u can fit 18" on astra SRi -T
have to be 225 x 18" x 8 or 8.5 with a load of (87Y) or better.roling diamitor of max 15mm and an off set of max 25mm (cant go more then 25mm wider on the trak from wheel to wheel).
and that will compy with the RTA but im still to measure the track of the SRo-T to make sure these wheels are 100% legal to get engineered :).