SCOTTSDALE, Ariz. — It’s been seven years since I first drove a Peterbilt Model 579. This particular truck was built right around the time the model was unveiled at the Mid-America Trucking Show in March 2012. It’s safe to say that the truck has only improved with the passing of time.
Designers added the Epiq aero-trim package in 2015, which included a roof fairing bridge to reduce the gap between truck and trailer. improved frame fairings. closing rubber wheels in the wells of the front wheels. a bumper barrage; and bumper-to-hood sealing to improve frontal profile. At the time, Peterbilt said this version would be up to 8% more fuel efficient compared to the Epiq-less 579. In 2019, after some further improvements, the Model 579 can now lay claim to being the most fuel efficient truck that ever wears the Peterbilt Badge.
While the classic Model 389 styling was top of the line back then—and still maintains a very strong following today—fleets and even owner-operator priorities have shifted to more aerodynamic cab designs. The Model 579 was destined to become the company’s flagship from the moment it was introduced.
This particular trip wasn’t so much a “new features” test drive as it was an opportunity to get out and experience the latest and greatest from Peterbilt. The truck was in Scottsdale, Ariz. for a sales meeting, so PR manager Nick Smith asked if I wanted to take a ride and deliver it back to the factory in Denton, Texas.
I flew in Thursday afternoon and picked up the truck Friday morning. Nick and I spent the next two days driving about 1,000 miles from Scottsdale to Denton. Our route took us on AZ 87 from Scottsdale to Payson, then AZ 260 and AZ 377 to Holbrook where we picked up I-40 heading east. We ran I-40 east to Amarillo, Texas, where we turned southeast onto US 287 and drove almost directly to the Peterbilt plant in Denton.
I don’t know what the engineers had in mind when they specified the truck’s drivetrain, but I suspect it was a fairly general truck application, operating on flat to rolling terrain at a cruising speed of 65-70 mph (105-115 km/h) with a gross combined weight of 80,000 pounds or less. At 65 mph, the engine revved to 1,100 rpm, just 100 rpm above a seemingly hard-wired downshift point. It was turning 1,200 rpm. at 70 mph, leaving the 200 r.p.m. before falling from maximum torque. I found the gearbox hunted a bit while at 65mph on rolling hills, but it held 12th gear well on level ground.
The 455/1,650 MX-13 did better than I expected on 6% and 7% grades through the Mazatzal Mountains in central Arizona. I pulled the grades on the AZ 87 in 8th or 9th gear at 30-40 mph (about 50-65 km/h) with a gross weight of about 77,600 lbs. Going down said grades, I ran 7th gear at 1,800-2,000 rpm, engine brake switching between first and second. The 579 has disc brakes all around so I wasn’t worried about stopping. I just wanted to ride the descents properly to see how well the 13 liter engine held me.
Overall, driving a Model 579 is a real pleasure. Visibility is fantastic. It’s really quiet (67.2dB on my iPhone’s sound meter app), and the Peterbilt air suspension shakes out all the rough stuff before it reaches the cabin and driver’s seat. And I always give top marks to the Peterbilt’s steering geometry. You can’t go wrong with a Sheppard steering system, and this 579 was no exception. Stable and confident on the highway and not too stable for maneuvering in tight places.
The physical aspects of driving the truck are very good, with all commonly used controls close at hand. This had cruise controls and a radio mounted on the steering wheel. Even the cupholders were in the right place, although the driver’s cupholder was a bit wobbly. My only beef with the dashboard layout comes from the flick of the headlight switch.
Call me old fashioned but I still like the lights to flash for a passing driver and the headlight switch is low on the left side of the A panel. To reach it you have to wedge your hand between the turn signal lever and the handle which is mounted on the front door pillar. It’s not a dealbreaker, just a little awkward.
The living spaces of UltraLoft
Even though I didn’t spend a night in the truck, I did get a good ride inside the 80-inch UltraLoft bed, and it’s all it’s cracked up to be. We had the double bunk version and the top bunk stayed put except to pull it down and take a few pictures. It unfolds easily and stacks with a fairly light push as only about 1/3 of the top bed flips down. In the top position with a single driver, this space becomes storage. Even with the top bunk down, the space above the bottom bunk isn’t cramped or claustrophobic.
The right hand side closet cabinet is quite high to begin with, but the shed bar is mounted even higher in the cabinet. Drivers can even hang shirts up there without worrying about clothes dragging to the bottom of the cupboard.
The microwave cabinet can accommodate an almost full-sized appliance, not the small shoebox-sized ovens we sometimes find in trucks. And speaking of larger appliances, the right wall can hold a 32-inch flat screen TV. This is huge.
