When I got my 33, I had a speedo chip convertor fitted which interfered with the Hicas although lots do not. I got a complete mph/kph speedo head from Hiteq, genuine Nissan part which looks the part too. Best solution in my book and only slightly more.
All that happened with mine was the HICAS light kept coming on after 10 - 15 minutes. When I put the HICAS into diagnostic mode, it reported that it wasn't getting (I think) a feed from the speedo, but I may worng on that last point, so long ago now.