We never got to see Grosvenor Arch! Please be advised that once you leave the black top road from Tropic you will get on to a dirt track. OK for a short while, this track RAPIDLY DETERIORATES, with damage from flash floods and rain water causing sudden steep gullies that any 2-wheel drive vehicle may have trouble with.
Although the track is graded by earthmovers, I would strongly advise AGAINST attempting this rough road. Even a good 4-wheel drive vehicle will prove to be a very uncomfortable ride.
We turned back after 4 miles, please don't try it by car at all.