OK, so the fuel economy number above isn't impressive. But don't get too excited: it's all in the name of science. Watch for an upcoming feature article to explain the circumstances of the above perfromance.
But it is accurate. In the course of several long-distance fuel economy trials with our 2007 Chevrolet Tahoe, we took to resetting this display at every fill-up. To the tenth, the figure above matched what we calculated by dividing the trip odometer by the gallons added, every time.
The "miles to empty" and "gallons used" displays, which we also reset at each fill-up, gave similarly accurate results. The needle may have cried wolf as it sat buried below "E", but we developed supreme confidence in the digital displays. If you reset each at the filling station, there'll be no excuse for running out of gas in this one. Kudos.
On the other hand, cylinder deactivation has a dark side: lame engine braking. The same mechanism that shuts-off 4 cylinders and closes valves to reduce pumping losses also removes the engine braking effect that said pumping losses provide.
So I find myself using the brakes in this Tahoe in situations where I normally would not have to. In traffic, a throttle lift by itself is frequently insufficient to bleed off speed to manage the gap. The Tahoe rolls on with seemingly undiminished speed. On cruise control going down a grade, even a slight one, speed picks up and exceeds the set point, requiring me to downshift or apply the brakes, which of course kicks-off the cruise control. For an extra mpg or so, I suppose its a livable trade-off. But reduced engine braking is there, and it is noticeable.
Dan Edmunds, Director of Vehicle Testing @ 12,643 miles
billt9 says:
10:44 AM, 05/10/07
You want engine braking?
brake pads are cheap, and instantaneous.
Engine braking is continuous sapping of gas for as long as the engine is running.
With brake pad braking, you'd still end up replacing the brake pads only once every 60,000-100,000+ miles.
In traffic,
Keep a longer followiong distance, as taught to you in driver's education on safety.
Coasting is what cars are suppose to do. The longer a car can coast, the better.
So get off the accelerator earlier, and know your vehicle is capable of coasting like it's suppose to.
Cruise control on an open road:
Depends on the cruise control software.
In Toyota's software, cruise control speed is maintained, upper boundry and lower boundry. Toyota cruise controls auto-brake the car.
In some other software setups, only the minimum speed is maintained.
Sounds like the Tahoe is the latter.
My experience in 2000 Honda Civic is also the latter.
Just a software preference, I wouldn't blame it on lack of engine braking.
You are annoyed by the cruise control software, and software is to blame.
tackepj says:
11:09 AM, 05/10/07
It's interesting how coasting changes in the quest for greater fuel economy. We have exactly the opposite scenario: a coast that is more labored than usual; much like a manual transmission. This is due to our car's regenerative braking system (a Saturn VUE Green Line). Coasting, foot completely off the accelerator, will charge the battery and increase coasting effort. By just resting your foot lightly on the accelerator, you eliminate regenerative braking and coast as normal.
You learn to anticipate upcoming traffic conditions and adjust your foot accordingly. The labored "charge" coast will probably lead to longer brake lining life since it will slow the car down without the brakes. It all works pretty seamlessly, but requires putting some attention into your car and how you drive it (what a concept!).
crashtestdingo says:
10:29 AM, 05/12/07
I have a 1990 Olds Toronado Trofeo and its "driver information center" offers readouts for fuel consumption and economy, too. I suppose it gets the fuel consumption figure from the ECU. But it doesn't reliably match the number on the gas station pump. I suspect that whatever pump I happen to fill up from stops pumping at a different gas tank volume level every time I fill up. I wonder what this Tahoe has that lets you "achieve" a consistent total amount of gas every time you fill up its tank.
actualsize says:
03:46 PM, 05/14/07
Since 1990, I think the DIC calculation method has improved.
And when we fill-up, we use a standardized procedure:
Use the slowest auto-fill setting on the nozzle.
Allow it to click / auto shut-off once.
Wait 10 seconds
"Burp" the nozzle's bellows (if equipped).
Continue filling, manually, for one more click.
And whenever possible, we stop at the same station and use the same pump.
jkavanagh says:
09:50 AM, 05/16/07
billt9, engine braking consumes no fuel. The engines in EFI-equipped vehicles will shut down the injectors completely when it sees a somewhat-rapid throttle lift.
You can feel it for yourself. Particularly in cars with a manual transmission when in lower gears. Try it-- get up to say 20mph in first.
Lift somewhat quickly. Feel the engine braking effect as you coast. When the engine speed drops to around idle, you will feel a slight 'bump' as the injectors come back on line. Engine braking is now over.
Now do it again, but lift off the throttle veeerrrry slowly. You will get a lot less engine braking because the injectors remain on the whole time.
Automakers calibrate engines this way intentionally for driveability--so when in slow-moving traffic you don't get overzealous engine braking.
I use engine braking whenever possible, expressly to avoid tapping my brake pedal and starting a chain reaction of unnecessary braking on the already-stupid SoCal freeways.
jerrywimer says:
05:28 PM, 05/18/07
I took a picture of my current readout with around 250 miles on this tank of fuel. I reset the trip, gallons used, and AVG MPG displays during each visit to fill er up- and I calculate the actual mileage manually and record that value in a log book that I keep in it. It turns out that the DIC is much more accurate in this new 07 than it was in any previous vehicle I've owned.