Depends on your car. I was advised to find the right balance between RPM and MPH, so get into your highest gear but lowest speed should be close to the optimal. For me that is around 2000-2200 RPM and between 45 and 50 MPH. But without instantaneous feedback via gauge it's hard to say until you fill up the tank.
(I believe it's also mentioned in the
100+ hypermiling / ecodriving tips to increase gas mileage - EcoModder.com)
Another caveat is that since I'm in an automatic, I don't really have the same control on when I can upshift/downshift as compared with a manual (who can downshift sooner and at more efficient RPMs)