Preface: The tuning menu is a important yet under-utilized feature in Horizon. Yes, the menu has a description of what each adjustment does, but its hard to understand without feeling or seeing the cars in motion. Its rather overwhelming if you are not a car person. Youtube videos like HokiHoshi’s Basics of Tuning Guide are great resources to learn and reference, but this is something the game should provide.
Suggestion: Add a Horizon Story that walks the players through the process of tuning a car. Have the missions let the player drive an untuned (or poorly tuned car) to feel the issue, then show what upgrades are need to unlock the tuning adjustments, and finally what aspects to tune for a specific race and drivetrain type. This would also be a good place to demonstrate how to use telemetry data to dial in a tune.
For example: FWD test car has bad corner entry understeer. Have the player drive a lap of the bad test car. Then allow them to upgrade race suspension, anti-roll bars, and differential. Have an NPC explain the reasons behind softening front ARBs, lower decel differential lock, and increasing front toe out to promote more understeer. Then allow the player to drive a lap of the tuned car to feel the difference.
One of the Donut Media stories briefly mentioned the tuning menu with regards to adjusting the final ratio. I believe this is a feature that could be fleshed out more. I personally get gratification from my car builds when I tune them myself instead of downloading someone else’s tune. This would be a simple way to add content to features already in the game.