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2011 Police Tahoe StabiliTrak Joins the 6-Speed

Written by PFM Staff

Room! Blessed, glorious room! For the fully outfitted officer, room for a radio and a TASER® on one side, pepper spray and a pistol on the other. For the fully upfitted police vehicle, room for all of the communications, enforcement and computing gear. Room for a prisoner partition that allows full driver seat track and seat back adjustments and plenty of space for the rear seat prisoner. Room in the back to stow all the gear most of us are required to carry. So, why a Tahoe for routine patrol? Room!

With very little fanfare, Chevrolet recently made two major changes to the police package, 2WD Tahoe PPV. One is a critical and long overdue safety feature, one that has been standard on the retail Tahoe for years—StabiliTrak. The other change greatly improves fuel economy in the police vehicle with the lowest fuel economy rating—a 6-speed automatic.

With the body-on-frame, V8-powered, RWD Ford CVPI going away after August 2011, and with the current search for a suitable replacement, the StabiliTrak and 6-speed changes make the body-on-frame, V8-powered, RWD Tahoe PPV well worth considering. This is especially true because the truck-based Tahoe PPV has a 250,000-mile duty cycle, making it one of the lowest total cost police vehicles available.

New 6-Speed Trans

The 6L80E 6-speed transmission is new to police work, but not new to the Tahoe. The retail 2WD Tahoe got the 6-speed auto in 2008 with the new 6.2L V8. The retail Tahoe with the 5.3L V8 used in the police SUV got the 6-speed in 2009. However, it was not until the 2010 model that the police version of the 2WD Tahoe got the 6-speed.

The 6-speed produced two immediate benefits: better fuel economy and faster acceleration. The 2009 2WD Tahoe PPV with a 320 hp, 5.3L V8; 4-speed trans; and 3.73 rear gears had EPA ratings of 14 mpg City, 19 mpg Highway and 16 mpg Combined. The 2010 2WD Tahoe PPV, with the same engine but 6-speed trans, is rated at 15 mpg City, 21 mpg Highway and 17 mpg Combined.

The important part of the EPA estimates is that the Tahoe PPV and Ford CVPI now have the same fuel mileage estimates. The LASD results confirmed the similarity in fuel use between these two vehicles—15.3 mpg for the Tahoe PPV and 15.8 mpg for the Ford CVPI.

In addition to EPA estimates geared for retail use, the best estimates for police use come from the Los Angeles County Sheriff’s tests. During a 100-mile loop with mixed urban and rural, low-speed and high-speed driving, the 2009 4-speed and 3.73 Tahoe got 13.6 mpg. The 2010 6-speed and 3.08 Tahoe got 15.3 mpg.

The change to a 6-speed made a 12 percent increase under these realistic driving conditions. This LASD protocol is a close estimate for fuel economy during admin use. Experience has shown that a marked patrol unit will get about 65 percent of this LASD estimate.

Some of the increased fuel economy is due to the new rear gear ratio. The 6-speed trans allows the use of a 3.08 rear gear. With a 3.08 ratio and double overdrive, the Tahoe cruises at 60 mph with the engine turning just 1500 rpm.

The 5.3L V8 police engine uses Active Fuel Management (cylinder deactivation). Under light throttle, four of the eight cylinders are shut off. It was extremely hard to tell when the AFM kicked in—just the slightest change in the exhaust tone. In fairness, AFM is seldom (perhaps never) activated in typical police use. It is not activated during idle.

During our nearly 1,100-mile test and evaluation with the Tahoe, we ran rural patrol and traffic enforcement, and spent time idling after responding to traffic collisions. We spent equal time on the interstate, suburban driving and crawling in rush-hour traffic. We averaged 16.3 mpg.

Faster Acceleration

The 6-speed also greatly improves the acceleration of the police Tahoe PPV. Since its 2007-model-year bump in power from 285 hp to 320 hp, the Tahoe has beat or tied the Ford CVPI in accelerating to 100 mph. A three-year average, the Tahoe was 0.3 seconds faster to 100 mph, which is essentially a tie. With the 6-speed, the 2010 Tahoe PPV is 1.9 seconds faster to 100 mph, and exactly 1/2 second faster to 60 mph. The Tahoe pulls especially hard above 80 mph. In other words, it accelerates from 80 mph to 120 mph just as briskly as it gets up to 80 mph. There are no flat spots in performance.

The Tahoe might be a big SUV, but it is a fast SUV. We had no problem catching 30-and 40-over speeders—even when braking from 60 mph and turning around in the median. With a top speed of 133 mph, the Tahoe PPV actually has a slightly higher top speed than the Ford CVPI. All other SUVs currently used in police service shut off between 98 mph and 106 mph.

