235/45R18 vs 245/45R18 tire size
A 245/45R18 is 0.35 inches (9 mm) taller and 0.39 inches (10 mm) wider than a 235/45R18. Fit them in place of 235/45R18 tires and the speedometer reads low: at 60 mph on it you’re doing 60.8 mph, 1.3% faster, and the vehicle sits 0.18 inches (5 mm) higher.
Sources checked
| Dimension | 235/45R18 | 245/45R18 | Change |
|---|---|---|---|
| Overall diameter | 26.33 in (669 mm) | 26.68 in (678 mm) | +0.35 in (+9 mm) |
| Section width | 9.25 in (235 mm) | 9.65 in (245 mm) | +0.40 in (+10 mm) |
| Sidewall height | 4.16 in (106 mm) | 4.34 in (110 mm) | +0.18 in (+4 mm) |
| Aspect ratio | 45% | 45% | |
| Rim diameter | 18 in | 18 in | |
| Circumference | 82.7 in (2,101 mm) | 83.8 in (2,129 mm) | +1.1 in |
| Revolutions per mile | 766 | 756 | −10 |
| Revolutions per km | 476 | 470 | −6 |
Check another speed or size
Metric, LT and flotation
At 60 mph on the speedometer you’re doing
60.8mph
+1.3% · the speedometer reads low
- Height
- +0.35 in (+9 mm)
- Width
- +0.39 in (+10 mm)
- Ground clearance
- +0.18 in (+5 mm)
- Each side, same wheel
- +0.20 in (+5 mm)
235/45R18 and 245/45R18 side by side
| Dimension | 235/45R18 | 245/45R18 | Change |
|---|---|---|---|
| Overall diameter | 26.33 in (669 mm) | 26.68 in (678 mm) | +0.35 in (+9 mm) |
| Section width | 9.25 in (235 mm) | 9.65 in (245 mm) | +0.40 in (+10 mm) |
| Sidewall height | 4.16 in (106 mm) | 4.34 in (110 mm) | +0.18 in (+4 mm) |
| Aspect ratio | 45% | 45% | |
| Rim diameter | 18 in | 18 in | |
| Circumference | 82.7 in | 83.8 in | +1.1 in |
| Revolutions per mile | 766 | 756 | −10 |
| Revolutions per km | 476 | 470 | −6 |
The working
- 235/45R18: sidewall = 235 mm × 45% = 105.8 mm = 4.16 in.
- Diameter = rim + 2 × sidewall = 18 + 2 × 4.16 = 26.33 in (669 mm).
- Circumference = π × 26.33 = 82.7 in; revolutions per mile = 63,360 ÷ 82.7 = 766.
- 245/45R18: sidewall = 245 mm × 45% = 110.3 mm = 4.34 in.
- Diameter = rim + 2 × sidewall = 18 + 2 × 4.34 = 26.68 in (678 mm).
- Circumference = π × 26.68 = 83.8 in; revolutions per mile = 63,360 ÷ 83.8 = 756.
- Speedometer. True speed = what it shows × new diameter ÷ old = 60 × 26.68 ÷ 26.33 = 60.8 mph.
- Ground clearance. Half the change in diameter: (26.68 − 26.33) ÷ 2 = +0.18 in.
Speedometer
A 245/45R18 turns 756 times a mile against the 235/45R18’s 766. With a speedometer set for 235/45R18, true speed is what it shows × 26.68 ÷ 26.33:
| Speedometer shows | True speed | Difference |
|---|---|---|
| 25 mph | 25.3 mph | +0.3 mph |
| 35 mph | 35.5 mph | +0.5 mph |
| 45 mph | 45.6 mph | +0.6 mph |
| 55 mph | 55.7 mph | +0.7 mph |
| 65 mph | 65.9 mph | +0.9 mph |
| 75 mph | 76.0 mph | +1.0 mph |
Clearance
- Ground clearance: +0.18 in (+5 mm), half the change in diameter.
- Top of the tread: +0.18 in (+5 mm), so that much less room under the fender.
- Each side, on the same wheel: +0.20 in (+5 mm), toward the suspension inside and toward the fender lip outside.
A new wheel with a different width or offset moves the tire again. The sidewall goes from 4.16 to 4.34 inches.
What tire makers publish
Inflated on the maker’s measuring rim; the percentage is against the arithmetic.
| Tire | Size as printed | Diameter | Width | Revs per mile |
|---|---|---|---|---|
| Toyo Extensa A/S II | 235/45R18 | 26.3 in (−0.1%) | 9.3 in | 789 |
| Toyo Proxes Sport A/S | 235/45R18 | 26.3 in (−0.1%) | 9.3 in | 789 |
| Toyo Proxes Sport A/S | 245/45R18 | 26.7 in (+0.1%) | 9.6 in | 779 |
Each size on its own
- 235/45R18 tire size: 26.3 × 9.3 in
- 245/45R18 tire size: 26.7 × 9.6 in
Other comparisons
Questions
Can I fit 245/45R18 tires instead of 235/45R18?
They fit the same 18-inch wheels. A 245/45R18 is 0.35 inches (9 mm) taller and 0.39 inches (10 mm) wider than a 235/45R18, so check three things first: that your wheels are within the 245/45R18’s approved rim width range, which tire makers list for each size; that it clears the fenders and suspension at full lock and full travel; and that its load index meets what your door-jamb placard asks for.
How far off is the speedometer with 245/45R18 tires?
With a speedometer set for 235/45R18, the speedometer reads low: at 60 mph on it you’re doing 60.8 mph, 1.3% faster. The odometer is off the same way: over 1,000 miles driven it shows 986.7.
Does 245/45R18 change the ground clearance?
By +0.18 inches (+5 mm), half the change in diameter, at the axles and at the lowest point under them. On the same wheel, each sidewall sits 0.20 inches further out, inside and outside.
Sources
- Bridgestone Americas: Determining tire size: P-metric, Euro-metric, LT-metric, C-type and flotation sizes, and what each number means (checked October 28, 2026)
- Toyo Tires: Extensa A/S II sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
- Toyo Tires: Proxes Sport A/S sizes and specifications: inflated overall diameter and width, revolutions per mile (checked October 28, 2026)
- Toyo Tires: Open Country A/T III sizes and specifications: P-metric, LT-metric and flotation sizes, with revolutions per mile (checked October 28, 2026)
- Nitto Tire: Trail Grappler M/T sizes and specs: inflated overall diameter and width, LT-metric and flotation sizes (checked October 28, 2026)
Spotted a number that doesn’t match your manual or a source? Tell us; corrections are logged on the sources page.