The question of 'how much bigger tyres can I fit' on your 4WD comes up regularly, and one of the most overlooked pieces of information is what the law is, and what the consequences are of running a vehicle that has tyres bigger than they are allowed to be.
VSB 14 states that you can increase the tyre size by 50mm. What it doesn't make clear though, is that in some cases you can actually go more than this.
Here's the exact wording "Must not be more than 50mm larger or 26mm smaller than that of any tyre designated by the
vehicle manufacturer for that vehicle."
If you are wanting to change tyre sizes, please have a read of this first - Fitting bigger tyres to your 4WD; what you need to know.
How can you fit bigger tyres?
You are allowed to go 50mm bigger than the largest tyres found in your model range. For example, if we look at our Isuzu Dmax, it came with 245/70/16 tyres, which are 749.4mm in diameter.
If you want a tyre size calculator, check this out - https://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx
Most people would naturally think that the maximum tyre size is 799.4mm. However, the LSU (a fancier model) came out with 255/65/17 tyres, which are 763.3mm.
Providing the changes between the models are purely cosmetic (chrome bits, fancy dashboards etc) you can go 50mm bigger than the biggest tyres in your model range. This means the maximum size is actually 813.3mm. This is perfect for us, as a 265/75/16 (one of the most popular tyre sizes around) is 803.9mm.
This exceeds the 50mm increase for the SX versions, but is within the 50mm increase for the LSU versions, and as a result we've gone for them. At 54.5mm bigger in diameter than the stock tyres they are a fair increase, but they actually correct the speedo and give us about 25mm more clearance everywhere under the vehicle.
Now, bigger tyres on your 4WD are good in many ways, but like everything, have a number of downsides too. Don't forget about them when it comes to fitting them to your 4WD.
Check with your local authorities
Please, before you fit bigger tyres, give your local road authority a call, and ask them the question. Stick within the regulations and you'll be much better off!