Why does driving slower save fuel or is more fuel efficient compared to driving faster to the same distance Isn't fastspeed n lesstime equal d same amt of fuelusage compared to slow speed n more time?

Driving faster uses more fuel for two reasons. First your engine is running at higher revolutions per minute, which uses more fuel. Second your car is encountering more air resistance the faster you go, which means your engine has to work harder and consume more fuel. Typically vehicles are most fuel efficient when well maintained and traveling at 45MPH, anything beyond that and air resistance multiplies exponentially.