ElpiAuto235/40R18 vs 255/40R18
255/40R18 is 2.5% taller (16 mm) than 235/40R18 — right at the edge of the ±3% swap band.
Two sizes
Also accepts 35X12.50R17 and 31x10.5-15.
Both sizes, drawn to scale
1 grid = 1 inWhat each number means
The two size codes broken down — which digit is which, and what it does on the car.
Size A — 235/40R18
- 235section width9.3″ across the tread, sidewall to sidewall
- 40aspect ratiosidewall is 40% of the width — 3.7″ tall
- Rconstructionradial — standard on every modern road car
- 18wheel diametermounts on a 18-inch wheel
In flotation sizing: ≈ 25.4x9.3R18
Size B — 255/40R18
- 255section width10.0″ across the tread, sidewall to sidewall
- 40aspect ratiosidewall is 40% of the width — 4.0″ tall
- Rconstructionradial — standard on every modern road car
- 18wheel diametermounts on a 18-inch wheel
In flotation sizing: ≈ 26x10R18
235/40R18 vs 255/40R18, measurement by measurement
Every dimension in inches, with the difference and which size is bigger
| Measurement | 235/40R18 | 255/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.40″ | 26.03″ | +0.63″ | +2.5% |
| Section width | 9.25″ | 10.04″ | +0.79″ | +8.5% |
| Sidewall height | 3.70″ | 4.02″ | +0.31″ | +8.5% |
| Circumference | 79.80″ | 81.78″ | +1.98″ | +2.5% |
| Revs per mile | 794.0 | 774.8 | −19.2 | −2.4% |
| Revs per km | 493.4 | 481.4 | −11.9 | −2.4% |
| Approved rim width estimate | 6.5–9″ | 7–9.5″ | — | — |
Overall diameter is the controlling number — clearance, gearing, speedometer accuracy and odometer reading all follow from it. Revs per mile/km is what your ABS, speedometer and odometer actually count.
What changes on the road
Direction and rough size of each effect going from 235/40R18 to 255/40R18. Geometry only — real tires vary by brand and compound.
Speedometer & odometer error
What changes on the dash when the overall diameter changes.
A speedometer doesn't measure distance. It counts how fast the wheels turn and multiplies by a fixed factory number for tire circumference. Change the tire and that factory number is now wrong.
Going to a bigger tire, each turn covers more ground, so at a true 61.5 mph the wheels spin at the rate the factory calls 60. The dial reads low — you are speeding without knowing it. A smaller tire does the opposite, and the odometer also over-counts miles you didn't drive.
Over 10,000 indicated miles, this change means the odometer under-counts by roughly 242 miles.
| Speedometer reads | True speed on 255/40R18 | Error |
|---|---|---|
| 20 mph | 20.5 mph | +0.5 |
| 30 mph | 30.7 mph | +0.7 |
| 40 mph | 41.0 mph | +1.0 |
| 50 mph | 51.2 mph | +1.2 |
| 60 mph | 61.5 mph | +1.5 |
| 70 mph | 71.7 mph | +1.7 |
| 80 mph | 82.0 mph | +2.0 |
With 235/40R18 as the original size. A taller tire makes the dial read low — the gap grows with speed.
Reading this comparison
The overall diameter is what matters here: 255/40R18 is the taller tire. It sets ride height, gearing and how far the tire travels per revolution — which is what the speedometer and odometer count. Width and sidewall change fit and feel (a wider tire needs a wider wheel and more clearance; a shorter sidewall sharpens steering) but not the gearing.
Figures are geometric estimates from the size codes; real moulded tires vary by brand by 1–3%. Confirm load rating, wheel width and physical clearance on your vehicle before fitting. To compare any other two sizes, use the tire size comparison tool; to decode one size and see what else fits its wheel, the tire size calculator.