My understanding of metric sizes is this.
The first figure (135, 145 185 or whatever) is the nominal tread width in
The second figure is the height of the side wall expressed as a percentage of
the tread width.
UK spec Spridgets, fitted with radials came from the factory with 135 tyres,
except for the latest '`77 on Midgets which had 145s
Where no profile is given they are nomominally 80, but actually 82. ie 82% of
the tread width. (odd figure?)
Some maths will tell you therefore that to go wider from 145 tyres (82%
profile) without changing the circumference you need a tyre with a lower
percentage tyre wall.
I don't think there is a 75 profile tyre made, so the next that gives the
same rolling diameter is a 165/70
I have these on my 1500 midget and they are fine.
You then go 175/65 (rare in UK), 185/60, probably too wide without some
bodywork modification, but depends on the wheel offset.
I believe that the widest recommended tyre for 4 1/2" rims is the 165
I once did the theoretical calculations, assuming a wheel radius of 164mm (6
1/2") plus the tyre wall heights calculated as a percentage of tyre widths,
and the figures actually worked out quite well. But remember that different
manufacturers will vary and of course the radius variation between a new tyre
and a well worn one are probably 8 to 10 mm.