Need Help on my Car

3 months ago my father told me to sell my car a nissan RS180, which was given to me by him and I've been using it since high school. He then gave me another second-hand sedan (I would like to have a new MPV, but what can A kid do.....sigh.....), anyway it's a Toyota AE111 Corolla Trueno, don't know about the year since the registration hasn't been given to me yet and it's under my father's name, but judging by it's look it shouldn't be older than 2000. At first it's a bit heavy, so then I tuned its gear ratio to reach 150 kph at 5th gear.
Last Monday when Holiday finally over, I drove it to the university, since it's outside the city right beside a highway (which happen to be the only road to there) and the highway is very empty (there are lots of motorcycles but they're on the slow lane, the car lane is well..........empty) I drove my car a bit fast at about 100kph, this is when the trouble occured, I have to make a U turn cuz the university is on the opposite lane, when I made the turn at about 45-55 kph to the right, I made the turn but then the front of the car swing to the left side by itself right after I release back the hand break, I almost hit the lane separator. Well this car sure is sometimes hard to turn on a curve when driving at high speed, but this time it's sure a serious problem, I usually done this kind of driving with my old car and nothing like this ever happened, I took it to balancing and spooring right after my father hand it down to me.
Please could you guys tell me the problem with my car, is it about the gear ratio setting that I did or something else?

It won’t be the gear ratio, I suspect it will be something to do with your suspension or bottom arm linkages.

If it was me I would get the car checked right away. That is one last thing you want to break on your car.