Alex mate, squeezing it onto the rim is nothing to do with it.
A 12.5" wide tyre will fit onto a 8" wide rim but the risk is that because it is being pinched in by the 4.5" narrower rim then an impact on the tyre when offroading could make the tyre pop off the rim... and then you're knackered :nenau
12.5" is hell of a wide anyway, equivalent to 315mm ie 50 mm wider than your current tyre. When it comes to mud the narrower tyre will be far better at cutting through it.
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