When you are on a hill, going down a hill.Never use it in heavy traffic, when it is raining, when towing, or when there is ice or snow on the road.It should not be used when:it's raining/snowingthere is a lot of trafficyou're driving through a neighborhoodyou drive through the cityUsually when the car is below 25-20 mph (miles per hour) but you should'nt use it when you are travelling on hilly terrain (going up and down hilly areas) the cruise will try to compensate by shifting to a lower gear(on a auto trans.) wich will lower M.P.G. to try to make the speed constant, if you are going uphill stay in the same gear if possible and wait till the downhill crest comes on to make the speed constant, gain as much speed as legally possible(usually 5 M.P.H. over depending on the strictness of the p