Well...

• You want the largest possible throttle opening (on naturally aspirated cars anyways) to reduce the vacuum and hence pumping, however, you don't want to force the computer out of closed-loop mode... So, roughly 50-75% throttle depending on the vehicle (assuming manual transmission as an automatic will probably grab a less efficient gear in those circumstances).

• You want the RPMs as low as possible as it truly is RPMs that really burn fuel (frictional losses with higher RPMs). However, if the RPMs are too low, then you don't get the benefit of timing advance...

So, "it depends" on naturally aspirated manuals. They give you the most control of how to optimize your fuel usage, but you need to know your specific car. :-) The easiest car to optimize for is the turbo automatic... Drive as slow as possible and hope for the best since you don't really have much control beyond that... :-)

Well...

• You want the largest possible throttle opening (on naturally aspirated cars anyways) to reduce the vacuum and hence pumping losses, however, you don't want to force the computer out of closed-loop mode... So, roughly 50-75% throttle depending on the vehicle (assuming manual transmission as an automatic will probably grab a less efficient gear in those circumstances).

• You want the RPMs as low as possible as it truly is RPMs that really burn fuel (frictional losses with higher RPMs). However, if the RPMs are too low, then you don't get the benefit of timing advance...

So, "it depends" on naturally aspirated manuals. They give you the most control of how to optimize your fuel usage, but you need to know your specific car. :-) The easiest car to optimize for is the turbo automatic... Drive as slow as possible and hope for the best since you don't really have much control beyond that... :-)