There are several ways to approach this particular issue.
One way - look at it from a clearance point of view. The bigger the better! The larger diameter wheel you have, the higher your diff is off the ground - so ruts and logs and rocks have that much further to leap up to nab the underside of your car. Add a 125mm Calmini lift and 50mm body lift to 315/85R16 tyres and you're really talking some height.
Another way is to look at it from an economy point of view. The standard tyres are designed to put the standard engine (no chip or other engine mods) in the top of the torque climb zone at marginally under the average highway speed in your area. So in Australia that's around 95km/h and in my 2009 D40 that's about 2000rpm or so. This torque point is NOT the peak torque area. This point I'm referring to is the top of the steep climb your torque curve goes through as the revs and boost rises. It should be about 1700rpm in the V9X, around 2000rpm in the YD25 D40 (the YD25 D22 has a different turbo, head etc and its point will be nearer 2700rpm). Let's refer to this point as the "sweet spot". From a more simple point of view, it's the point in the RPM range where the engine is producing the most torque it can for the fuel going in. Put in less fuel, you'll get MUCH less torque. Put in more fuel, you'll get a LITTLE more torque but the fuel rate is higher than the torque increase rate - so you're LESS economical beyond that point.
SO, after all that, if you size your tyres based on the cruise speed that YOU want to do at those RPM, you'll do well economically, with the caveat that the engine has to actually have enough power to hold the car's weight (and wind resistance) at that speed. Choose a tyre that's too large, the engine will have too much difficulty turning it over to get you the speed and you'll use more fuel. Use a smaller tyre and you'll not go as fast as you might have, but your car will have a much easier time of it. Balance that with the ground clearance you're seeking, of course!
Offset is something else, and doesn't affect the economy much although you could argue that the increased width of the vehicle increases the wind resistance so going for a more negative offset increases fuel consumption. What it WILL do is increase your vehicle's stance - that's excellent for stability, not so good for parking lots or tight garages. I'd love to increase the offset on each side by 45-50mm (giving me 90-100mm of increased width) and flare the guards with
Arctic guards. Go on, click on the link, the cars look awesome.
You don't ever really want to reduce your offset (increase the positive offset) because your tyres will sit closer to the centre of the vehicle increasing the chance of scrubbing against the guards and interference with the brakes.
Here's a website that explains offset with pictures.
Some people choose two sets of tyres - one "standard" (or near standard) for around town, fitted with long-lasting tyres (highway "HT" usually) and another set of ginormous great big honking muddies (see the AT38 Navara on the Arctic website). Most don't actually go THAT far, but it's possible!
