Noticed I was the only one to unlock this so thought I'd do a guide.
Not as tough as it might seem at first here's the basics;
Final Drive Ratio (Fleetari) - Highest 4.625
Air/Fuel Ratio - 13.1
Wheels/Tyres - Racing slicks front, Rally rear
Performance Radiator,Exhaust, Carbs, Seat (driver only)
No Roll Cage (should be ok if its already installed)
Wheel Alignment - Toe out all the way (wheel stops moving) then 61 clicks back in
Valve Tuning - Do a basic valve tuning first, plenty of guides out there. Then, scroll wheel down each valve until the valve taps, then scroll wheel up slowly one click at a time until tapping stops and leave it there.
After that its a simple matter of shedding weight so remove/don't install anything that's not 100% necessary ie. fenders, hood, rear seat, tailgate, doors , etc. Even gauges, every ounce you shave will help your time.
The most important weight to remove is your fuel, so keep the bare minimum in your tank, I saw a huge improvement doing this. You can also smash out your windscreen (H), not sure how much this helps though.
My best time was under 16 secs with an Xbox one controller, but have also got under 17 using keyboard.
Any questions, ask away.