Ford

I recently bought a ford contour. its a nice car. after i got it i thought that there might be some kind of transmission problem in it but there was not. i figured out the the o/d button was pushed. when i pushed it again, it turned off and the transmission alongwith the car got smooth. so does anyone know why does o/d make the car go less smoothly? and why does it run smoothly with o/d off?another question was about the mileage. if i run it with o/d off, will it consume more or less gas?anothe one about the coolant. what coolant should i add as it says on the radiator that add 50 % water to the coolant.please help me.thanks.

Od your engine is turning slower than your tire,,,so less engine noise on high speed,,,so you save gas,, your od is working if the light is off, when buying coolant,it usually is stated whether it is 50%water use same color as the old one,,