It's probably a safe bet that our 2008 Subaru WRX STI's trip computer has never displayed this number before. But then, our Subie had never gone 331.8 miles on a single tank, either.
Here's how it went down. There's a stretch of the I-5 between Sacramento and L.A. where gas stations don't come along very often. I was on my way back from a Thanksgiving jaunt to Oregon (more on that later), and I didn't feel like stopping when I passed a station with 270 miles on the trip gauge and an indicated 1/8 of a tank remaining. I'd been getting about 300 miles between fills, so I figured there had to be another station within the next 30 miles or so.
10 miles later the gas light came on, and there was no sign of civilization.
I did some quick math. The STI's gas light comes on when there's 2.6 gallons left, and I'd been averaging around 21 mpg. That gave me about a 50-mile range. I was pretty sure there'd be a gas station well before I hit the 330-mile mark, but I really didn't want to be wrong about that. So I went into full-on hypermiling mode -- sixth gear, A/C off, 50-55 mph, minimal throttle inputs, neutral on downgrades -- and reset the secondary trip gauge (Trip B) just for kicks.
At 308 miles on Trip A, I passed a sign that read: "NEXT GAS 24 MILES." Hypermiling suddenly seemed like a brilliant idea. When I finally pulled into the rest stop with 331.8 miles on the clock, my Trip B fuel economy was jaw-dropping. The STI had managed a Focus-like 34.8 mpg over the final 52 miles or so -- and before you go accusing the trip computer of inaccuracy, note that it was actually about 1 mpg pessimistic over the course of the entire trip.
As it turned out, the STI only drank 15.104 gallons at the pump. With a capacity of 16.9, that means there were nearly two gallons left in the tank. Something doesn't quite add up here -- if I had 2.6 gallons left at 280 miles, and I got 34.8 mpg from there on out, I should have gone through an additional 1.5 gallons, which would have left me with 1.1 gallons at the station. It seems the STI's fuel light might be coming on a little earlier than Subaru claims.
Nonetheless, who would've thought 34.8 mpg was possible in an STI? Certainly not me. I spent much of the trip cursing the Subie's prodigious thirst during ordinary (70-80 mph) highway driving.
Josh Sadlier, Associate Editor, Edmunds.com
skisupreme says:
01:29 PM, 12/ 3/08
Now I was schooled on here a few months ago that it's better on fuel economy to let the engine brake on downgrades vs. shifting into neutral. Something about negative pressure using no fuel vs. engine idle.
texases says:
01:45 PM, 12/ 3/08
Yep, no need for neutral, you probably use more gas that way. Most cars shut off fuel injection above idle speed with a closed throttle.
boxermike says:
01:51 PM, 12/ 3/08
"Now I was schooled on here a few months ago that it's better on fuel economy to let the engine brake on downgrades vs. shifting into neutral. Something about negative pressure using no fuel vs. engine idle."
Correct. I was hoping I'd be the first to pick that one out. When you're doing that you can even indulge the guilty pleasure of using the A/C as the injectors are off, the added load is on gravity, not the engine.
-mike
chavis10 says:
01:59 PM, 12/ 3/08
I wonder if that's true for a manual though. For an automatic, if you coast in gear- the PCM/ECU cuts fuel delivery and the wheels keep the engine turning.
ayao says:
02:04 PM, 12/ 3/08
I tried to do that sort of math with my 04 WRX and ended up (barely) coasting into a gas station on fumes. Wife not pleased. It was surprising because I ran out of fuel -way- before the fuel capacity and warning light would have suggested, despite instituting economy maneuvers.
The nasioc.com forums are littered with similar anecdotes and I think the consensus was that the numbers -do- lie. Also, I had read that letting levels drop below 1/4 tank can be bad for the fuel pump which (I think) uses ambient-temperature gasoline to cool.
Dunno. YMMV.
SadButTrue says:
02:24 PM, 12/ 3/08
Chavis, I was wondering the same thing. Our resident engine guru says that the transmission type doesn't matter -- so all else being equal, it's more fuel-efficient to coast in gear than in neutral.
However, all else wasn't equal in this case, because the downgrades were gradual, so keeping up my forward momentum was an issue. My strategy was to squeeze the throttle a bit at the crest of a hill and then shift into neutral for the duration of the downgrade. Coasting in this way allowed me to maintain my speed for a longer duration without throttle inputs. If I'd left it in 6th gear at 50-55 mph on the downgrade, the engine speed of 2,000 rpm or so would have produced enough engine braking to require additional throttle inputs (compared with coasting) in order to maintain my speed. So given that my goal was to maintain a constant 50-55 mph, the squeeze-n-coast method seemed preferable.
