The Citroen SM tyre is 205/70VR15 Michelin XWX. There are other tyres in this size but none of them suit the Citroen SM the way the XWX does.
When the Citroen SM first came out it ran on carburettors. These first Citroen SM carburettor models actually fitted 195/70VR15 Michelin XWX which are no longer available. The tyre to fit now to the carburettor Citroen SM is the 205/70VR15 Michelin XWX; the same as the SM 2.6 Injection and the SM 2.9 injection models. When the injection model SM was introduced in 1972 other than the engine the only other change that would effect the cars drive was the introduction of the slightly larger tyres. So the earlier SM carburettor models are improved by fitting the 205/70VR15 Michelin XWX. This size is also appropriate for the Citroen SM Chapron Mylord and Citroen SM Chapron Opera.
Recommended Products for Citroen SM
Other Options for Citroen SM
The fuel-injected Citroen SM was equipped with 205/70x15 tyres as standard. Here are the options available for your Citroen SM. But in reality the only tyre to fit would be the XWX from Michelins Collection range of tyres.