Electronic Stability Control

With all of this performance, GM Fleet added StabiliTrak (electronic stability control) to the Tahoe PPV to help keep drivers out of trouble. StabiliTrak is long, long overdue in this aggressively driven, high roll center SUV. StabiliTrak was an option for the 2003 retail Tahoe. StabiliTrak was standard on all 5.4L retail Tahoes for 2005 and all retail Tahoes for 2006.

Most members of the GM Law Enforcement Product Council have urged the use of StabiliTrak on the police Tahoe for years, even if only the conservative retail software was used. The concern from a few police fleet managers was that the driving dynamics would be hurt too much by the intrusiveness of the stability control.

The Dodge HEMI Charger, which has always had stability control, proved to everyone that aggressive driving and stability control can co-exist. Even the most skeptical now see the benefit of this safety device. Electronic stability control is such a significant safety device that the nation’s chiefs and fleet managers convinced Dodge to delete the OFF button. You can push a button and open up the Dodge parameters before stability control engages, but you can’t turn it off.

Stability control is mandated by federal law on all 2012 vehicles. As a running change on the late-2010 police Tahoe PPV and all 2011 police Tahoes, StabiliTrak is finally standard. And police officers driving the Tahoe are safer because of it.

What Does It Do?

All electronic stability control systems work basically the same way. It is built on top of already existing ABS and traction control systems. The new parts include a yaw sensor, a lateral acceleration sensor and a steering wheel position sensor. These tie into the wheel speed sensors in the ABS and TC systems.

Stability control senses a skid and the loss of control and automatically applies one or more brakes to push or pull the vehicle back into the line the driver intended. With severe oversteer (the rear of the car coming around), the brakes on the outside front wheel are applied. With severe understeer (the front of the car pushes straight ahead), the brakes on the inside rear wheel are applied. StabiliTrak also reduces engine torque to slow the vehicle down.

Stability control helps to keep the vehicle on the road and inside the correct lane. It may sound obvious, but if the vehicle stays in its own lane when cornering, it is less likely to hit oncoming vehicles. If it stays on the road when cornering or during emergency maneuvers, it is less likely to roll over. Check out www.gmfleet.com for a video link to the police Tahoe-StabiliTrak demo.

For late 2010 and 2011, the police Tahoe PPV uses exactly the same stability control software and parameters as the retail Tahoe. This is the same as the StabiliTrak system used on the special service package 4x4 Tahoe 5W4.

On late-2010 and all 2011 police Tahoe PPVs, one push on the stability control button turns the traction control off. Some wheelspin may be necessary to “rock” the Tahoe to get it unstuck in mud, sand or snow. If you push the button again and hold it for 5 seconds, both traction control and stability control are completely deactivated (i.e., completely shut off).

In this situation, the new Tahoe PPV reverts to a condition similar to older Tahoe PPVs. However, modern software separate from StabiliTrak prevents the rear wheels from spinning up wildly. This alone works to reduce throttle-on oversteer. Of course, it does nothing to prevent understeer and throttle-off oversteer. With each key stroke, each time the engine is turned on, StabiliTrak defaults to its original condition (i.e., traction control on, stability control on).

For 2012, the police Tahoe PPV will get unique StabiliTrak software. The officer will have the option to turn traction control off with the first button push. With the second push and hold, the stability control will go to a more open, less restrictive, police-calibrated mode. In 2012, the stability control will not be able to be shut completely off.

This 2012-model-year, police-specific stability control logic will only be available on the police 2WD Tahoe PPV. The special service 4x4 Tahoe 5W4 will continue to use retail-calibrated stability control.

Separately, for 2011, GM has developed a City Brake option for the Tahoe. The 1LR brakes are designed to give improved pad life in city driving. The 1LR pad material wears better at lower (patrol, not pursuit) temperatures. However, the pad “is not optimal for track performance.” The 1LR rotor does not have the radial drilled holes, which are on the PPV rotor for better thermal performance under track applications. The brake performance is likely to decline with repeated high-speed stops (track performance). The benefit for those vehicles not involved in this kind of driving is longer pad life in urban scenarios.

Driving Impressions

We put more than 1,000 miles on the new police Tahoe PPV StabiliTrak and crossed a lot of medians to stop a bunch of cars. However, we decided it was better to put the StabiliTrak to its ultimate test at GM’s Milford Proving Ground rather than to find ourselves wheels up in a ditch on US 41.

A wide open cone course was set up on Milford’s famous “Black Lake,” which is a 67-acre pad of absolutely flat asphalt. The cone course allowed straight-line acceleration up to 100 mph and high-speed sweeping turns. The cone course also included a lower speed, sudden lane change; a medium-speed serpentine; and high-speed braking around both sweeping turns and tight turns.

Basically, this course tested everything important in either an urban or a rural police car: high-speed stability, aggressive braking, steering and suspension response during sudden emergency maneuvers, full throttle acceleration, and what level of aggressive driving the electronic stability control would allow.

