Tuning advice

I'm working on a miata in c class. I have every first level upgrade on the stock engine. I was able to put level three upgrades on engine block, pistons and exhaust. Is that a good strategy while building a c class car. I try to bunch upgrades that work
together, next I was gonna see if doing all air system upgrades is better than the engine block. Its hard to tell which is better, your pretty limited in c class. So far I noticed a big difference tuning the differential to stop the back from sliding out.
But ihad to go way down to like 30%, is that too much.

Oversteer is when the car turns more than expected in a corner. Understeer is when a car turns less than expected in a corner. Tuning them out is pretty car specific and depends on a whole host of factors.

To determine how to stop the back end from sliding out we would need to know what car and class you're building and also what upgrades you're using on the car (upgrades that change weight have effects on handling that impact tuning).

If you're looking for a good, general tuning guide - look here:
forums.forzamotorsport.net/turn10_postst221_Tuning-Guide.aspx

I meant to reply to this yesterday but work was friggin crazy and I had pretty much no free time. Crunch is right. To add to that, front engine, front drive cars suffer from understeer and front engine rear drive cars (generally) suffer from oversteer.

Yes, there are some tuning calculators that help you get a starting point but you will still have to fine tune to get the most out of a car. Also the biggest thing isn't always the tune, it is the driver. Coming from me, who is one of the slowest competitors
in my Forza club, have used the same exact tune as some of the faster guys and I still am much slower than them so yes I suck but don't think that a tune will turn you into a speed demon. My advice is to learn the tracks and practice your braking and exiting
lines/points.

It all depends if you want to spend time tuning cars. The calc can be useful, but more often that not, you're better off letting a guy with 10 years experience online tune it. THE only way to start to understand the dynamics of tuning is to do it yourself.
If not buy tunes off the AH, but be careful because you just might waste money.

Years ago I knew nothing of tuning, but I put the time in and now people drive my cars and say... feels great!

No one here is forcing anything on anyone, but using the calc is not making anyone better. Talk to some guys on the leaderboards and ask them. Don't believe me.

I am here to give advice and if I have to shove it in your face I will. The whole point of the forums is to help, not hurt.