While many diesel-engine lovers are swayed by performance and fuel-economy numbers, commercial operators typically look long and hard at another critical though less-sexy metric in their competitive analysis: maintenance needs.

That's because the costs of upkeep, including such mundane chores as changing engine oil, can add up to a pretty penny over the life of a diesel engine.

And for some operators, it's those numbers — and not the outlandish torque ratings — that win them.

Take the Cummins 6.7L turbodiesel powering Chrysler Group LLC's Ram pickups, for example.

While it placed last in the Best Engines Diesel Shootout in acceleration times, compared with Ford Motor Co.'s home-grown Power Stroke and General Motors Co.'s Duramax, it nabs the top spot outright in terms of lower maintenance lifecycle costs.

While oil, fuel-filter and coolant change intervals are almost the same among the three OEMs, diesel exhaust fluid (DEF) replenishment needs and life-to-overhaul metrics aren't even close.

For starters, the Cummins engine uses an oxides of nitrogen trap to comply with exhaust emission rules, eliminating the need for the selective catalytic reduction technology used by Ford and GM.

That means no fill-ups of DEF, an ammonia-based liquid that is 67.5% water. DEF costs about the same as a gallon of diesel fuel, which is roughly $2.93 per gallon at the moment.

So, for Ford, that equates to $16.11 on average to refill its 5.5-gallon (21L) DEF tank, and GM comes in at $15.52 due to its smaller 5.3-gallon (20L) tank.

How many miles the truck is driven per year, of course, dictates how much DEF gets used annually. Trucks in heavy use will need several fill-ups annually.

How soon an engine will need an overhaul is another critical issue for owners.

The Ram Cummins diesel offers far and away the longest interval here, boasting a 350,000-mile (563,255-km) life-to-overhaul timeline, while Ford comes in at 250,000 miles (402,325 km) and GM at 200,000 miles (321,860 km).

GM notes the 200,000 figure is based on severe operating conditions, so more sedate drivers might get more miles for that service interval.

Diesel Exhaust Fluid Refill

Ford: 5.5-gallon (21L) tank needs refilling about every 7,500 miles

GM: 5.3-gallon (20L) tank needs refilling about every 5,000 miles (8,047 km)

Chrysler: Uses a NOx catalyst so no DEF fluid required

Miles to Overhaul

Ford: 250,000 (402,325 km)

GM: 200,000 (321,860 km)

Chrysler: 350,000 (563,255 km)