A COE (cab-over-engine) has less total surface area. A COE has packaged all components as efficiently as possible.
The best was an early/mid-90's Pete cabover that had a "front" with the slightest V-shape to it. Subtle.
1991 Peterbilt 372
1991 PETERBILT 372 Cabover Truck w/ Sleeper For Sale At TruckPaper.com
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 2004.0 7,360-lb DODGE Ram QC/LB 2500 2WD/6-speed Cummins 305/555 ISB. Stock, except LEER bed topper and twin muffler exhaust. 180,000 miles at 4,625-hrs @ 39 mph. 15-cpm solo, 25-cpm towing. (22-mpg avg past 37k)
Fuel Log
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