Cold Air Intakes
 

Are cold air intakes worth buying? Do they really improve a vehicle's fuel efficiency and horsepower?

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No, cold air intakes are generally bad for efficiency. Instead we fabricate our own hot air intakes or use grill blocks, which tend to raise under hood temperatures which also increases air intake temperature.

Only in a diesel.

Only in a boosted diesel because boost equals heated intake. Not boosted diesel engines prefer warm also

When my diesel was non turbo I found it got +2 to +3 mpg when I restored the cold air during warmer temps.
When it was well below freezing warm air was better for +1 to +2mpg.

Yeah but it is a density thing not a vaporizing thing like on gasoline. Cold air gives more compression which increases efficiency. Warm air increases vaporizing efficiency. Warm engine, cold air is best of both for diesel. 250 is happiest AFTER it starts on winter day

Yes on horsepower, no on efficiency.

Others bought a CAI to improve the sound of their engines.