The simple truth is that any more than certain small deviations from standard may be illegal without an engineer's certification.
While the majority of the time people will get by just fine, it becomes nasty and bitter when an accident occurs and there's a significant issue - like life, or large $ - presented to the courts. If you're in a vehicle that is not roadworthy and you are the one that put the vehicle into that condition, then you alone are responsible for having the vehicle there and will most likely bear the full blame for whatever happened.
There are some areas of safety that are impacted by wheel changes. The offset (axial centre of the road surface to the point where the bearing sits) must be within certain limits, as changing this may place additional strain on the bearings and additional torsional loading on the axle. The road diameter also has this effect, as the wheel tries to rotate up into the wheel housing, it's actually imparting a bending moment on the axle. The larger the diameter, the larger the force.
As for speedometers, ADR requires that the speedo MUST NOT read lower than the speed the vehicle is actually travelling at and MUST NOT indicate more than 10% over the speed the vehicle is travelling at.
In other words, if you are travelling at exactly 100km/h, your speedo is NOT allowed to read 99km/h and is allowed to read anywhere from 100km/h up to 110km/h. A defect notice can be issued if the speedo reading is outside these limits.
Putting it all together - if the police see massive tyres on a 4WD that are just awkwardly large, they have two possible grounds to take that vehicle off the road. If you're hooning, they'll do exactly that. If you're just driving sedately, there's every chance they'll leave you alone.
Just never, ever forget that lawyers will get involved if there's a court issue raised and if they can pin something on you for non-compliance, they'll run with it.
