The correct size that should be on is 380 85 28. I have 12.5 inch wide rimsYou need to find the spec sheets for the existing tyres and find out the rolling circumference of your tyre. Then match up a wider lower profile tyre to get a similar rolling circumference. Also check the width of your wheel rims, with the wider tyres recommended rim width. All the info is readily available online.
Many tyre dealers have agreed that a 420 will fit on the rim