| Here is what I found it takes to install 315's on a TT. 1. Required before you start: aftermarket coil overs and deleted HICAS. Stock rear struts are bigger and would hit the rim/tire. I replaced the HICAS ball joints with NA bushings and modified toe rods but this may not be required. Tools and Materials Required:
MIG welder
12 gauge steel sheet metal (approximately 0.1’’ thick)
Grinder with cutting and sanding discs
Plasma cutter or cutting torch 2. Roll fenders. I rolled the inner lip of the fender fully flat but did not pull the fenders. The outer body line is stock. 3. The following wheel and tire specs are what was used: 18 x 9’’ with +35mm offset and 315-30-18 tires. 4. Send wheels to Eric Vaughn to increase the width by 2’’ on the inside. Typical cost is $300 each. The wheels will now be 18 x 11’’. Eric Vaughn Machine, (626) 358-6413, 111 N. Myrtle Ave. Monrovia, CA 91016. He does amazing work. I think this now makes the wheel offset +60.4mm. 5. The front section of the lower control arms will need to be cut and boxed to clear the wheels. One method to determine how much is to bolt the wheels on before they are widened. Attach a 2.25’’ section of cardboard to the inside of the wheel with tape and spin till it is up against lower control arm. Keep in mind that when the wheel move up through the suspension stroke the lower control arm will get closer to the wheel than when full extended on jack stand. This is why the extra .25'' on the cardboard is needed. I missed this the first time and knocked off all my inner wheel weights and had to redo the LCA. 6. Remove lower control arms and cut off marked section. It should look approximately like the picture below: 
7. Box in lower control arm. One method is to start with a piece of cardboard and trim to fit as shown below. Once piece fits correctly, use this to trace out the shape on the sheet metal. Use plasma cutter to cut out shape. Tack piece into position 
Use carboard again to cut out shape needed for bottom of control arm. Cut shape from sheet and tack weld to lower control arm. 
Once section are tack welded to control arm, reinstall on car, install widened wheels and disconnect coilovers from lower connection point. Move the wheel through the full suspension travel and confirm there is proper clearance between wheel and lower control arm. Keep in mind stick-on wheel weights may be added to the inside of the wheel and may be knocked off by the lower control arm if there is not enough room. If so motivated you can continue to box in the entire lower control arm to further strengthen the part. 
Fully weld all new pieces to control arm. Run short beads on difference section at a time to prevent heating the part up too much. Be particularly careful around the rubber bushings. Primer and paint parts when they are cooled down. 8. Remove the protective cover from the fuel fill tube on the driver’s side. I am running 3 deg camber and it was necessary to remove the actual fuel tube and dent it just below the mounting point to the body. This can be done just by placing it in a vice at the appropriate positions. In the picture below you can see the vice marks to the right of the bracket. I didn't modify the tube at the start and when entering some steep driveways from the right the tire would rub the tube. This didn't cause any problems but was annoying. When you modify the tube wait until you are almost out of gas - trust me on this. 
9. Install the mounted wheels and tires, unbolt the coilover from the spindle and actuate the wheel through the entire suspension travel. It is difficult to determine if the wheel will rub the fuel fill line. To determine if and where it may rub you can drive the car. If there is not enough clearance the tire will rub the paint of the section but not cause any actual damage. 
I have been driving around with these for about 6 months without any issues. If you can't weld you could always cut the LCA's and take them to a local shop to box everything up for you.
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How to install 315-30-18's on a TT - 
