Standard RRC wheels have 33mm positive offset, which means the center line of the tyre is 33mm inboard of the face of the hub the wheel fits to.
The modular and 8 spoke wheels are a common source of increased track width, though some people have turned the centeres on standard discovery steels (+33mm can end up around -33mm). Most of the 16"x7" modular and 8 spoke wheels I've seen have around +10mm offset, which moves the tyres out approximately 1" each side. You can get these wheels with neagtive offset to push the tyres out even further, though they tend to be the wider wheels (eg 8" or 10").
