Bro i second that of driving too slow is bad for FC....for me i feel the optimum speed range is 80-100km/hr on ur highest gear ratio
attach is what i am getting in sg for my half tank 
Hi Bro fri13th/all, hv try to go for very slow speed (targetted speed ~100km/h and will vary from 70 ~110km/h depending on up slope or down slope) on NSHW last weekends. Pls. find below for the updates with photos taken:~
1. With 90% HS and 10% town area -> able to hit 170km+ when reaching 3/4 mark
2. Continue to drive at kampung+city area (100%) -> hit 310km+ when reaching 1/2 mark
3. On my way back to Sg, again on slow speed, hit 480km+ when reaching Skudai toll (=> able to hit >200km from 1/2 mark to 1/4 mark with this slow speed) and hit 503km+ (b4 reaching 1/4 mark) when top up petrol at station which is very near to JB, abt 2km from JB chkpts.
**In short, with above, hit 16.85km+/l and again, I'm able to hit >19km/l should I consider 100% NSHW driving (which I did if b4 on my way back from Penang during CNY period).