RX8 fuel consumption is only marginally better than FD-FC. Some articles note that the RX8 has fc that is within acceptable limits due to better efficiencies but no one has been able to verify this.
My understanding is RX8 has a 60liter gas tank. It does 360-400 KMs per full tank (and that's the upper limit). That's 6.5km/litre, not much diff from FD.