Test driving Peterbilt’s Model 579

SCOTTSDALE, Ariz. — Seven years have passed since my first drive in a Peterbilt Model 579. That particular truck was built right around the time the model was being unveiled at the Mid-America Trucking Show in March 2012. It’s safe to say 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; enhanced chassis fairings; rubber wheel closeouts on the front wheel wells; a bumper dam; and a bumper-to-hood seal to improve the frontal profile. At the time, Peterbilt said that version would be up to 8% more efficient compared to Epiq-less 579. In 2019, following a further few refinements, the Model 579 can now claim to be the most fuel-efficient truck to ever wear the Peterbilt badge.

While the classic-styled Model 389 was top-of-the-heap back then — and still maintains a very strong following today — fleets and even owner-operator priorities have shifted to the more aerodynamic cab designs. The Model 579 was destined to become the company flagship from the moment it was introduced.

This particular trip wasn’t as much a “new-features” test drive as an opportunity to get out and experience the latest and greatest from Peterbilt. The truck was in Scottsdale, Ariz. for a sales meeting, so public relations manager Nick Smith asked if I wanted to go for a ride and deliver it back to the factory in Denton, Texas.

I flew out Thursday afternoon and picked up the truck Friday morning. Nick and I spent the next two days driving about 1,655 km from Scottsdale to Denton. Our route took us on AZ 87 from Scottsdale to Payson, then AZ 260 and AZ 377 up to Holbrook where we picked up I-40 eastbound. We ran I-40 east to Amarillo, Texas, where we swung southeast on US 287 and rode that practically right to the Peterbilt factory in Denton.

I don’t know what the engineers had in mind when they spec’d the truck’s powertrain, but I suspect it was a fairly generic truckload application, operating on flat to rolling terrain with a cruise speed of 65-70 mph (105-115 km/h) at a gross combination weight of 80,000 pounds or less. At 65 mph, the engine turned 1,100 rpm, just 100 rpm above a seemingly hardwired downshift point. It turned 1,200 rpm at 70 mph, leaving 200 rpm before dropping out of peak torque. I found the transmission hunted a bit while at 65 mph in the rolling hills but held 12th gear nicely on flat ground.

The 455/1,650 MX-13 did better than I expected it would on the 6% and 7% grades through the Mazatzal Mountains in central Arizona. I pulled the grades on AZ 87 in 8th or 9th gear at 30-40 mph (around 50-65 km/h) at a gross weight of about 77,600 lb. Coming down said grades, I ran 7th gear at 1,800-2,000 rpm, toggling the engine brake between first and second position. The 579 has disc brakes all around so I wasn’t worried about stopping. I just wanted to drive the descents as they should be done to see how well the 13-liter engine held me back.

Overall, driving a Model 579 is a real treat. The visibility is fantastic. It’s really quiet (67.2 dB on my iPhone sound meter app), and the Peterbilt air-leaf suspension shakes out all the rough stuff before it gets to the cab and the driver’s seat. And I always give top marks to Peterbilt’s steering geometry. You can’t go wrong with a Sheppard steering gear, and this 579 was no exception. Firm and sure-footed on the highway and not too firm for maneuvering in tight quarters.

The physical aspects of driving the truck are very good, with all the commonly used controls within easy reach. This one had steering-wheel mounted cruise and radio controls. Even the cup holders were in the right place, though the driver’s cup holder rattled a little. My only beef with the dash layout comes from reaching for the headlight switch.

Call me old-fashioned, but I still like to flash the lights for a passing driver, and the headlight switch is down low on the left side of the A-panel. To reach it, you have to squeeze your hand between the turn-signal stalk and the grab-handle mounted on the forward door post. Not a deal breaker, just a little awkward.

The UltraLoft’s living quarters

While I did not spend a night in the truck, I did have a good poke around inside the 80-inch UltraLoft sleeper, and it’s all it’s hyped-up to be. We had the double-bunk version, and the upper bunk remained stowed except to pull it down a take a few pictures. It deploys easily and stows with a fairly light push as only about 1/3 of the upper bed flips down. In the up position with a single driver, that space turns into storage. Even with the upper bunk in the down position, the space above the lower bunk isn’t cramped or claustrophobic.

The wardrobe cabinet on the right side is tall enough to begin with, but the hangar bar is mounted even higher in the cabinet. Drivers can even hang tall shirts in there without any worry about the garments dragging on the cabinet bottom.

