245/40R18 vs 245/45R18 tire size
A 245/45R18 is 0.96 inches (25 mm) taller than a 245/40R18, and the same width. Fit them in place of 245/40R18 tires and the speedometer reads low: at 60 mph on it you’re doing 62.3 mph, 3.8% faster, and the vehicle sits 0.48 inches (12 mm) higher.
Sources checked
| Dimension | 245/40R18 | 245/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.72 in (653 mm) | 26.68 in (678 mm) | +0.96 in (+25 mm) |
| Section width | 9.65 in (245 mm) | 9.65 in (245 mm) | 0.00 in (0 mm) |
| Sidewall height | 3.86 in (98 mm) | 4.34 in (110 mm) | +0.48 in (+12 mm) |
| Aspect ratio | 40% | 45% | |
| Rim diameter | 18 in | 18 in | |
| Circumference | 80.8 in (2,052 mm) | 83.8 in (2,129 mm) | +3.0 in |
| Revolutions per mile | 784 | 756 | −28 |
| Revolutions per km | 487 | 470 | −17 |
Check another speed or size
Metric, LT and flotation
At 60 mph on the speedometer you’re doing
62.3mph
+3.8% · the speedometer reads low
- Height
- +0.96 in (+25 mm)
- Width
- 0.00 in (0 mm)
- Ground clearance
- +0.48 in (+12 mm)
- Each side, same wheel
- 0.00 in (0 mm)
245/40R18 and 245/45R18 side by side
| Dimension | 245/40R18 | 245/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.72 in (653 mm) | 26.68 in (678 mm) | +0.96 in (+25 mm) |
| Section width | 9.65 in (245 mm) | 9.65 in (245 mm) | 0.00 in (0 mm) |
| Sidewall height | 3.86 in (98 mm) | 4.34 in (110 mm) | +0.48 in (+12 mm) |
| Aspect ratio | 40% | 45% | |
| Rim diameter | 18 in | 18 in | |
| Circumference | 80.8 in | 83.8 in | +3.0 in |
| Revolutions per mile | 784 | 756 | −28 |
| Revolutions per km | 487 | 470 | −17 |
The working
- 245/40R18: sidewall = 245 mm × 40% = 98.0 mm = 3.86 in.
- Diameter = rim + 2 × sidewall = 18 + 2 × 3.86 = 25.72 in (653 mm).
- Circumference = π × 25.72 = 80.8 in; revolutions per mile = 63,360 ÷ 80.8 = 784.
- 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 ÷ 25.72 = 62.3 mph.
- Ground clearance. Half the change in diameter: (26.68 − 25.72) ÷ 2 = +0.48 in.
Speedometer
A 245/45R18 turns 756 times a mile against the 245/40R18’s 784. With a speedometer set for 245/40R18, true speed is what it shows × 26.68 ÷ 25.72:
| Speedometer shows | True speed | Difference |
|---|---|---|
| 25 mph | 25.9 mph | +0.9 mph |
| 35 mph | 36.3 mph | +1.3 mph |
| 45 mph | 46.7 mph | +1.7 mph |
| 55 mph | 57.1 mph | +2.1 mph |
| 65 mph | 67.4 mph | +2.4 mph |
| 75 mph | 77.8 mph | +2.8 mph |
Clearance
- Ground clearance: +0.48 in (+12 mm), half the change in diameter.
- Top of the tread: +0.48 in (+12 mm), so that much less room under the fender.
- Each side, on the same wheel: no change.
A new wheel with a different width or offset moves the tire again. The sidewall goes from 3.86 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 Proxes Sport A/S | 245/40R18 | 25.7 in (−0.1%) | 9.8 in | 808 |
| Toyo Proxes Sport A/S | 245/45R18 | 26.7 in (+0.1%) | 9.6 in | 779 |
Each size on its own
- 245/40R18 tire size: 25.7 × 9.6 in
- 245/45R18 tire size: 26.7 × 9.6 in
Other comparisons
Questions
Can I fit 245/45R18 tires instead of 245/40R18?
They fit the same 18-inch wheels. A 245/45R18 is 0.96 inches (25 mm) taller than a 245/40R18, and the same width, 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 245/40R18, the speedometer reads low: at 60 mph on it you’re doing 62.3 mph, 3.8% faster. The odometer is off the same way: over 1,000 miles driven it shows 963.8.
Does 245/45R18 change the ground clearance?
By +0.48 inches (+12 mm), half the change in diameter, at the axles and at the lowest point under them. The width doesn’t change.
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.