Fitting a Larger dia rim/tyre combo will raise your 'low gear', so off roading may be adversely impacted by that..... but the larger rolling dia will probably make it easier to roll over obstacles IF your engine can drive it thru the taller gearing. That can make for better fuel economy on the road, at least theoretically; but the bigger wheel dia will also lift the vehicle a little (by 1/2 the dia increase) so that's likely to increase the wind resistance at hwy speeds your engine hasta shove thru the air, & that means that unless you've already got tonnes of torque on tap right across the rev range, you usually end up with no better fuel economy at absolute best, & often it'll end up a whole lot worse. But that might just be 'seems' - cos your tyre rolling dia has changed, the speedo & odo won't be reporting distance or speed the way they were, so while you might think you're doing the same speed & travelling the same km as you used to, but you'll actually be travelling faster (& working the engine harder) for any given speed AND doing fewer km (less revs per set distance) so it'll only LOOK like your fuel economy is much worse unless you've corrected for both speed & odo inaccuracies.
About speedo correction/accuracy, pretty much all vehicles leave the factory with speedo's that read somewhat faster than your actual speed & have odo's that suggest you've travelled a few more km than you actually have - our ADR's allow them to read as much as 10% over/more but not under at all - so by fitting taller tyres (meaning they need to do fewer revs per km) you might actually make your speedo & possibly odo too somewhat closer to accurate; just so long as you don't go any bigger'n a 10% increase, cos then they start reading out the other way!! Btw, Many people 'think' their odo's are dead accurate from the factory, but usually that's just because it's so much harder to discern a 10% discrepancy in 1 or 2 km (10% max is a max discrepancy of less than 10m out of every 100m!) than it is to see the difference between 100 kph & 110 kph - and no matter what you might 'think', both speedo's & odo's are driven by the same rotating machinery & if you change the 'revs per km' that the tyre makes you travel over the ground, then both indicators also hafta change!
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