 in). 255/40R19 sidewall is 102 mm (4.02 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
- Is 255/40R19 a safe plus-size upgrade from 255/45R18?
- A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.
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