Is there a guide on how to properly tune cars in this game for faster stage times?

There's no guide as far as I know, but the basics are that you need to adapt the settings to the stage itself - for example:

• Tires are the big one, obviously use the tire best suited to the stage (e.g. Abrasive tarmac on a Germany stage with Abrasive/Rough tarmac). And with mixed stages, like Stage 3 in UK, just try out different options, so if it's Gravel+Tarmac I'd run the stage once with Tarmac tires and then once with Gravel tires to see which is best!

• High & Soft suspension for Greece, Australia, Finland and some Sweden stages.

• Low & Hard suspension for smoother, tarmac stages such as Germany, UK & Japan.

• Longer gears for certain cars on long, tarmac stages (like the Metro 6R4 which has weirdly short gearing).

And the other options like steering and differential tuning are sort of down to you in my opinion - I prefer lighter steering just because that's what I've always used.

Okay, it turns out my tunes are basically on point then. Although I noticed that I use the shortest possible gears for all Classic cars, as well as the Evo and the Peugeot 206.

I just slam my car to the ground, max strength rollbars. Absolute pain to control on Finland 01, but oh does it feel great to fly past the final check point in a low rider.

Generally speaking from my experience, messing with the gearbox setup seems to always be detrimental and lose tons of time, default just never fails.