-Sadlier
ddoouugg says:
02:27 PM, 12/ 3/08
That is definitely true for mauals. I didn't know it was true for automatics though.
mustang5507 says:
08:57 PM, 12/ 3/08
Hmm...maybe that's how I squeezed 19.6 mpg from the mother's LR3 coming back from Vegas. Hills create such a love-hate relationship. Hate going up...LOVE going down. I wonder though, how much extra fuel is used coasting in neutral vs coasting in top gear, having to touch the throttle once in a while..
estreka says:
11:31 PM, 12/ 3/08
If you guys ever figure this out, let me know. I've always coasted downhill rather than ride the clutch. Speaking of which, is it bad for the clutch to coast in neutral? Someone told me it was, but I cannot imagine how.
dougtheeng says:
06:00 AM, 12/ 4/08
Having such a short range in a vehicle would drive me nuts.
actualsize says:
08:26 AM, 12/ 4/08
Coasting in gear with your foot off the clutch going downhill or when you want to slow down is the fuel saving technique here, so clutch wear isn't an issue.
Josh encountered the "tipping point" where the downhill run wasn't sufficiently steep to maintain speed in high gear, but it was steep enough in neutral. In this case coasting in neutral may save fuel.
Coasting in gear with the clutch pushed in doesn't wear the clutch friction material directly, but it adds stress to the clutch release bearing and the cover plate spring fingers, which are loaded whenever you have the pedal down. And every time you push the pedal in you have to let it out again, which creates another moment when the clutch material and flywheel come back together, an event that does produce wear, especially if you don't match revs. If you do rev match, those throttle blips consume fuel.
Does it add up to a new clutch every four years instead of every five? I can't say. But if you always coast up to signals holding the clutch in, you just might do your release bearing, your mpg and your brakes a favor if you leave it in gear with your foot off the clutch whenever practical.
greenpony says:
11:47 AM, 12/ 4/08
I've actually taken a semi-scientific approach to this. A few months back I alternated driving style with each tank of gas. One or two tanks with neutral-coast, then one or two tanks with in-gear-coast. My hypothesis was that DFCO would kick in under in-gear coasting and save a few mpg relative to neutral coasting.
The car I used for this test is my daily driver, a '07 Focus 5MT. My one-way commute is about 45 miles of gradual hills in northern Illinois/southern Wisconsin.
I didn't use cruise control for this study, since I felt it would a pain in the ass to have to re-engage my cruise after every neutral coast. No a/c either. Tire pressures all set to nominal +3/-1. My results showed no significant gain or loss in fuel economy between coasting in-gear and coasting in neutral. I could get up to about 40 mpg and down to about 34 mpg with either technique.
I recently got a Scangauge II and it shows that I actually get BETTER mpg in neutral coast than in-gear coasting. Still haven't been able to verify it to any level of confidence with actual measurements at fill-up.
actualsize says:
02:34 PM, 12/ 4/08
My tests have taken place in the more mountainous regions of California and Nevada. Long stretches of downhill exist. We can desecend from 10,000 feet to sea level in a single tank of gas. Heck, there are places where you can drop 6,000 feet in 20 miles. In these situations the benefits are tangible.
But over an entire tank they probably amount to no more than 2 mpg, especially for a car that's frugal to begin with, like your Focus. There is only so much downhill out there.
I've found that the differences show up most easily (and are the most reliable) in the ScanGauge's instantaneous readout while you're in the moment, switching between technique "A" and "B".
joefrompa says:
07:09 AM, 12/ 5/08
Here's the deal:
When you lift off the gas pedal in gear (manual transmission) the engine actually shuts off the injectors (no fuel being used) and the force of the mechanical connection between wheels/engines keeps the engine running. There is no need for fuel use during this time, since the engine is turning as it's connected to the transmission/drivetrain/wheels, which are spinning with momentum.
The downside is that by keeping the engine connected to the wheels, you are adding mechanical drag on that momentum....thereby reducing the distance the car will travel without using more fuel.
When you put the car in neutral, the engine automatically injects fuel necessary to keep the engine spinning at idle RPM speed. The wheels are no longer connected to the engine.
There's no extra mechanical drag using this method, but the engine is using unloaded idle-speed fuel while coasting in neutral.
I only found this out in the last 6 months. I did an experiment in my 06 SI.
Normal driving, shifting at 4500-5000 rpms in gear 1-4, aggressive starts, driving at 75mph, and using lots of engine braking when going down hills or coming to stoplights rather than brakes or neutral. 28-30mpg consistently in the same type of driving.
Or, driving never above 3000 rpms, maintaining a highway speed that would keep the engine under 3000 rpms (about 68mph), and all shifts occuring below 3000 rpms. I used neutral more often to "coast". I got 29-30mpg consistently.
In short, I found that in my car I should enjoy life and I'll still get great fuel mileage :)
Joe
P.s. Now that gas is down to $2 a gallon, who the hell cares? I'm rich again! :)