The microwave cabinet can accommodate a near-full-size appliance, not the little shoe-box sized ovens we sometimes find in trucks. And speaking of larger appliances, the right-hand wall can fit a 32-inch flat-screen TV. That’s huge.

The other feature I really liked was the new thermal insulation package. While it’s said to maintain the internal temperature longer, it’s also a great noise attenuator. It’s quiet in there, even parked next to a roaring reefer. It also helps with the road noise generally, making the driving environment extremely quiet. In fact, the external profile of the sleeper, coupled with the aero cab and hood, nearly eliminates wind noise. I was doing some narration for a video with the driver-side window up and down at highway speed and you could barely tell the difference. That lack of wind noise is a hallmark of a well-designed aero-shaped cab and sleeper.

This trip from Scottsdale to Denton was the longest run I’ve made 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 get anymore 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, rather than kicking stones at it as I walked away.

Spec’ Sheet

2019 Peterbilt 579 Epiq

• 80-inch Ultraloft sleeper (upper & lower bunk) platinum interior
• Peterbilt LX driver premium driver and passenger seats (leather)
• Premium sound system
• Thermal insulation package
• Premium pocket spring mattress

Powertrain

• Paccar MX-13 455 hp @1,600, 1,650 lb-ft @900 (2017 emissions)
• Paccar 12-speed automated trans. 0.77 OD (11th gear direct)
• Meritor RPL 25 SD driveline

Front end

• Meritor MFS+12E 12,500-lb. w/ 3.5″ drop
• Taper leaf springs, 12,500-lb., w/ shocks
• Sheppard HD94 power steering
• Air-disc brakes, pre-set aluminum hubs
• Bridgestone R 283A 16-ply 295/75R 22.5

Rear end

• Paccar 40,000-lb. drive axle, 2.64:1 ratio
• Peterbilt low air-leaf suspension 40,000-lb., 52-inch spread
• Air-disc brakes, pre-set aluminum hubs
• Bridgestone R 283A 14-ply 295/75R 22.5

Weight reduction and fuel savings

• Aluminum frame and crossmembers
• Holland no-lube aluminum 5th wheel
• AMT neutral coast enabled
• Aerodynamic 123-inch BBC aluminum cab
• Aero close-out under cab/sleeper sides
• Aero-optimized sun visor
• Roof fairing close-out and trailer bridge
• Aero bumper, cab fairings w/18-inch extenders
• Chassis fairings quarter fender to back of sleeper w/ rubber skirts, FlowBelow rear-axle fairings and wheel covers

Wheelbase: 236 inches

Dry weight: 17,600 lb.

GCW as driven: 77,380 lb.

Some phenomenal fuel mileage

This truck was obviously spec’d for fuel efficiency with a downsped driveline, modest power, and a plethora of aero devices. The trailer was decked out in aero, too, with a mustache-type gap reducer, side skirts, a trailer tail and rib tires. Over a 505-mile (808 km) leg on the second day of the trip, the average fuel economy shown on the dash display was 10.2 mpg (23.1 L/100km). During that leg, I had the mileage up to 11.1 mpg (21.2 L/100km) for more than 150 miles.

I ran that leg mostly without cruise control and kept the speed close to 65 mph (105 km/h) despite the 75-mph (120 km/h) speed limit. The gross combination weight of the truck was 77,380 lb. (35,100 kg), weighed upon our arrival at the factory in Denton with less than a quarter of a tank of fuel.

The previous day, running through the mountains in Arizona and cruising on 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 which had a greater impact on my fuel economy, the hills or the extra speed, but clearly the combination of the two had a detrimental 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, N.M. to 675 feet in Denton, a drop of nearly 4,000 feet over 800 km. We also had a stiff tailwind of about 20-25 mph (30-40 km/h) for much of the first two-thirds of the trip, but the winds diminished in the evening over the final 240 km. Those conditions certainly didn’t hurt my fuel economy.

I drove the truck pretty gently, taking my foot off the pedal on any downslope to gain some free, gravity-fed momentum and letting the truck slow down on the uphill side, backing off the throttle a little rather than powering up to maintain 65 mph (105 km/h). I like to think this driving style had some impact on the 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 turn in some great fuel consumption numbers, and the truck rose to the occasion.

You’ll find an extended story about my 11.1 mpg trek across west Texas here.