I find the mpg almost the same in 4th and 5th if going up a steep hill and for a long hill recommend the lower gear with a little more rpm just for better timing advance and oil cooling the piston bottoms with more spray and better water pump speed. At light throttle higher gears are better over 1000 rpm which in 5th is about 20mph although 4th gives the same mpg but 3rd lowers mpg at 20mph. Long putting reduces lubrication in the engine so once in a while higher rpms will help mpg even throwing a rev with the clutch in helps.
Less friction less bull****, even climbing mild hills in 5th is awesome because for my car the speed bleeds MUCH slower than opposed to trying to climb 1500 rpms in 4th as opposed to 1200 rpms in 5th which suprisingly is the real winnar!
Look at it this way, in city even at speeds as low as 45 km/h in 5th my engine load only goes as max as 30 and that's max. ON the highway at 100 km/h with rpms at 2800 my engine load is as high as 50 and that's constant!
Areodynamic is a *****.
If your reading this, then good for you, your saving some gas because your here.