Detroit DD15
The fuel economy leader in most head-to-head fleet testing, particularly in the integrated Freightliner drivetrain with the DT12 automated transmission and a downsped rear axle. The amplified common rail fuel system is the technical differentiator.
Output
400-505 hp
1550-1850 lb-ft peak torque
Fuel per mile
$0.529
at 7.7 mpg and $4.049 a gallon
Fuel a year
$63,514
over 120,000 miles
In-frame overhaul
$24k-$34k
rough, and shop rates move it
In the exhaust
DPF and SCR, one-box
The first greenhouse gas phase targets the whole vehicle rather than the engine, so this era is as much about aerodynamics, tyres and axle ratios as combustion. Downspeeding becomes standard: tall gears, low cruise revs.
Known issues
Aftertreatment one-box sensors, DEF pumps, and on earlier builds the turbo actuator. The 2010 to 2013 units had more one-box trouble than the current ones.
Verdict
If fuel is your largest line and you run a Freightliner, this drivetrain is the reason people buy Freightliners. Best matched as a factory set rather than mixed.