Tire Size Calculator

How to read a tire size like 225/45R17

Every metric tire size is three measurements glued together. In 225/45R17 the 225 is the section width in millimetres, measured across the widest part of the inflated tire. The 45 is the aspect ratio: the sidewall height is 45% of the section width, so 225 x 0.45 = 101.3 mm. The R means radial construction, and 17 is the wheel diameter in inches. Nothing in the code tells you the overall diameter directly — you have to build it: 17 x 25.4 = 431.8 mm of wheel, plus a sidewall at the top and another at the bottom, so 431.8 + 2 x 101.3 = 634.3 mm, or 24.97 inches. That number is the one that matters for everything else on this page.

Worked example: 225/45R17 to 235/40R18

This is the most common upgrade on a Golf, a 3 Series or an Accord, so run it end to end. The current tire: sidewall 225 x 0.45 = 101.3 mm, overall diameter 634.3 mm, circumference pi x 634.3 = 1,992.7 mm, and revolutions per mile 1,609,344 / 1,992.7 = 807.6. The new tire: sidewall 235 x 0.40 = 94.0 mm, overall diameter 457.2 + 188.0 = 645.2 mm, circumference 2,026.9 mm, revs per mile 794.0.

The diameter difference is (645.2 - 634.3) / 634.3 = +1.72%. That is comfortably inside the 3% window, so the speedometer, ABS and shift points stay within the tolerance the manufacturer designed for. Ride height rises by half the diameter change, 5.5 mm — barely visible, but enough to eat into the fender gap on a lowered car. The speedometer now under-reads: at a true 60 mph the needle shows 60 x 634.3 / 645.2 = 59.0 mph. You are going 1 mph faster than the dash claims, which also means your odometer accumulates miles slightly slower than reality.

Why 3% is the line everyone quotes

Your car has no idea how big its tires are. It counts wheel revolutions and multiplies by a diameter stored in software. Change the diameter and every derived value drifts by the same percentage: indicated speed, trip odometer, ABS wheel-slip comparisons, traction control intervention thresholds, and on some cars the transmission's shift map. UK and EU rules require that a speedometer never reads below true speed and never more than 10% plus 6.25 mph above it, so factories deliberately set the needle to read a few percent high. A tire that is 3% taller eats most of that built-in cushion; one that is 5% taller can push the indicated speed below true speed, which is the direction that gets you a ticket.

Speedometer error at a glance

The relationship is linear, so the arithmetic is easy to carry in your head: indicated speed = true speed x (old diameter / new diameter). A 2% taller tire at a true 70 mph indicates about 68.6 mph. A 5% taller tire indicates 66.7 mph — over 3 mph optimistic in the wrong direction. Go the other way with a shorter tire and the needle over-reads by the same proportion, which is harmless legally but makes the odometer log miles you never drove, and that quietly shortens warranty and service intervals.

Plus sizing without changing rolling diameter

Plus sizing is the trick behind almost every wheel upgrade: add an inch of wheel, subtract enough sidewall to keep the overall diameter the same. One inch of wheel diameter adds 25.4 mm overall, which means each sidewall has to shrink by about 12.7 mm. On a 225-section tire that is roughly 5 to 6 points of aspect ratio, which is why 45 becomes 40 when 17 becomes 18. Widening the section at the same time, 225 to 235, adds a little height back, and that is where the extra 1.7% in the example came from.

What you gain is a stiffer sidewall that resists rolling under cornering load, quicker steering response, and clearance for larger brake calipers. What you pay is comfort — less air means less cushion — plus wheel damage risk on broken pavement, higher tire prices, and often more road noise. Beyond plus-two on a road car, most people find the trade goes negative.

Load index, speed rating and the things this calculator cannot see

Diameter maths is only half the fitment question. The load index and speed rating stamped after the size, such as 91W, must meet or exceed the placard values on your door jamb, especially on anything that tows or carries weight. A wider tire also needs a rim within the manufacturer's approved width range, and it needs somewhere to live: check clearance to the strut, the inner fender liner and the fender edge at full lock and full suspension compression, not just sitting still in the driveway. Cars with direct TPMS sensors keep working after a size change, but the system's pressure targets still come from the placard, and a different size may want a different pressure to carry the same load. Finally, the calculator uses nominal figures — real measured widths and diameters vary by a few millimetres between brands and rim widths, so treat anything inside about 0.5% as a tie.