235/45R18 vs 245/40R19
Switching from 235/45R18 to 245/40R19 changes the overall diameter by +9.9 mm (+1.5%) — inside the commonly used ±3% guideline. Every figure below is calculated from the two size codes with the standard formulas.
| Measurement | 235/45R18 | 245/40R19 | Difference |
|---|---|---|---|
| Overall diameter | 668.7 mm | 678.6 mm | +9.9 mm (+1.5%) |
| Overall diameter | 26.33 in | 26.72 in | +0.39 in |
| Tire width | 235 mm (9.25 in) | 245 mm (9.65 in) | +10 mm |
| Sidewall height | 105.8 mm (4.16 in) | 98.0 mm (3.86 in) | -7.8 mm |
| Wheel (rim) diameter | 18.0 in | 19.0 in | +1.0 in |
| Circumference | 2101 mm | 2132 mm | +31 mm |
| Revolutions per mile | 2407 | 2372 | -35 |
| Revolutions per km | 476 | 469 | -7 |
Speedometer error
Calibrated for 235/45R18, the speedometer under-reads or over-reads once 245/40R19 is fitted. This is your real speed at each indicated speed:
| Speedometer shows | Actual speed on 245/40R19 | Error |
|---|---|---|
| 20 mph | 20.3 mph | +0.3 mph |
| 40 mph | 40.6 mph | +0.6 mph |
| 60 mph | 60.9 mph | +0.9 mph |
| 80 mph | 81.2 mph | +1.2 mph |
| 100 mph | 101.5 mph | +1.5 mph |
How these two sizes are worked out
235/45R18 breaks down as a 235 mm tread width with a sidewall of 45% of that width (105.8 mm) on a 18.0-inch wheel, giving an overall diameter of 668.7 mm (26.33 in). 245/40R19 works out to 678.6 mm (26.72 in) the same way. The standard formulas are the same ones used on the tire size calculator page: diameter = rim × 25.4 + 2 × sidewall, circumference = π × diameter, revolutions per mile = 63,360 ÷ diameter in inches.
Nominal sizes vary slightly between brands and models, and tread wear shrinks a tire over its life. These figures follow the size codes exactly; your vehicle manual or a professional tire shop is the authority for what fits your vehicle.
More comparisons
Frequently asked questions
Can I replace 235/45R18 tires with 245/40R19?
The diameter change is +1.5%, which is within the commonly used ±3% guideline. Diameter is only one part of fitment — wheel width range, load index, speed rating and physical clearance all have to check out too, so confirm with your vehicle manual or a professional tire shop before buying.
Will 245/40R19 make my speedometer read wrong?
Yes, by the diameter ratio. When the dial shows 60 mph you are really doing 60.9 mph on 245/40R19. The full table above covers 20 to 100 mph.
Which tire is taller, 235/45R18 or 245/40R19?
245/40R19 is taller — 678.6 mm against 668.7 mm overall diameter, a difference of 9.9 mm.