Guide to Calibra Wheel Offset
From Calibra Wiki - Vauxhall, Opel and Holden Information and Technical Resource
Calibra Wheel Offset
Wheel Offset is the measurement between the centreline of the wheel and the mounting face. It is important to maintain the correct offset when fitting aftermarket wheels to avoid upsetting the handling of the car.
Most Vauxhall cars use an unusually high offset of 49mm. If you ignore this when fitting aftermarket wheels, you can run the risk of upsetting the handling/stability of your car and/or causing the wheels to rub on the bodywork when going over bumps etc.
On the subject of wheel offset, Courtnay Turbo say this: "When GM introduced their first front wheel drive car in 1981 they introduced their innovative 'Negative Scrub Geometry' which gives excellent stability in the event of uneven braking due to eg. gravel in the verge or a blow-out. At the core of the design was the adoption of a unique offset of 49mm negative. Aftermarket wheel manufacturers and retailers almost universally ignore the requirement for GM fwd cars to run on this offset alone and normally supply the more common 38-45mm ranges. Because we are specialists and aware of the importance, we will not supply a wheel which is not ET49."
It is worth mentioning that Irmscher, the suppliers of the Vauxhall approved aftermarket accessories, initially specified an offset of 42 - 49mm for their wheels for the 16 valve Calibra. However, on speaking to Courtnay about this, they explained that Irmscher, being German, have different requirements for offset. This is because they drive on the other side of the road and the camber affects the car differently. For the UK market, they have redefined the requirement to be an offset of 49mm only.
Note: If fitting wider tyres than standard, offset is particularly important. Wheels with a lesser negative offset than recommended will 'stick out' more than standard wheels. This will make a wider tyre more likely to rub on the wheel arches than if a wheel of the correct offset is fitted.
Courtnay say that they never have trouble with 215 tyres fitted to a Calibra if the correct offset is used. It has been known for engineers to machine 1mm off the mounting face of a wheel to increase the offset by 1mm in order to get a 48mm offset wheel to the correct offset for GM cars. We would not recommend that you machine any more than this else you run the risk of over-weakening the wheel.