I actually tried it out of curiosity just to see how it acts - this is what I found (for what its worth to whoever is interested): You have to be in the right gear at any speed so as not to load the engine unnecessarily - being in too high a gear at a low speed means you have to press the accelerator down a lot more... which means your
mpg will suffer. So, is there a specific rpm for best
mpg? No - the secret is being in the right gear (rpm) for the speed you are traveling. If the sound of your engine gets deeper and deeper but you're not going any faster, but maybe getting slower, then your
mpg is suffering - downshift... get into a lower gear, and your
mpg will increase...
Hope that makes sense...
Having said that, and did that, now I just drive.. to heck with
mpg... I drive for the thrill (safely)...
edit:
mpg instead of 'mph' 
!!