changing rim size calculator Page 577

Here’s the cheat sheet while you hunt for a rim-size calculator: keep the overall rolling diameter about the same so your speedo doesn’t fib. The formula is Overall diameter (mm) = (Rim diameter in inches × 25.4) + 2 × (Tyre width in mm × Aspect ratio ÷ 100). Example: 225/55R17 ≈ 679 mm; step to 245/40R19 ≈ 679 mm—near identical, so your guard liners won’t file a complaint. Aim to stay within roughly a couple of percent, check brake and suspension clearance, and keep load/speed ratings equal or higher. After the maths, get a proper fitment check, because rubbing is only funny when it’s a lamp and a genie pops out.