For Aston Martin Third Series tyres the 525/550x18 Dunlop D2/103 is what Longstone Tyres thinks will suit your car best.
History of the Aston Martin Third Series
Built between 1934-1936 the Aston Martin Third Series or MK II was a development of the Aston Martin Second Series, intended as a more usable but faster version. There were several different versions, available with either a short or long-chassis with either tourer, two-door saloon and drophead coupe bodies. The short chassis version of this car coupled with a lightweight body would ultimately become the Aston Martin Ulster.