Hey everyone. Quick question:

With these high gas prices, I'm trying to grab a few extra MPGs out of my EJ6 by driving better. I've read all over that coasting in gear, our Civics use no fuel; the injectors cut off. This simply doesn't make sense to me. If I'm sitting on flat ground and slowly let out the clutch in first, I can begin to move without any throttle. I can shift to second and get up to 10-15 mph. How would this be possible if no fuel was being delivered once the clutch is all the way out? I'm just trying to clear up the facts to get the most mpgs out of my driving.

 

I've read this 3 times and don't understand WHAT you are asking.

 

When your in gear and you let off the gas your vacuum goes way up, so the engine is being forced to turn by the momentum of the car. In that case the injectors turn off. At idle the engine has load, its keeping itself going and also as you let off the clutch the idle air control valve will open slightly to try and maintain idle.

 

yeah, i've been doing this in my EF. it helps a bit but you won't notice HUGE gains in your fuel mileage. it's a useful tool to use when you're going down hill or approaching red lights. another thing i've been doing is slowing down before i reach a red light so that the car is still rolling when it turns green. it takes a lot of energy to move your car from a stand still.

 

Yeah, I've been doing that for a while too. It's hard to judge the improvements because when I used to live in N.C. I got much better mileage as compared to Miami traffic and I still drive up there/have mixed results, etc. I'm also running wider tires than I used to so I'm just trying to compensate a bit.

I understand the momentum thing turning the engine and all, but it still doesn't really make sense that the car can go forever in first on flat ground with no throttle (which it can) if it isn't getting any fuel, it seems like it must be delivering enough to maintain idle speed. It's just something I've been curious about lately.

 

Many people on here have said that if you're driving and you take your foot off the throttle completely while in gear (with the engine still running) and coast the injectors deliver no fuel. For example, when you're approaching a red light, you might just let off the gas way in advance and just coast until the light turns green. I don't think this makes sense/is possible, and was just looking for some explanation.

 

best way to see if you are improving is accurate measurement of your fuel usage. you probably know this already, but the proper technique is to:

1. fill tank to the top, set trip meter to zero.
2. drive till you need to fill up
3. record exact miles travelled and exact amount of fuel used to fill tank again
4. calculate your mpg

for my own purposes i try to fill up at the same gas station when possible because the pumps are all calibrated differently. i've noticed over the last half year or so that my mileage is fairly consistent and hovers around the 7.00L/100km range (~35mpg) for the EF.

i also check my tire pressure every time i fill up and keep the tires at 32psi. before, i kept them at 35psi and DID get better mileage due to the reduction in rolling resistance. however, i was also using 13" and 14" steelies back then; i'm rolling on 15" rims/steelies now.

 

The factory ecu does a good job with this, when you go aftermarket It gets a bit weird if its not 100% tuned correctly. I work on a military base and drive about 5 miles at 25mph everyday. It does this weird jerky thing when the injector go off to on under light throttle. I have had this happen with a few cars I have had tuned. The last one I had it turned off, with the stock ecu I was getting 27 mpg after the tune I would avg 30mpg, as long as I stayed out of the throttle.

 

Yes, this is true, and if you learn how to use it well, you can actually make significant improvements in mileage. I managed to get 42mpg out of a tank on a 2000EX with the stock d16y8.

There's also an RPM threshold involved here... below a certain RPM, the ECU keeps the injectors in to keep the engine turning. Above a certain RPM, while coasting in gear, the injectors are 100% unnecessary to keep the engine turning because the wheels and the transmission are turning over the engine. So, for example, when you coast down from 60mph, you use no fuel until you drop below about 2000 rpm.

I've verified this a few different ways. I have a ScangaugeII mounted in my DIN pocket that shows MPG. When I coast in gear at highway speeds, it shows 9999mpg. If I attempt to coast down to below a few thousand rpm without downshifting, it will jump down to like 40mpg when the injectors cut back in, but if I downshift to 3rd then, it will jump back up to 9999mpg.

I've also verified that the engine is not firing by coasting a REALLY long time and watching the coolant temp drop accordingly. Here's what happened:

I drove up to the top of a mountain on the Tennesee/NC border. At the top of the mountain, my engine was at full operating temp... about 205* (this is on my B18C1). Then, I headed back down the other side.

Now, driving down a mountain, the best thing you can do is coast the entire way and control your speed by shifting. If you ride the brakes the entire way, you'll overheat them. So, I'm coasting down this mountain in 4th and 3rd. About 5-10 minutes in, I realize that the heat is blowing cold. I look down at my coolant temp (on the Scangauge), and it's showing about 110* and still dropping... actually got down to about 95* by the bottom of the mountain, and the engine was running the entire way.

That won't happen at idle. At idle, the engine is generating heat because it's using fuel and firing. When coasting however, it's not.

