how about this one?Do they fit on gs 300 20" F10+31 76 mm R12+19 114 mm let me know also how wide in the back do i need
thanks guy

 

20x10 +31 up front is pretty aggressive, equivalent to around 9" +19. To fit this you will need some stretched tyres, shaved guards (or rolled flat totally), and a fair bit of camber at least -2.5. Or guard work...pulling etc

20x12 +19 is equivalent to around 10" -4. You will need either aggressve as stretch & like -5 camber & shaved flat guards, or most likely guard work e.g. pulling