[RPM]

I do believe that it is a lean burn indicator. It's just that you're in lean burn all the time, unless you are almost flooring it or exceeding the lean burn rev limit. An AFR gauge would be more useful because being in learn burn doesn't necessarily mean you are using the leanest possible mixture the motor is capable of (21:1?), i.e. 16:1 is still lean burn.
To maximize FE in my car you have to interpret the subtle changes in the accelerator's response in order to use the leanest possible mixture for the current driving conditions.