My car cooled from 205* to 110* in 10 minutes by coasting down a long hill. The injectors weren't firing, so the engine wouldn't maintain its heat, but was still being cooled by the ambient air. I drove down the entire mountain with the engine technically running, but using no fuel.

Before that experience, I knew that's how it worked, but actually seeing it like that made me a believer.

As far as improving your mileage in everyday driving - coast to a stop whenever possible. Learn how to pulse and glide. You should get at least an extra 2-3 mpg over the course of a tank if you get really good at it.

 

@ spoon.ek9: Thanks, I know how to calculate my mileage and everything, it just isn't very consistent because I drive a ton of really short trips and the traffic here is pretty rough, but it's usually bad (high 20's), I'm just sort of waiting for a full tank when I'm up in N.C. again to compare with different driving habits, etc.

A Vindicator, thanks, that was a great explanation as usual! People have said on here many times that they just shut off when coasting in gear, however once you said it accounted engine speed it makes perfect sense. That's pretty cool that you have evidence to prove it too.

 

I'm also doing the whole coast to a red light in Neutral bit in my 98 Sedan
It's an auto though, so I'm not sure if I were to do the same thing down a mountain like you did would net me the same results since I can't really manually control which gear I'm in
(Yeah it has D4, D3, and 2, but come on-not as easily controlled as a manual)

Something else I've figured out thru trail and error involves the A/C system:

I've seen reports and even an episode of Mythbusters that claim that driving with your A/C on at above a certain speed is ALWAYS better than with the windows down

My only gripe about the Mythbusters episode:
They were using a big V8 SUV
That big motor will hardly feel the pull of the A/C clutch engaging, so of course it'll be better than driving with the windows down (which makes the already un-aerodynamic brick that much more un-aerodynamic)
XD

I say do this same test with a smaller displacement motor like ours and you'll likely see different results

Now, I'm not saying to drive with the windows open
On the contrary, I have a totally different solution:

To save the MOST MPG in regards to the A/C system, at any speed, drive with the windows up, the A/C off, and the VENTS open, with the fan blowing and the slider (or **** for you newer Hondas ^^ ) at the coldest setting

The fan pulling in cooler air, coupled with your vehicle's speed will be plenty cool enough AND save you on gas mileage because of no "open window drag" and no A/C compressor load on the motor.
(just make sure you have a cabin air filter unless you enjoy exhaust fumes and ingesting leaves)
XD

And no, I'm not gonna yank out my A/C system for the best savings:
It's gets freakin hot here in Texas in the summer
XD

 

Coasting to a redlight in neutral is a bit of a different proposition.

When coasting in neutral, the engine is running at idle speed, and using fuel to keep itself running at 750 rpm. Coasting in gear however, the engine is using no fuel at all to keep itself running. The injectors completely shut off. The engine is kept turning over by the motion of the wheels, and the injectors start back up again when throttle is applied.

There are still occasional fuelsaving advantages to coasting in neutral. When coasting in neutral there's far less drag on the drivetrain because there's no engine brake effect. If you coast 2 cars down a hill, one in neutral and one in gear, the one in neutral will go faster.

So, sometimes coasting in neutral can be more fuel efficient... like if you want to accelerate down a hill to get back up the other side. If you coast in gear, the engine braking will maintain your speed going down the hill. If you coast in neutral though, there is no engine brake, and you'll gain speed going down the hill.

For anyone really serious about trying to improve mileage, the best thing you can do is get a computer that shows realtime mileage as calculated from the speed sensors and injectors. They help you to really see how different behaviors affect your short-term mileage. From there, you can experiment and adjust those behaviors to see your mileage per tank averages go through the roof.

 

I hypothesize that coasting in neutral can be really beneficial in hilly areas, as you can often gain enough momentum going down a hill to then go over the crest of the next hill without using any throttle. I don't really have a great way to test the merits of this versus cruising down the hill in gear and then using a little bit of gas to get back up the next hill. I mostly try to drive in such a way where I can give the car as little gas as possible. I'll be moving out of Miami this summer, so I'm probably going to try to take gas saving driving and modification to the next level, mostly just for fun. Maybe then I'll get a computer to monitor fuel use and try to figure out the best combinations for optimal fuel economy.

 

I usually just let out the clutch when I'm going down hills or when I notice that I have somehow gotten considerably above the speed limit. Basically, anytime I don't need the gas, I let the clutch out. The engine drops down to idle, though I hope I'm not ruining my clutch. It was new when I bought my car.

 

it's kind of funny.. some of the best numbers i've recorded were in spring/summer weather with the driver's side wide open the entire tank. it's simply too hot to drive my EF in hot weather with the windows rolled up. even with the temp setting at the lowest, it blows warm air into the cabin making it literally like a sauna! i have the same problem in my EK.

 

I fill up the tires a little more and I get slightly higher mpg. Basically less contact and a stiffer tire seems to raise it up some