245/45R18 vs 255/40R18: Height, Width and Speed

Calculated dimensions and changes from the original tire

The first row, 245/45R18, is the original reference. All dimensions come from the tire geometry engine. Candidate percentages and speed results use that same original; figures are rounded only for display.

A broader passenger tire with less sidewall proportion

The two input changes work against each other in the sidewall formula. More width tends to deepen a sidewall at a fixed ratio, while the lower ratio reduces it. Only the full calculation establishes the final diameter. That is why a description such as one step wider is incomplete. A wider replacement can put its upper tread surface closer to the wheel center, changing the geometry in two different directions.

A lower sidewall proportion may be part of the visual goal, but it is not a measured handling improvement. The actual tire construction, wheel and operating conditions are missing from the code. Use the calculated sidewall to describe the package, then keep any steering, impact or comfort claim tied to relevant product information. The table cannot choose a preferred product from proportions alone.

The calculated change from 245/45R18 to 255/40R18

Using 245/45R18 as original, 255/40R18 changes nominal diameter from 26.68 to 26.03 inches. The difference is -16.50 mm, or -2.43% of the original diameter. Nominal section width changes by +10.00 mm and one sidewall changes by -8.25 mm. Those figures are calculated from the complete markings. They describe different dimensions, so a diameter result cannot stand in for a lateral clearance result.

The ideal radius difference is -8.25 mm (-0.32 inches). This is the geometric contribution to wheel center height, not a verified loaded ground clearance change. At an indicated 60 mph, the candidate gives a theoretical 58.54 mph actual speed, or -1.46 mph actual minus indicated. The assumption is calibration to the original circumference with no other instrument error; actual vehicle readings require their own evidence.

What this pair means for the actual assembly

If the wheel is retained, its mounting range still needs checking for the candidate. If the wheel is changed, offset can have a greater effect on one local clearance than the nominal width step suggests. Inspect the inner and outer space separately. The tire's ideal radius and nominal section width are useful starting dimensions, but neither locates the installed shoulders relative to the body.

A shorter nominal tire also changes the circumference used in reference calibrated speed and distance calculations. Read the signed result instead of assuming that every wider tire makes actual road speed higher. The same direction applies throughout the pair, with the first size fixed as original. Reverse the original and candidate in the calculator if you are assessing the opposite move.

Keep circumference, calibration and mounting separate

Circumference is calculated from diameter, and the candidate to original circumference ratio gives the theoretical actual speed multiplier. A taller circle travels farther per revolution, while a shorter one travels less. Reversing original and candidate changes the percentage denominator. Do not reverse this comparison by simply changing the sign of a rounded result; enter the actual original in the calculator and compute the other direction directly.

These calculations describe an ideal circle rather than a loaded rolling test. Exact product dimensions, wear, inflation and load can affect the real assembly. Wheel diameter matching does not establish an approved rim width, offset or vehicle clearance. Keep the actual tire model and complete service markings in the comparison record, and check the vehicle instructions if the dimensional change is outside its specified equipment.

Frequently asked questions

Can the wider candidate reduce ideal axle height?

Yes, if its smaller sidewall proportion produces a smaller overall diameter. The table calculates that balance directly. A width increase alone does not establish the direction of the radius or speed change.

Do the speed and radius figures establish real vehicle fitment?

No. They describe nominal geometry under a stated reference assumption. The exact tire and wheel, vehicle specifications and clearance through movement must be assessed separately; loaded dimensions and instrument behavior are not measured here.

What changes if 245/45R18 is the replacement rather than the original?

Reverse the inputs in the calculator. Percentage change uses the original diameter as denominator, and theoretical speed uses candidate circumference divided by original circumference. A reversed comparison therefore needs a fresh calculation rather than only a changed sign on the displayed percentage.

Continue your comparison

• Calculate a comparison using your documented original tire
• Read the metric size conversion method