When you turn on cruise that cable holds its place just like when you keep your foot at the same spot when cruising down the highway. I think you can actually use it at like 40mph or maybe way less, but I dunno cause I never tried below that.
You can use it at less than 40mph, I know because i was tailed by a city cop going thru a small town where the speed limit was 35, that was for about 2 miles.
He tailed me so close he could eat off my car, I set CC to 35 and kept going till I was out of the city limits when he turned around.
Weird...I was in a 2004 V6 and the cruise set at 25(I live in Cleveland where there are tons of cops) and it was the actual speed limit. Maybe the new cars are lower... dunno
the lowest the CC will go is 25 mph on our mustangs and i think almost all other fords. and just because i know....the lowest the CC on a chrysler product will go is 35.