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The Lucky8 17" steel wheels have a lower offset so you'll get some extra clearance and they fit over the stock v8 brakes. Might solve your issue if the weight limit is high enough on them.
Calling Mack....
Have you read about the 17" steel wheel provided by mudtech ? I am in conversation with them about wheel options having wider offset, wider rim width and also the 17" they have fitting TDV6 2.7 engine/lr3.
I am not sure yet if they have tested the steel on a V8 lr3 though. It's 17x8 ET 30 but it sounds like it may not work with the v8 brakes.
Also of interest from them are two other wheels:
18"x8.5" alloy ET 45 that will also fit lr4 same as the compo wheel
There's also a 9" wide x 18" ET 20 with optional rock rash ring. It may only fit on the lr3 without spacers though.
These are all tested on their own lr3 and lr4.
My idea would be to use the 17" steel if it will simply bolt on to the lr3 hub without issue. Then test fit the 33" bfg km2 and possibly commit to the lucky8 bump stop limiter. 255/80-17, 10" section. However, your 30.8" size would still provide good ground clearance if using the lucky8 strut top spacers.
Normally I would not want to loose any up travel, but in this design I think you regain enough downward travel to equalize the trade offs.
In reality the limiting thing on tire diameter becomes the fit against the slider ends at both tires. This is where tucking a narrow tire up front is most important. Although, the bfg 305/60x18 would look so incredibly bad ass... You might only be able to go straight

or at least revert to steering radius similar to a large tire on a Disco2.
From the website for referencing the brakes sizes:
"It always comes back to the question, do not fit even 18-inch wheels on the Discovery 4? The answer lies in the size of discs. The following sizes built Land Rover Discovery at 3 and 4 in the standard equipment:
Discovery 3 TDV6 (2.7), Discovery 4 TDV6 2.7, Discovery 4 V6: front 317 mm, rear 325 mm
Discovery 3 V8: front 337 mm, rear 350 mm
Discovery 4 TDV6 3.0, Discovery 4 V8: front 360 mm, rear 350 mm
For all models with a front disc diameter of 360 mm, commercially available 18-inch wheels can be raised. Please make sure here at the fine print in the ABE or the opinion, even if is executed and linked to the vehicle listing of Discovery. 4 The pads contain i.d.R. the note "Do not permissible on vehicle model with the following brake system on axis 1: 360x30 mm internally ventilated brake discs".
Only on the Discovery 4 models with the 2.7-liter diesel engine and in which not sold in our V6 gasoline engine fit 17- and 18-inch wheels. The 2.7-liter engine is no longer available from model year 2012, and was also previously built only in small numbers during Discovery. 4
When 3.0-liter TDV6 diesel engine at least 19 inch rims are required. Because there is no suitable choice of road tires in this size, we recommend to go here on 20-inch wheels.
From 2013, however, we can offer a specially designed 18-inch rims, which can also be installed at the 3.0 TDV6. The Compomotive rims in 18 "was specially developed for the Nemesis Bowler, a Range Rover Sport rally car, geferting. This rim can with any necessary adjustment and Eizelabnahme be driven on Discovery 4. When interested please contact us.
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