Diesel-fuelled vehicles are also affected. Suppliers gradually blend Winter diesel to prevent fuel gelling. They do it according to the historic temperature normal for the region. This blend negatively affects the fuel efficiency, so it is done only as necessary. It is therefore imperative that low-mileage diesel vehicles don't linger too long on their tanks of fuel or they could be in for a rude surprise when the outdoor temperature drops. Another aspect of diesel vehicles is that they have to run an initially richer fuel-to-air blend to get up to minimum operating temperature; At the first drive-off it should not come as a surprise if the diesel engine produces some black smoke, especially in sub-freezing temperatures.