Saaaaamon, nice to see a driver wanting to get the best fuel economy from his boss’s lorry 
Ignore Carryfast, he talks from his sphincter, I toatlly disagree with his methods and like it or not, I used to be a road tester of lorries and I have been taught how to drive them properly by the best in the business. I also have a fair bit of experience driving V8 Scanias and I was paying the fuel bills too, so driving them for economy was very much in my best interests 
So, start off in 1hi with no throttle, as soon as you start to move and have fully engaged the clutch (pedal at the top) go into 3lo, then take the rpms up to almost the top of the green band with a reasonably light throttle, pretend you have a raw egg under your right foot and you don’t want to break it, then go up a full gear into 4lo, do the same to get you into 5lo and then 6lo and then change into 6hi at 1300rpm. That is the most efficient method of getting up to cruising speed 
Now, when you’re at cruising speed back off a couple of kms, not only will this save fuel, you will also have a much more relaxed drive. If you’re on the level, use cruise control, but as soon as you get into the hills, switch that off, it will cost you. It cannot see the road, so all it does is throw fuel at the engine in an attempt to get it back up to its set speed. Never ever use the resume function for those reasons 
If you drive properly and let momentum help you crest hills and gravity help you accelerate down them then you will save a lot of fuel. Anticipation is the key here, especially in traffic, try to let momentum do as much as possible and avoid using the throttle unless you have too, for example, in traffic keep moving at a steady pace, don’t try and stay 20/30/40/whatever feet from the vehicle in front, speeding up and slowing down, try and keep rolling at a constant speed all the time. If you’re approaching a roundabout, traffic lights or coming off a slip road, come off the throttle or cruise as soon as you reach the point where momentum will get you to the point where you have to stop. Any time you have your foot off the throttle you will be using any fuel at all. It may seem like you’ll never get anywhere, in theory at least, in practice however your journey times will not suffer and the driving is much more relaxed
If the lorry has a turbo boost guage, use it, keep boost as low as you can. The same applies to climbing a big hill, it’s tempting to let that mighty V8 roar past everything, but for maximum fuel returns you want to be going up the hill in a completely different way. Instead of, for example, going up a long hill in 5lo at 1100rpms with your foot flat to the boards, instead go up it in 4hi with 3/4s throttle, by this I mean go up it so that you have a bit more throttle left and could go a little bit faster if you want to, this keeps turbo boost (& fuel use) down
Finally when pulling up hills that require a few downshifts, don’t split the gears coming down the box, go down in full gears and go back up in half (split) gears, again, it saves fuel
