Fuel economy has never been more of an issue than it is today. Carriers run more miles and fuel prices stay at record highs. At the recent Truckload Carriers Association fuel forum, Steve Graham, director of fuel and tire systems for Schneider National, happened to mention in conversation that <+>1/<->10th of a mile per gallon was worth $8 million to his company each year.

And if it's important to the carriers, you can bet it's important that the truck manufacturers deliver fuel economies. They must conduct extensive testing in order to know what they're dealing with and to be confident of the benefits they gain.

To this end, Kenworth runs a sophisticated fuel economy program out of its Renton, Wash., facility, headed by Senior Project Engineer Tom Leithen, ably assisted by engineer and longtime friend Gary Ziebel.

I first met Ziebel 21 years ago at the public launch of the T600, shortly after he and a couple of colleagues had feather-footed the new model over some Texas testing. They managed to squeeze 10 mpg out of the revolutionary looking "Anteater." That year, I drove that truck with Ziebel at the Mid-America Trucking Show. My best memory of the driving experience was listening to all the questions and cat-calls over the CB.

One in particular I should paraphrase: "I don't care if it gets 20 mpg. You won't find me driving an ugly sucker like that."

So here I am again, at the wheel with Ziebel beside me, in the latest iteration of the T600 – a sleek and, to many eyes, fine-looking AeroCab sleeper.

It was a customer truck, in white with a 475-horsepower ACERT Cat and Super 13 transmission, with the latest sumptuous interior package.

To add a little spirit of competition, two journalists were invited to drive this T6. The other was Jim Park, editor of the Canadian owner-operator magazine HighwayStar, a sister publication to our own RoadStar magazine. Park said he was out to gain the economy title. An experienced driver and past owner-operator, Park would prove to be a worthy competitor.

There was a double purpose to the exercise. One was to examine the latest in truck interiors and drive the T600 again. The second purpose was to run the fuel economy route – the mountain one – and see what Leithen and Ziebel do to make fuel economy testing an art form. For one thing, they always run the route with two trucks: One is the test vehicle, the other a "control" truck that has covered the route dozens of times. This control truck allows for corrections for weather, time of day and a host of other variables.

Also correcting for conditions is Moses Luyombya, a mathematically minded engineer and fuel economy guru, who looks at things such as fuel temperature, ambient temperature and humidity and corrections derived from the control truck. This was vital to our contest, because Park and I drove on different days under quite different conditions.

The information is gathered on specially designed and constructed "Silverleaf" data-loggers. These collect a host of engine operating conditions, sorting it into real-time information for the driver, including rpm, speed, engine load and a handy fuel economy display indicating turbo boost.

The route we were driving is very familiar to the engineers, a scenic 496-mile drive, south out of the Seattle area on I-5 to Portland, then inland along the Columbia River on I-84 to The Dalles. After a quick break there, the trailers are switched and the trucks turn around and run back to Seattle. Most of the route is four-lane, but there's some two-lane and some busy traffic, reflecting the sort of highway conditions most customers see on a daily basis.

The route is consistent, so the engineers always stop at the same places for the same amount of time. Engines are started, idled and stopped at the same time so the engine hours are identical. And at the start and finish of the runs, the trucks are fueled in precisely the same position on the same flat concrete pad from fuel kept in the back of a special test-facility trailer.

They have developed a test probe that is lowered into the tanks to get the fuel level consistently accurate. Then the fuel is carefully added and recorded to within 4 ounces. Fuel temperatures are recorded, as well as ambient temperature and humidity to allow for later corrections.

Post trip, the data is downloaded, compiled and analyzed by Luyombya. He's looking for anything out of the ordinary on the control truck to ensure the integrity of data on the test unit. The trips have been done so often, the data can be analyzed for different sections of the route. They have even profiled every mile of road so they can pinpoint what the trucks are doing at any particular moment. As a result, even though Park and I drove on different days in different weather, the team had confidence in the corrected figures.


This was the latest T600 AeroCab, spec'd with a one-piece windshield that likely adds to the aerodynamic efficiency, and for certain adds to the visibility. DayLite doors add to the overall view out. Kenworth also has done a great job with its aero mirrors. The cowl-mounted brackets are unobtrusive and hold the mirrors steady. The adjuster for the mirrors is conveniently set in the armrest/door pull.