The other feature I really liked was the new thermal insulation package. While it is said to keep the internal temperature longer, it is also an excellent noise attenuator. It’s quiet in there, even parked next to a roaring reef. It also helps with road noise in general, making the driving environment extremely quiet. In fact, the exterior profile of the sleeper, combined with the cockpit and hood, almost eliminates wind noise. I did some narration for a video with the driver’s side window up and down at highway speed and you could barely tell the difference. This lack of wind noise is a hallmark of a well-designed aircraft cabin and sleeper.
This trip from Scottsdale to Denton was the longest trip I’ve taken since I stopped driving in 1998. With two 500 mile days, I probably deserved to be tired at the end of the shift, but I wasn’t. That’s a walk in the park for most drivers, but I’m fat and happy now behind my desk and rarely spend more than a few hours in a test truck. The 579 is a very nice truck to drive, and I turned my back on it wanting a little more, instead of hitting rocks on it as I drove away.
Specification sheet
2019 Peterbilt 579 Epiq
- 80′ Ultraloft Platinum Bedroom Interior (Upper & Lower Bunk).
- Peterbilt LX Driver’s Premium Driver and Passenger Seats (Leather)
- Premium sound system
- Thermal insulation package
- Premium mattress with pocket springs
Drive system
- Paccar MX-13 455 hp @1,600, 1,650 lb-ft @900 (2017 emissions)
- Paccar 12-speed automatic transmission. 0.77 OD (11th gear direct)
- Meritor RPL 25 SD drive line
Front
- Meritor MFS+12E 12,500-lb. with a 3.5″ drop
- Conical leaf springs, 12,500-lb., with shock absorbers
- Sheppard HD94 power steering
- Air disc brakes, preset aluminum hubs
- Bridgestone R 283A 16-ply 295/75R 22.5
Rear end
- Paccar 40,000-lb. drive shaft, 2.64:1 ratio
- Peterbilt 40,000 lb low leaf air suspension, 52 inch spread
- Air disc brakes, preset aluminum hubs
- Bridgestone R 283A 14 ply 295/75R 22.5
Weight reduction and fuel savings
- Aluminum frame and cross beams
- Holland aluminum 5th wheel oil free
- AMT neutral coast enabled
- 123 inch BBC aluminum aerodynamic cab
- Aero close-out under cab/bed sides
- Aero-optimized sun visor
- Closing roof fairing and trailer bridge
- Aero bumper, cab fairing with 18 inch extension
- Frame fairing quarter fender to rear of bedroom with rubber aprons, fairing under rear axle and wheel covers
Wheelbase: 236 inches
Dry weight: 17,600 lbs.
GCW as driven: 77,380 lbs.
Some amazing fuel mileage
This truck was obviously specced for fuel economy with a reduced driveline, moderate power and a plethora of aero gadgets. The trailer was also airbrushed, with a whisker-type vacuum reducer, side skirts, a trailer tail, and beaded tires. On a 505-mile (808 km) leg on the second day of the trip, the average fuel economy displayed on the dashboard screen was 10.2 mpg (23.1 L/100 km). During this leg, I got my mileage up to 11.1 mpg (21.2 L/100km) over 150 miles.
I ran this leg mostly without cruise control and maintained a speed close to 65 mph (105 km/h) despite the 75 mph (120 km/h) speed limit. The truck’s gross combined weight was 77,380 pounds (35,100 kg), which it weighed when we arrived at the Denton factory with less than a quarter of a tank of fuel.
The day before, running through the mountains in Arizona and cruising down the highway at 70 mph (112km/h) and occasionally up to 75 (120) through New Mexico, my average for the day was 7.8 mpg (30 .1 L/100 km) . I’m not sure what had a bigger effect on my fuel economy, the hills or the extra gear, but clearly the combination of the two had a negative effect.
The terrain on the second day was mostly flat with sections of rolling hills. We descended from an elevation of 4,600 feet above sea level in Santa Rosa, NM to 675 feet in Denton, a drop of nearly 4,000 feet in 800 km. We also had a stiff tail wind of about 20-25 mph (30-40 km/h) for much of the first two-thirds of the trip, but the winds died down in the evening for the last 240 km. These conditions certainly didn’t hurt my fuel economy.
I drove the truck pretty gently, taking my foot off the pedal on any downhills to get some gravity-fed free momentum, and let the truck slow down on the uphills, backing off the throttle a bit instead of powering up to maintain 65 mph (105 km/h). I like to think that this driving style had some impact on overall fuel economy, but without a side-by-side comparison there’s no way to determine how much. If I had to guess, I’d say about 1-1.5 mpg overall.
We had ideal conditions to deliver some great fuel economy numbers and the truck rose to the occasion.
You’ll find an extended story about my 11.1 mpg trip to west Texas here.
