Over the years driving and racing different types of cars, there have been some broad principles that apply across the different disciplines. These are in no particular order, and more a collection of thoughts stolen from people much wiser than me!
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Spend on the Driver Modification
Throwing parts at a car to make it faster without considering the driver is similar to flushing money down the toilet. A way that you can test the gap between your skills and what your car is capable of is by letting a experienced driver take the wheel and note the gap. Spend time in simulators, or better yet, on formal driver training courses.
Walk & Analyse The Track
There is a temptation to assume that the ideal line for every track can be observed from apex to apex. However, walking the track allows for deeper observation of the track condition itself.
Print a map of the track and mark the most important corner first, and go in reverse order until the least important. Attention must be paid to the corner leading to the straightaway where exit speed usually matters.
A good racing line maximises available grip. Mentally calculate which corners would benefit from more rear slip angle (next corner to corner), and ones where exit speed (next straights) should be prioritised. Races are won in the straights, not corners.
Tires, Tires, Tires.
All the technology you see in cars are transferred to the road on 4 small contact patches. The quality of tires that you use will have the single greatest impact on lap time on a track.
It's pretty common to see thousands spent on modifications to engines, on a car with a worn set of tires trackside. Find the class winning tire of that year, and change it often.
Smooth yet Deliberate Inputs
The less you do with your controls the less chance for error.
The less you turn your steering wheel the faster you will go.
Speed comes from the timing and release of the brake pedal.
Everytime you change gear or brake, you are upsetting the balance of the car.
The amount of time taken to move the right foot to the brake and back to the throttle pedal can be saved by left foot braking. This will allow a driver to overlap the brake and throttle instantaneously.
Approach, Corner Entry, Mid Corner & Exit Speed
Overlap your braking cornering and acceleration forces
Corner entry speed is more important than late braking.
The apex is the closest point of any corner which a vehicle passes through in order to reduce lap time. The faster the corner, the closer to the geometric line you should drive (clip the corner in the middle), the slower the corner, the closer to the ideal line you should drive (clip the corner at the end). Generally, the larger the radius of the corner, the later the apex. As you can imagine this reduces the effective track length by a small distance, and this will result in greater speed.
Focus on where you want to go, not where you are. Always look ahead.
Use reference points - good for consistency, and if errors can correct.
awareness by debriefing increase