Let's Talk Tire Pressure / Tires

[sina27]

Hello,

Got a 96 Tracker 2WD 5 Speed. 
Since it has been raining here in California lately, the tracker has been slipping / sliding on the wet roads, especially when
I'm going downhill when in neutral (a light touch on the brakes easily causes the car to lock brakes and slide).

I'm wondering if this is because my tire pressure is off - The recommended stock tires are supposed to be 195-75-15, and the recommended pressure is supposed
to be 23 psi. 
The tires that are on my car are 215-75-15 (slightly oversized), and the pressure is around 27 psi. Everything else (like suspension) is stock. 

Since my tires are oversized, should I be inflating the tire pressure above the recommended pressure of 23 psi? Or should I keep it at the recommended 23 psi? 

Most previous cars I've owned have usually recommended around 30 psi, why is the recommended pressure lower for the Tracker? 

Would it be better if I went back to the recommended tire size next time I change my tires? Any real benefits when using the 215-75 size? I never take the car off road, and I drive in the city. 

Thanks for the suggestions