Ive found that I can get excellent fuel economy from my VSV6 by keeping everything serviced etc. Engine oil and filter, auto trans filter and fluid, air filter, fuel filter, plugs, leads, the 3 ignition coils and the 02 sensors. Keeping the MAF clean with MAF cleaner, keeping the throttle body clean and cleaning/replacing the fuel injectors every 150,000 or so km. All these things mentioned will make hardly any, or only a very small difference on their own, but when all the small differences are added up, it makes one big considerable difference, especially to fuel usage.
Wheel alignment also makes a huge difference to fuel usage over time. Get the alignment set to factory specs with the measurment (caster angle) absolutely dead even on both sides, and this will help a lot with using less fuel. Keeping the tyres pumped up to maximum pressure will also help. Also check for dragging brakes (including handbrake shoes) or binding wheel bearings and uni joints..
Ive also found that using 98 octane fuel (even with the standard tune) makes a big difference to the fuel ecomony of my VS and my VY wagon, and I go further on the 98 octane fuels, even though I get less of it at the bowser as its more expensive..
If your vehicle is not running 100% correctly to begin with, then you probably wont notice any difference at all, if you like to plant the right foot a bit, then this is where the 98 fuels really come into their own, and the vehicle will use way less fuel than it would if were driven the same way, but on 91 octane fuel..
I can get anywhere from 6 to 8 litres per 100 km out on the highway, and anywhere from 8 to 12 litres around town driving normally. I only ever use 98 octane fuels, and with the tune im running in the VS, the ignition timing values are bumped up a lot higher over the standard values, this is so the engine can take even more advantage of the 98 octane fuels, it also holds the timing for longer when under load before the ECU detects any knock and then retards the timing. When on 91 fuel, it drops way more timing than it does on 98 fuel, but as I said above, even with the standard tune, you should see a fair difference between 91 and 98 fuels, thats if your vehicle is running 100% correctly to begin with of course, but increasing the timing values in the tune just tops it off nicely, and, as well as saving fuel, it also makes the engine a lot more responsive and just better to drive. Ive also got pacemeker tuned length extractots fitted, which again, helped with fuel usage. I also had problems with standard passenger side exhaust manifolds cracking, I went through 4 or so of them, that was one of the main reasons I fitted the pacmakers, and havent had a problem with it since.