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225/45R17 is 24.97 inches tall and turns 808 times per mile. Jump to 235/40R18 and your speedometer reads 59 at a true 60 — inside the 3% rule.
The code on your sidewall — 225/45R17 — is three numbers: width in millimeters, sidewall as a percentage of width, rim diameter in inches. This calculator decodes any size into overall diameter and revs per mile, then compares a second size so you can price the speedometer error before buying wheels — because diameter is what the speedo, ABS, and gearing all assume.
Drivers plus-sizing wheels use it: 225/45R17 is 24.97 in tall; jump to 235/40R18 and the speedo reads 59 at a true 60 — within the 3% rule.
Three numbers in two unit systems, because tires are globally weird: 225 is the section width in millimeters; 45 is the aspect ratio — sidewall height as 45% of width (225 × 0.45 = 101.25 mm); R means radial construction (nearly everything now); 17 is the rim diameter in inches. Overall diameter = rim + two sidewalls: 17 × 25.4 + 2 × 101.25 = 634.3 mm = 24.97 inches. Variations you'll meet: LT sizes (LT265/75R16, load-rated for trucks), flotation sizes (35×12.50R17 = diameter×width, off-road style), and the trailing load/speed index (94V: 1,477 lb per tire, 149 mph rated). The aspect ratio history lesson: 70-series was standard in the 1970s; today's 35–45 series exist because bigger brakes and looks demanded shorter sidewalls.
Keep the new overall diameter within ±3% of stock — because your speedometer, odometer, ABS, stability control, and transmission shift points are all calibrated to stock diameter. Bigger tires make you go faster than indicated (reads 60, actually 61+); smaller make you slower. Beyond 3%: speedometer tickets you didn't earn, ABS faults, incorrect shift timing, and potential rubbing on full lock or full suspension compression. The plus-sizing recipe that stays safe: +1 inch of rim needs −5 points of aspect ratio at similar width (225/45R17 → 225/40R18 keeps diameter within 1%). Check clearance too — wider isn't just taller math: 10 mm of extra width can rub the strut or fender liner even when diameter matches.
Steering sharpens, everything else pays. Shorter sidewalls flex less, so turn-in response and cornering stability genuinely improve — that is why sports cars ship on 40-series and lower. The costs: ride harshness rises (the sidewall is the first suspension), pothole damage risk jumps (less air cushion between rim and road — bent 19-inch rims are a pothole-season ritual), tire prices climb steeply with rim diameter, and winter performance usually suffers. Range matters on EVs: the same car on 21-inch wheels can lose 5–10% of rated range versus 18s — Tesla and Rivian owners learn this at delivery. The sweet spot most engineers privately recommend: the middle trim's wheel size — the one the suspension was actually tuned around.