Glancing forward, the new dash arrangement is pretty impressive. There is a small peak over the large speedometer and tachometer, with stylish panels for the round supplementary instruments. A row of rocker switches spreads across the lower edge of the dash, and there are rotary controls for the heating and ventilation. A very useful multifunction switch is part of the turn signal stalk and, on the test unit, a smart wheel puts cruise and retarder functions right at the fingertips.

The passenger side features a good-sized and nicely trimmed locker – much more than a glovebox – and because of the sensible sleeper layout, the Kenworth AirCushion premium seats recline quite nicely.

It truly is a handsome setup. But then, so is the rest of the trim. The top level is the Diamond VIT. It manages to capture some of the traditional diamond and buttoned trim that has been a mark of Kenworth for a long time, but presents it in a thoroughly modern way. The door trim is especially nice and includes an integrated pocket and wood accents that are repeated throughout the interior.

The sleeper has more than 7 feet of headroom and a well-arranged interior that is bright from all the overhead windows. Despite all the glass, there's still good storage and nice features such as the flip-down working surface that hides behind the right side upper compartment door.


Kenworths are drivers' trucks, with a well-engineered, well-connected feeling to the controls. This one did not disappoint, particularly with its Eaton Fuller 13-speed. The 475 ACERT is a strong puller and we had little difficulty maintaining speed, mostly using cruise control, seeing as that's the most consistent way to get good fuel economy. I did occasionally let the truck run on some of the downgrades rather than hold speed back. This is all recorded in the data. I was just a shade faster than my competitor, Jim Park.

As you can see from the summary panel, the top speed I attained was also a shade more. But the number I like is the fuel economy: It's not on the table, but I beat Park by just a bit. My overall figure was 6.89 mpg. His, after correcting for rain and ambient temperature and humidity, was 6.78. That's only 0.11 mpg over 500 miles, but the clue is in the other numbers.

Despite being a hair quicker on the road and with a higher top speed, the actual load factor on the engine was lower, as were engine rpm and turbo boost. That meant I was able to keep it in the big hole just a little more, and keep my foot out of it just a shade more, showing that going slow isn't the only factor to consider.

To borrow from Park's account in his magazine: "Manifold pressure represents power demand on the engine and aggressive throttle applications results in correspondingly high manifold pressure," explains Ziebel. "Using less throttle – sacrificing a little acceleration – uses less fuel overall at a given speed, so the driver who is gentler with the throttle pedal is going to get better mileage."

The chart figure that explains this is the "Acceleration, True Mean," and it is a measure of the power demand. Park's is 11.742 where mine is 7.289, accounting for the slightly better fuel economy.

According to post-testing conversation, it appears that fuel economy guru and ex-Caterpillar tester Jim Booth – who is retired but still out there trucking – is consistently in the 3-4 range for these acceleration figures. And that buys him more than 9 mpg on a T2000 with a virtually identical powertrain to this test unit.


It was a thoroughly fascinating exercise, both from the perspective of driving the latest of the long line of T600s, and from understanding why there is a difference in fuel consumption when two guys are trying their best to beat the other.

The updates to the interiors – common on all the AeroCab models, W900, T800 and C500 as well – are exceptional, making a comfortable and quiet truck even more so. The road manners of the T600 have never been in doubt, and the combination of new interior and old values makes the latest generation of T600 a very fine truck indeed. And a handsome one, whatever that critic might have said on the CB more than two decades ago.

This drive test reaffirmed the lessons I had learned from driving with Booth over the years. And if I can do it, so can other drivers.

There's nothing particularly magic about the data collection here. The team goes to a lot of trouble to analyze it. But you can get virtually the same information from the ECMs on the engines in your own fleet. Engine speed and demand histograms are available and can be downloaded on a regular basis to compare different drivers. Armed with this sort of information, trainers can work with their drivers to get fuel economy numbers up.

You're never going to find another Jim Booth. But Jim Park and I are true believers, having experienced the evidence of this drive.

You don't necessarily have to slow down – within reason – but you do have to keep your right foot out of it.