"Engine Wear and Tear Letting an engine idle actually does more damage to the engine than starting and stopping. Running an engine at low speed (idling) causes twice the wear on internal parts compared to driving at regular speeds, which can increase maintenance costs and shorten the life of the engine. Generally, fuel consumption during engine start-up is equivalent to about 30 seconds of engine idling." From: EPA New England Something fleet mechanics have known forever. Some folks believe that there is a linear correlation with hours idleing to driven mileage (or engine hour run time), where there are many more destructive variables involved with idleing hours than driving or running the engine at an efficient rpm and load. One destructive variable is condensation in the exhaust and also into the crankcase, which builds up acids, which are destructive to the exhaust system as well as the internals. The enging temp is lowered at idle rpms, the condensation does not cook efficiently out of the oil causing sludge. Inefficient combution results from lower cylinder pressures causing carbon buildup in the top end sparkplugs, valves, o2 sensors, while also finding its way into the crankcase through blowby causing accelerated wear. The increase in pollution from this is up to 10 times the pollution from an engine under driving conditions. Lowered oil pressure from idle causes less hydrostatic pressure on bearing surfaces increasing oil shear and consequently wear. Excessive idleing is very stressful to an engine and related systems. Fleet owners are now starting to require that truckers reduce idleing of their trucks by sustantial amounts (there are now idleing monitors that can be installed) to save on fuel and maintinience costs. The side benefit is cleaner environment. Of course there will be a$$brains that will think that 'hours are hours' no matter what rpm.