What the numbers on the sidewall actually mean
A tyre size written as 225/45R17 packs three separate measurements into one string, and reading it correctly is most of the battle before comparing two sizes against each other. 225 is the section width in millimetres, roughly how wide the tyre is across its tread when mounted and inflated. 45 is the aspect ratio, not a distance but a percentage: it's the sidewall height expressed as a percentage of that section width, so a 45 aspect ratio on a 225mm wide tyre gives a sidewall of just over 100mm. R means radial construction, which is what almost every modern car tyre uses. 17 is the rim diameter, and it's the one figure in the whole string that's genuinely in inches rather than millimetres, a leftover from imperial rim sizing that never got converted. Feed all three numbers for two tyre sizes into the calculator above and it works out the overall diameter, the circumference and the sidewall height for each, plus what changing between them does to your speedometer.
Why a size change moves the speedo, not just the ride
A car's speedometer doesn't measure speed directly. It counts wheel revolutions and converts that count to a speed reading using the rolling diameter the factory tyre was built around. Fit a tyre with a genuinely different overall diameter and that conversion is now wrong, because a bigger tyre covers more ground per revolution and a smaller one covers less, while the speedo keeps counting revolutions as if nothing changed. Fit a taller tyre and the car is travelling faster than the speedo says at any given reading, because each revolution is covering more distance than the speedo assumes. Fit a shorter tyre and the reverse happens, the car is travelling slower than the speedo claims. That direction matters and it's easy to get backwards, which is exactly why this calculator states it plainly for whatever two sizes you've entered rather than leaving you to work out which way the error runs.
The odometer runs off the same revolution count, so it moves the same way. A bigger tyre means the car is actually travelling further than the odometer records for a given trip, and a smaller tyre means the reverse. Over a long trip that's a real gap between the distance the odometer claims and the distance actually driven, running in the same direction and roughly the same proportion as the speedo error above.
Gearing and the fuel figure on the sticker
A tyre's rolling diameter is effectively one more gear ratio sitting between the gearbox and the road, even though nobody thinks of it that way. A taller tyre means the engine turns fewer revolutions to cover the same distance at a given speed, which can shave a little off fuel use at highway speed and can also mean the engine is labouring slightly harder from a standing start, since the same torque is now working through a taller effective gear. A shorter tyre does the opposite, quicker off the line, marginally thirstier and busier at highway speed. None of this is usually dramatic on a modest size change, but it's real, and it's part of why the fuel figure on a car's compliance sticker was measured on the factory tyre size, not on whatever's currently mounted.
Rim width is the limit nobody reads off the sidewall
The three numbers in a tyre size say nothing about whether that tyre will actually sit properly on a given rim, and that's a real, physical limit rather than a legal formality. Every tyre section width has a manufacturer-specified range of rim widths it's designed to be mounted on, published by the tyre maker, not derived from the size code itself. Mount a tyre outside that range and the tyre's profile distorts on the rim, changing how it actually performs and wears compared to what the size numbers alone would suggest, and in some cases it won't seat and seal correctly at all. If you're changing to a size with a noticeably different section width, that's a question for the tyre shop fitting it, working from the rim you actually have, not something this calculator, which only knows the three numbers you've entered, can tell you.
The placard on the door jamb is where the real answer lives
Every vehicle sold in Australia carries a compliance placard, usually inside the driver's door jamb, sometimes in the glovebox or fuel flap, stating the tyre size (or sizes, where more than one is approved) the manufacturer actually designed the car around. That placard is the reference point for whether a size you're considering counts as a minor change or a genuinely different setup, and it's worth checking before assuming a popular aftermarket size is a safe swap for your specific vehicle. A change that reads as a small percentage difference in this calculator can still be outside what a particular car's suspension geometry, guards or speedo calibration were designed to tolerate, and the placard, not a general rule of thumb, is where that gets checked first.
Why there's a legal tolerance on speedo accuracy at all
A speedometer isn't required to be perfectly accurate, and Australian vehicles are built to a specific tolerance rather than an exact reading. Australian Design Rule 18/03 (Instrumentation) sets the rule: a speedometer must never indicate a speed below the vehicle's true speed, and it must not exceed the true speed by more than 10 percent plus 4 km/h. In practice that means factory speedos are usually set to read a little high, never low, so a driver sticking to the number on the dash is never actually speeding relative to the true figure. A tyre size change that pushes the speedo the wrong way, reading low rather than high, moves the car outside the margin that rule exists to guarantee, which is a meaningful safety reason to check the numbers on a size change, not just a curiosity. See the full instrument at legislation.gov.au.
Plus-sizing: why the wheel gets bigger and the tyre gets shorter
A lot of size changes people run through this calculator aren't random, they follow a pattern the tyre trade calls plus-sizing. Moving from a factory size to a wider tyre on a larger rim, while dropping the aspect ratio to compensate, is a deliberate way to change a car's look and grip without changing the overall rolling diameter by much at all. A well-executed plus-one or plus-two change keeps the diameter close enough to the original that the speedo barely moves, which is exactly what the percentage difference in the result above is useful for confirming. Punch in the factory size and the proposed new one and a genuinely well-matched plus-size change should come back showing a small percentage difference, usually under two or three percent. If it's coming back a lot higher than that, the two sizes aren't as closely matched as the seller might be suggesting, and it's worth reconsidering the aspect ratio on the new size before buying rather than after.
ABS, cruise control and other systems that trust the wheel speed
The speedometer isn't the only system reading wheel revolutions. ABS, traction control, stability control and adaptive cruise all use wheel-speed sensors that were calibrated against the factory rolling diameter, the same way the speedo was. A modest size change usually sits well within what these systems tolerate without any noticeable effect, but a bigger change can shift how they behave at the margins, since they're all working off the same underlying assumption about how far the wheel travels per revolution. This isn't something a driver can feel or test for themselves in a driveway. It's another reason a size change bigger than a small plus-sizing step is worth running past someone who fits tyres for a living rather than deciding on measurements alone.
Reading the result
Enter your current tyre size and the one you're considering, then a true speed you'd actually like to check, a common one to try is 100 km/h. The result tells you what the speedo will show at that true speed once the new tyre is fitted, plus the overall diameter, circumference and sidewall height for both sizes and the percentage difference between them. A small percentage difference, a couple of percent either way, is common on a plus-sizing change and rarely worth worrying about beyond confirming the rim width and the guard clearance with the shop fitting it. A bigger jump is worth checking against the placard before committing to it, and worth a phone call to a tyre shop that can confirm fitment on the actual vehicle, not just the arithmetic.
What this calculator doesn't cover
This tool works from the three numbers in a tyre size and nothing else, so it can tell you what happens to diameter, circumference, sidewall height and speedo reading between two sizes. It doesn't assess whether a specific tyre is safe to keep using, tread depth, general condition, age or damage all matter for that and none of them are captured in a size code. Roadworthiness is a separate assessment again, covering the tyre and a lot more besides, and it's handled by whoever's doing the actual inspection, not by a size comparison. Treat this calculator as the first step, working out whether a size change is a sensible one before you buy, not the last one.