Tyre size change calculator
Change a tyre size and this works out everything that follows as pure arithmetic: overall diameter, how far the axle rises, what your speedometer and odometer now read, how your reported fuel figure shifts, what happens to gearing and pulling force, and whether the change sits inside Australia's VSB14 diameter and width limits.
It deliberately stops where the arithmetic stops. It will not predict your fuel consumption, and it will not tell you the tyre clears your guards, because neither follows from the numbers on a sidewall.
VSB14 sets a different diameter limit for each, and it excludes road-biased AWD "soft roaders" from the off-road category even when they are MC-certified.
Optional inputs
What this does to your fuel figures
We will not tell you your new fuel consumption, because nobody can calculate it from a tyre size. Rolling resistance is a property of the compound and casing, and two tyres of identical size can differ by more than 20 per cent. What IS exact arithmetic is the damage a tyre change does to the number your car reports, because the odometer is the denominator of every economy figure on your dash.
Two effects pull in opposite directions and people routinely confuse them. A bigger tyre usually burns more fuel, through mass and rolling resistance. It also makes the reported figure look worse than reality, because the distance is understated. Only the second one is computable, and it is the one shown here.
Against the Australian rules
+54.5mm exceeds the 50mm increase VSB14 clause 4.2.4 allows for this vehicle class.
Limit: +50mm / -26mm
Needs the manufacturer's widest optional tyre width for this model, which is often wider than the size on the car. Without it this cannot be judged.
Limit: 1.3 x the manufacturer's widest optional tyre
The larger diameter makes the speedometer under-read: an indicated 100km/h is a true 107.0km/h. VSB14 clause 4.2.4 requires speedometer accuracy to be maintained to ADR 18.
Limit: A speedometer must never indicate less than the true speed
Changing tyres to tow or carry more? Run the load itself through our towing and payload calculator, and see what the extra rolling resistance does to a real trip in the fuel cost calculator.
Common questions
Do bigger tyres use more fuel?
Usually yes, through added mass and higher rolling resistance, but that increase cannot be calculated from the tyre size. Rolling resistance is a property of the compound and casing, and two tyres of the same size from different makers can differ by more than 20 per cent. What can be calculated exactly is the effect on the fuel figure your car reports, because a tyre change corrupts the odometer that every L/100km reading is divided by.
Why does my fuel economy look different after changing tyres?
Because the odometer is now wrong. A larger tyre covers more ground per revolution, so the car records less distance than you actually travelled. Your trip computer divides fuel used by that understated distance, so the reported L/100km reads worse than the truth per real kilometre. A smaller tyre does the reverse and flatters the figure. This calculator shows the exact percentage for your change.
How much bigger can my tyres be in Australia?
VSB14 clause 4.2.4 allows a passenger car or passenger car derivative a tyre no more than 15mm larger or 26mm smaller in overall diameter than one the manufacturer designated for that model. For 4WD passenger vehicles specifically designed for off-road use, 4WD goods vehicles and medium goods vehicles, the allowance is 50mm larger or 26mm smaller. Road-biased AWD soft-roaders are excluded from the off-road category even when they are MC-certified.
How much wider can my tyres be?
VSB14 caps tyres on passenger cars and passenger car derivatives at 30 per cent wider than the manufacturer's widest optional tyre for that model. The comparison is against the widest option offered, not necessarily the size fitted to your car, so you need that figure before the check means anything.
Do bigger tyres make my speedometer wrong?
Yes, and in the direction that is never permitted. A larger tyre makes the speedometer read lower than your true speed, so an indicated 100km/h might be a true 110km/h or more. VSB14 clause 4.2.4 requires speedometer accuracy to be maintained to ADR 18. A smaller tyre makes the speedometer over-read, which is the permitted direction, though the size of the over-read still has to sit inside the ADR 18 tolerance.
How much lift do bigger tyres give?
Half the diameter change. A tyre 100mm larger in overall diameter raises the axle centre, and everything bolted to the body, by 50mm, because only half the extra diameter sits below the hub. That is a ride-height figure, not a guard-clearance figure: whether the tyre actually clears the arches, suspension and steering depends on the vehicle and cannot be derived from tyre sizes.
Will bigger tyres slow my car down?
They reduce the pulling force at the contact patch for a given engine torque, because force is torque divided by the rolling radius. A 17 per cent larger diameter cuts tractive force by about 15 per cent before any added mass is counted, which is why a big tyre blunts acceleration, hill-climbing and towing. It also lowers engine revs at a given road speed, which is effectively taller gearing.