bzchi wrote:
I'm lost.. why do you need to engineer wider than 6"? the limitation you are referring to sounds like an insurance company rule, not an engineering requirement.
Previous rules in most states had a limit of 26mm width increase, with 26mm overall track increase (13mm per side). Overall diameter increase was 15mm max.
I've just checked the new NCOP LVM rules and found that they've dropped the max rim width requirement, and instated a 30% tyre width limit, with a note saying that rim width must merely 'suit the selected tyre size'.
NCOP cliffnotes:
4.2.3 Clearance
No part of the wheel must touch any part of the body, chassis, steering, braking system or suspension under any operating condition. To check this, the vehicle must be fully laden and capable of negotiating raised obstacles that would normally be encountered whilst driving such as speed humps and driveway entries. This test should be conducted from lock to lock without any part of the rim or tyre contacting any other part of the vehicle. Test weight for passengers is 68kg plus 15kg per person for luggage where luggage space is provided.
The wheels must be contained within the bodywork, or mudguards (including flares) when the wheels are in the straight ahead position.
Steering and/or suspension stops must not be modified to provide clearance for wheels.
4.2.4 Overall Nominal Diameter
The overall diameter of any tyre fitted to a passenger car or passenger car derivative must not be more than 15mm larger or 26mm smaller than that of any tyre designated by the vehicle manufacturer for that model.
4.2.8 Maximum Passenger Car Tyre and Rim Width
Tyres fitted to passenger cars or passenger car derivatives must not be more than 30% wider than vehicle manufacturer’s widest optional tyre.
The rim width must not exceed the recommendations for the tyre fitted.
For example, if the original widest optional tyre is 185mm, the maximum tyre width is 1.3 times 85mm = 240.5mm, i.e. a 235mm wide tyre. The maximum rim width for a 235mm tyre is 9 inches if the aspect ratio is 60 or below.
4.2.9 Passenger Car Wheel Track
The wheel track of passenger cars (or derivatives) must not be increased by more than 25mm beyond the maximum specified by the vehicle manufacturer for the particular model. This means that the rim offset must not be changed by more than 12.5mm.
Reduction in wheel track must not be performed without approval of the relevant Registration Authority.
So the answer is that if you live in NSW (who have already implemented NCOP LVM) then there is no limit on wheel width as long as you obey the rules on track increase and body protrusion.
Suzuki MightyBoy - Three pots and a snail.
Obviously, I'd also have to do that without increasing the track by more than 12.5mm per side 
