Thanks both, so I need to make sure my rolling radius is the same as current to avoid issues? sorry complete novice with tyres and stuff
Pretty much;
- Rolling radius dictates the overall size of the wheel in terms of diameter, so keeping it the same means it should fit in the wheel arch still (subject to the next point) and will also mean your speedo will still be accurate as a single turn of the wheels will equal the expected forward distance travelled (bigger wheels mean your speedo underreads, smaller wheels vice versa)
- If you go for different rims than supplied on a Cali, you need to make sure the rim width and offset are suitable. If the offset is wrong the wheel could end up clashing with stuff inside the wheel arch (if the wheel sits too far inboard) or hitting the arches (if it's too far out). Neither are good news!
- If you got genuine Cali 18" wheels and fitted more off-road type tyres, you'd be in the safest territory but you need to watch the rolling radius and width again as off-road tyres tend to have taller sidewalls. You need to check speed ratings and load ratings too; some off-road type tyres have surprisingly low speed ratings and I seem to recall insurers "expect" the speed rating to match the capability of the vehicle, not the actual speeds it gets driven at (most Calis can do at least 110mph, some over 120mph, which is more than a lot of "knobblies" are rated for)
I'd probably ask what you're trying to achieve; looks or capability? The former almost always comes with compromise (as listed above, plus things like handling, traction on the road, ride quality, noise etc..). Capability; unless you're planning on mud-plugging, all-seasons will be reasonable or even better, have two sets of wheels, summer tyres and winter tyres (the latter are very impressive in snow/ice and way more capable than a generic off-road knobbly tyre!).
It is a minefield . . . of course, you could just throw caution to the wind and many do, but this is a nice vehicle, not a £1500 Disco wreck.