>new tyre for my 59 5Ta.
>current tyres are Dunlops sized 4.10 V18 (front) and 4.10 VB18 (rear)
>biggest size of the mitas I liked is 3.25 18 (ribbed) front and 3.25 18 (Universal) rear.
>is it ok to drop that size.
You aren't "dropping" anything. 3.25 and 4.10 are just different tyre cross-sections. In reality, 4.10's very fat for something as dinky and low-powered as a 5TA.
Original tyre sizes on all '59 C-range were 3.25x17 front and 3.50x17 rear. Modern tyres are a little wider overall for the same section width so you could probably drop to a 3.00 front with the 3.25 rear. Or increase the rear width to the original 3.50.
The 18" rims are already bigger than the 17" fitted originally. The overall diameters of the various sections mentioned above are pretty similar, just the overall widths will be less.
>Akrond 18" 1.85 40
1.85 is the same rim width as the original WM2.