in mpg efficiency, shifting into a higher gear close to stall. which means running the engine with the least rpms possible. not too good for the engine in the long run. but it doesnt use as much gas. ( idont think.. correct me if im wrong)
and for acceleration, read
http://www.allpar.com/eek/hp-vs-torque.html which comprehensibly teaches why shifting at redline would be the fastest way to accelerate