So on the open road you got 12 to 13 litres per hundred. That's not really TERRIBLE. Is this thing auto or manual? Any modifications? Were you carrying much load?
Don't trust the "litred used" on your trip computer as gospel truth, it can often be inaccurate. Always trust how much fuel you put in at the next servo stop.
You can fill it up to the brim if you want, but I just use first click. So long as you use the same method every time, you'll get a reasonably accurate reading.
If you want to throw a few dollars at the car in the hunt for some more economy, get a new coolant temp sensor (the two-pin sensor if they ask which one), new manifold temp sensor (they're not that dear any more), and a new throttle position switch. Should set you back in the order of $150, which isn't bad since they will probably need replacing at some point anyway.
What sort of oil are you using?
What temperature does your car tend to run it, has a cooler thermostat been fitted? Reducing the running temperature of your car can also adversely affect fuel economy.
I think you could recover most of your fuel economy using good driving habits, like gaining speed on downhills and allowing your car to lose speed on uphills. Coast for as long as possible before a known stopping point, but most importantly just try to keep your engine load consistent.
Keep in mind that any increase in speed above 80kph is going to be exponentially reducing your fuel economy, so if you're belting down the highway at 130kph expect bad results. Air drag is the number one killer of fuel figures.
If you want to get really stuck into it, you can buy tyres with less rolling resistance, fit lighter wheels, lower ratio highway diff gears, throw a good toneau over the ute canopy, and begin to fiddle with undercar air effects if it really interests you.
With a few simple mods I was able to get 8-9 litres per hundred on the highway out of my V8 Statesman, which is a heavier car than your ute.
