The GeForce GTX 285 replaces the GTX 280, so we used the GTX 280 as the basis for comparing performance-and-efficiency differences.
The GTX 285 we tested today beats the GeForce GTX 280 by almost 9%, but remember that our sample came from XFX with a slight overclock. We noted that the performance gain due to overclocking was likely around 2 to 3% on average, so the typical GeForce GTX 285 performance advantage over the GTX 280 is probably closer to 6%.
The GeForce GTX 280 and our mildly-overclocked GTX 285 have similar peak power consumption, but the GTX 285 saves around 5 W at idle. A non-overclocked GTX 285 would have lower peak power consumption, but the difference is likely only a few watts.
Efficiency is a ratio of energy to performance. Because the GeForce GTX 280 consumed the same peak power as our GTX 285 did, performance improvements translate directly into increased efficiency.