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Engine maker Cummins Inc. said Wednesday it has revised its previously announced fourth-quarter and full-year 2012 operating results down by $20 million, with net income reduced by $12 million, or 7 cents per share.

There’s no real mystery as to why otherwise intelligent fleet operators so often negotiate full-service leases using only big, round numbers. Reason No. 1 is that they simply don’t know their actual, detailed maintenance costs.

American Axle & Manufacturing Holdings’ fourth-quarter net income rose to $319.9 million, or $4.21 per share — which included a $337.5 million tax benefit — from $31.1 million, or 41 cents, a year earlier.

Independent engine maker Cummins Inc. extended its lead as the primary supplier to the North American heavy-duty diesel truck market last year, growing its market share to 40.4%, from 39.1% in 2011, WardsAuto.com reported.

Last summer’s record heat scorched much of the United States, with evidence of the high temperatures visible on the nation’s highways in the form of chunks of broken tires and separated treads, so-called “road gators.”

Cummins Inc.’s fourth-quarter operating profit and sales fell on weaker North American trucking and construction markets, the engine maker said Wednesday.

Eaton Corp. said Tuesday its fourth-quarter earnings fell 50% on charges related to its $13 billion acquisition of Ireland-based Cooper Industries.

Used Class 8 truck sales declined in December from a year earlier as vehicle prices increased and average mileage trickled lower, according to ACT Research.

Meritor Inc. reported a loss for its fiscal first quarter, citing weaker conditions outside of North America.