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I’ve used it a few times in the mountains in Utah & NE Pennsylvania. I think the upper limit is 20 mph but it was a big help when I was in a steep long hill descent on secondary roads towing our Landmark 5er. Not off roading but steep & narrow enough that it came in handy. Just pressed the button & took foot off accelerator on the decline. When the hill levels out & you accelerate it kicks out but then will reactivate on the next descent once below 20.