Fuel consumption remains constant as air filter restriction increases until it reaches a certain point. Testing by independent companies suggests a slight increase of fuel consumption with a diesel engine when air filter restriction reaches 17-18 inches water vacuum. However, the fuel consumption may not be severe.
The 17-18 inches restriction is only seen at full load conditions, which may be an infrequent condition. Full load condition includes hard throttling, pulling a trailer, or passing another vehicle. Air filter restriction reduces when these conditions end. The engine may be operating most of the time at lower restriction levels cruising at highway speeds, which would not affect fuel consumption. If there are concerns about excess fuel consumption, the air filter can be changed at lower restriction levels.
