There is a simple test you can do to find it.
1- First, drive around a bit until the engine is completely warmed up. Completely warmed up means that the fan should kick in and shut down twice; this usually takes 8-10 km.
2 - Find a long, fairly level road with almost no traffic. It should be somewhere where you can safely maintain 90+ km/h. Maybe GT Road before sunrise?
3- Accelerate normally to 50 km/h, upshift 5th gear, and reset your on-board mileage meter. Maintain this speed (in 5th gear) for at least 1.5 - 2.0 km, and note your mileage.
4- Increase your speed by 10 km/h, and repeat step 3. Do this until you hit the legal speed limit of the road, or whatever you speed you feel is safe.
You will know which speed is the most fuel efficient. I am willing to bet that you will get the best fuel economy between 60-70 km/h. It will keep decreasing slightly until 90-100 km/h, and above that it will drop off sharply. Do a little experimentation, and see what you are comfortable at.