Low-speed "cruise control" using idle speed controller?

[Bearleener]

When I'm cruising at a constant, low speed of 30-40 km/h (18-24 mph) I'm usually in 5th gear, which results in a lower RPM than the engine's idle speed controller would like (for a moving car), so the engine automatically increases the throttle and the car cruises by itself at about the desired speed with my foot off the gas. That's about 1000 RPM (the engine tolerates down to about 600 RPM without balking).

What's more efficient: the above "cruise control" method or should I be in 4th, so that I have to add a little throttle myself? What have you ScanGaugers and MPGuinos found? And will the engine's idle speed throttle motor wear out faster?

Thanks!