There are Vauxhall Astra GTC options from Tyres.co.uk with Michelin Pilot Sport 5 and Pirelli P Zero 235/50 R18 for 2012 modelled Sport, SRi and BiTurbo versions with 18-inch factory fitted wheels. Be sure to check the load index, speed rating and XL construction against the placard on the driver's door frame and registration.
For all the information about tyre size and pressure for the Vauxhall Astra GTC, check the placard on the front left side of the car's door frame. For original tyre size and load, and speed ratings as well as cold pressure, check the passenger front door. For differences in the factory wheel options in different models, check the placard on the tyre sidewall, and the registration.
The Vauxhall Astra GTC, being a front wheel drive vehicle, does not require all four tyres to be changed at once. The safety recommendation advises that it is best to match pairs on the same axle. When two new tyres are put on, it is best to comply and put them on the rear axle according to safety recommendation.