We ran the long course three ways: default StabiliTrak, traction control off and StabiliTrak off. By the time stability control activated, we had so nearly lost control that we really wanted the help from the stability control. Because when the traction control was off it allowed wheelspin, and the 320 hp V8 was more than capable of producing wheelspin, the best overall driving performance came in the default StabiliTrak mode.

The Michigan State Police confirmed this drivability and performance: They were 2 seconds faster around the Grattan race course in the Tahoe with StabiliTrak than the Tahoe without StabiliTrak. StabiliTrak stops excessive (time-wasting) wheelspin and stops the slides that waste time in steering wheel corrections. No kidding—vehicles with stability control are faster, easier to control, and safer than vehicles without stability control.

StabiliTrak takes a lot of the guesswork out of aggressive driving. It makes a good driver out of a mediocre one. It keeps almost all drivers out of trouble under almost all situations. The Tahoe PPV with the default StabiliTrak handles so well under so many conditions, the StabiliTrak allows such aggressive driving, that no one will notice the stability control until it kicks in to save them from completely losing control of the vehicle.

The bottom line is that StabiliTrak allows aggressive driving but keeps drivers out of trouble when they drive recklessly or when they must make a sudden emergency or accident avoidance maneuver. StabiliTrak allows you to drive harder with more confidence.

You can, of course, still lose control. StabiliTrak does not set aside the laws of physics nor improve the coefficient of friction between the tire and road. But frankly, you have to drive like an idiot to lose control in any stability control vehicle. The best part about StabiliTrak in this big, heavy SUV is how quickly it corrects initial understeer.

StabiliTrak is about more than maintaining control during aggressive or evasive driving. It is also about maintaining control during normal driving on wet, snow-covered or ice-covered roads; when driving on gravel or dirt roads; and when driving off-road on mud, sand or grass. Finally, StabiliTrak maintains control when cornering on uneven or washboard road surfaces and broken pavement.

Reality Check

The police Tahoe PPV has slightly better acceleration than the Ford CVPI and the same braking distances. However, even with a lowered and heavy-duty suspension, the Tahoe PPV is no road racer. The Tahoe PPV has always run a road course in times comparable to the police Impala. The Tahoe is big and heavy and has a high roll center. There is only so much that can be done.

The Tahoe has some SUV-type body roll (lean left-right), and considerable jounce (up-down body movement) and wallow (wiggle left-right). That said, for what it is, the officer can indeed drive the Tahoe PPV aggressively. It can be bossed through a cone course or road course with confidence.

Ergonomics and Office Space

The Tahoe stands head and shoulders, literally, above all other current police package vehicles in another area—room for the driver. Year after year, the Tahoe has earned the top rating from the Michigan State Police during its 28-point Ergonomics and Communications evaluation. The MSP reviews the front seat, rear seat, instrumentation, vehicle controls, visibility and accessibility for installation of communications gear.

In 2010, the three police sedans got scores ranging from 204 points (Impala) to 217 points (Ford CVPI). The Tahoe, however, earned 234 points. In terms of the officer’s office, the Tahoe is as superior to the Ford CVPI (17 points more) as the Ford CVPI is superior to the Impala (13 points more).

Time of Transition

We are now entering the largest transition of police vehicles ever. Not even the loss of big block V8s in the late 1970s caused the kind of changes we will see in the next year. The Ford CVPI goes out of production after August 2011. That means 80 percent of the police vehicle market is up for grabs; 80 percent of the police vehicles will change.

Once fleet managers have found a suitable replacement, based on past practices, they will continue to buy that vehicle well into the future. That raises the question: What is the future of the Tahoe? Will it be available long enough to justify a transition?

The future of the Tahoe looks like the past—the time before the boom sales of the 1990s. The retail Tahoe peaked in sales in 2002 and is now selling at about one-third of the peak. However, that 1980s level of demand is plenty to keep a vehicle assembly plant busy. And Chevrolet owns 70 percent of the full-size utility market.

The demand has fallen from “want to have a full-size SUV” to “need to have a full-size SUV.” And that demand is enough to keep the Tahoe in production for the foreseeable future. A styling change is already in the works for the NextGen Tahoe.

As of the coming year, the Ford Explorer and Dodge Durango are both sedan-based, full-size crossovers. The Tahoe remains a truck-based, full-size SUV. If the need is for a full-size SUV for police work, keeping in mind that it is 2WD, the Tahoe PPV is definitely worth a closer look. We hated ­giving it back…

Published in Police Fleet Manager, Jan/Feb 2011

Rating : 10.0


Comments

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Tahoe Competitors

By CS

Neat article, but the last paragraph is just a bit off the mark- the new Durango is not in any way sedan based- it is a unibody construction, but this is basically second-generation Mercedes ML architecture, which has always been a purpose-designed off-roader. The new Explorer is indeed sedan-based, as it derives from the current Taurus which evolved from the previous Taurus/500 which itself grew out of Volvos largest platform D3 I believe

Submitted Mar 10 at 1:23 PM

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