When I bought GAM in January it badly needed 5 new tyres. The OEM size would have been 5.9 (150mm) x 15, but this was no longer available. The perceived wisdom here on the forum was that the best fit in modern sizes would be a 165/80R15 tyre. I wonder whether that is strictly true.
Here is a photo of a 1961 2 door Amazon with 5.9x15 tyres (taken from my Volvo Amazon - The Complete Story book by Richard Dredge):
... and here is one GAM with the original red wheels and 165/80R15 tyres:
... you may notice the wheel wells are rather more filled (to an extent due to the track spacers, but they don't change the diameter of course).
Now have a look at GAM with the silver wheels and 175/60R15 tyres:
... I think the spaces around the wheels are more similar.
A little simple mathematics shows that:
4.9x15 - equates to a rolling diameter of 24.6"
165/80R15 - equates to a rolling diameter of 25.25"
175/60R15 - equates to a rolling diameter of 23.4"
... so the two modern sizes are about an inch either side of the OEM tyre. In GAM's case this is fortunate in that the slightly smaller tyre fixes the speedometer inaccuracy issue very cheaply and simply.
In the case of a standard Amazon transmission (so either a M40 with a 4.1 rear axle, or a M41/OD and a 4.56 axle) then 175/70R15 would be give a closer rolling diameter (24.8") to the OEM tyre.
With hindsight (isn't that a wonderful thing) I'd have probably fitted 175/60R15 tyres to GAM from the outset if I had known the above, and would certainly go for 175/70R15 tyres for standard gearing.
:-)