If you want economy find out the factual cruise speed for your ride and use it, but as a rule that will be too slow for most people.
If all you want is to cut back on fuel used while keeping fast thats a different story, Funked could well be correct if all your gonna do is reduce rpm BUT keep map at max.
In that case i would go for a map reduction over rpm, if you check my link below you will see at 12000 ft reducing rpm to minimum and keeping map high is the worst result possible for economy.
Have some excel stuff that i ran up for the F6F-5 but the result is that engine Map and rpm both have an effect on fuel economy when in cruise mode.
Heres a rough version,
www.cavalier.cwc.net/dlfiles/f6f5fuel.htm Aim was to use the F6F-5's recommended cruise speed and see if a strike was feasible for a scenario we were involved with.
NOTE the scenario ran a 1.5 burn rate but the results are still valid for the purpose of showing the huge differences involved at different settings.
Check the values for 12000 ft, rpm was altered for each run and map was adjusted to maintain the IAS of 168.
This was a fully loaded F6F-5 2 1kg bombs + Roks.
The 100% fuel readings relate to the wing tanks, aux was burned off at that point.
It also proves fuel efficiency is better at 12000ft than 6000ft if you compare at 12000ft it covers less distance in the early stages per fuel % used while climbing, but once at cruise altitude it soon recovers.
Not shown here is the fact that its much more fuel efficient to do the climbout on wep than at Mil or any other reduced map.
The longer time spent in the cruise mode by getting up there faster is more important than trying to climb on any reduced map and taking longer to achieve the cruise alt.
