1. You can fit a 245 -40 -20 tyre on all 4 rims but this will result in an inaccurate speedo reading as the total diameter of the tyre will be increased by 24 mm.
This also means the tyre will extend into the wheel well (in height) by a further 12 mm compared with the current tyres so if the car is lowered you would need to check clearances.
2. In NSW (as in other States) it is mandatory to retain speedo accuracy and if you fit the higher profile tyre (245 40 20) you will have to have the speedo recalibrated .
3. It is also mandatory in NSW that there is NO fouling of wheels/tyres on the body work or suspension or wheel well materials/fittings etc under all operating conditions (full steering and suspension movement) so make sure you can accomodate the larger diameter tyre. If the car has not been lowered this should not be an issue. If lowered I suspect you will have a problem and you may not be able to accommodate a tyre of that diameter.
In NSW these are the current guidelines you need to comply with:
http://www.rta.nsw.gov.au/registration/downloads/vsi/vsi09_rev4.pdf
4. I would definitely not look at going to a tyre with a larger diameter than 245 40 20 (approx 704 mm )and this should give you a slightly better ride with the higher profile if it will fit your car without fouling.
5. To check the offset of the current wheels remove the wheel and it should be stamped on the inside of the rim somewhere (if not stamped on the outside). If it is a GMH 20 x 8 inch wheel it should be +48 mm offset. If an aftermarket 20 x 8 inch wheel it will probably be in the range +35 to +45 (many are +45)
6. If you are not happy with the ride the better option (rather than fitting a larger 20 inch tyre with a 40 profile) is to go back to the original SSV 19 x 8 inch rims (or equivalent ) and the standard SSV 245 -40 - 19 tyres for that rim size. This will give you the correct tyre diameter and a higher 40 profile. You will not then have clearance issues or any need to recalibrate the speedo.
