If you follow the walkthrough without reading this page, you will have an easy time unlocking all of the achievements. With that said, there are a few hints and tips that may help you even further.
For starters, there is just one achievement in the main game that requires any specific difficulty. There is an additional achievement in the Horizon Rally Expansion DLC too. Apart from these, you can play on whatever difficulty you wish. Having a hard time? Turn the difficulty down! Finding it too easy, and want more money? Turn the difficulty up!
Difficulty can be changed either before a race, or whilst in Free Roam by pressing Start and selecting, My Profile, Options, Difficulty. Here you can also find a description of each settings. Here is a list of the settings and the race bonus associated with each.
Braking. Assisted (0%), ABS On (+10%), ABS Off (+20%).
Steering. Assisted (0%), Normal (+10%), Simulation (+20%).
Traction Control. On (0%), Off (+15%).
Shifting. Automatic (0%), Manual (+15%), Manual w/ Clutch (+20%).
Driving Line. Full (0%), Braking Only (+10%), Off (+20%).
Cosmetic Damage. Off (0%), On (0%).
Rewind. On (0%), Off (20%).
Opponent Difficulty. Easy (0%), Medium (+5%), Hard (+15%), Insane (+25%).
You may notice that Cosmetic Damage does not nothing for your bonus. Forza Motorsport veterans will also notice that 'Damage, Fuel & Tyre Wear is not here. That's right, you can crash as many times as you want, and drive for as long as you want. There is no pitting, and your car's functionality will never change, no matter how badly you drive.
A particularly useful feature is the Rewind function, especially on the Rally Expansion DLC, as this enables you to re-do any bad turns/sections, simply by pressing Y. This will pretty much guarantee you victory, if used correctly, as you can use 'trial and error' to perfect your racing style.
Instead of buying new cars, you can just upgrade your old ones. In many cases, this can save you a lot of money! In fact, if you are wise, and search for all 100 Discount Signs early in the game, you can earn yourself FREE Upgrades for all vehicles. There's also an achievement for doing so. Whilst this is described in the walkthrough, you can also use this image to see what will be required of you.
To work on your popularity, you should aim to create large skill chains in every race. Firstly, avoid crashing heavily in to walls and other cars, as that will lose all skill points that have not been awarded yet. Secondly, try to perform lots of different skills in a short space of time. You can easily extend a skill chain by drafting behind other cars, smashing in to sign posts and drifting around corners. If in a street race, or in Free Roam, you can also extend your skill chain by near misses of other non-racing cars.
Note that skill chains in Multiplayer races work in very much the same way, and can earn you a up to a 1.5x bonus to your EXP after the race.
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