it isn't about a specific rpm. it's about where your right foot is. get a scangauge and go from there.

 

Even though injectors may not be actively firing, the engine cranking over will still pull fuel through the system, as the fuel pump does not stop. Using my OBD dongle I can see fuel flow is higher staying in gear downhill than coasting in idle. A car can idle for days on a full tank, so the fuel being used is absolutely minimal. If you have A/C on (or defrost), the injectors shouldn't shut off anyway.

I've found I use the least amount of fuel in rolling hilly terrain by coasting in neutral, as generally the car can gain a lot more momentum downhill and therefore you'll need to use a lot less gas to get uphill, as you can coast significantly further up the hill. It's the uphill you need to worry about, the further you can go uphill without using throttle the better, and you can always get further uphill going fast in neutral.

I usually pop out of gear before cresting the hill, and there are certain stretches in my commute I can coast 2 miles or so before dipping back to the speed limit. If I left the car in gear, I'd have to throttle sections of those stretches to stay above speed limit, so even if the car was using zero fuel downhill (which it doesn't), I'm still using more fuel over that stretch than I am coasting at idle.

 

I've found the exact opposite true from both the OBC gauge and my ScanGauge II. Coasting in gear and gph registers nothing but slip into neutral to idle engine and then gauge starts to show fuel use.

Fuel pump runs to keep system pressurized at all times. How does fuel get used if not coming out the injectors?

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Thank you.

 

Wait, are you saying that leaving it in gear while coasting consumes less gas?!

 

Less gas yes, but more specifically it uses NO gas. As long as the engine rpms stay above ~1,200 it uses no gas whatsoever as injectors completely turn off within a few seconds of taking your foot off the gas. I've even downshifted to raise rpms and remained coasting using zero gas longer. But between downshifts you will consume some gas as immediately when clutch is engaged idling gas usage starts and doesn't stop until a few seconds after clutch is disengaged again.

A/C usage does not effect this either.

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I used to think that too but if that was true then starting your car uses no fuel until it goes into closed loop. Short term fuel trim is medium to high loads and Long term fuel is low to medium. It does use less fuel but not sure its zero. There is also a fuel enrichment went the AC is on added to long term fuel trims. Seeing the ST lower then the LT means the engine will run more effecient at high rpms giving better mpg.

Its better to leave it in gear because the momentum is controled by engine braking even if the rpms are higher its using less fuel.

The difference would be like running 22 to1 comapred to 14.7 at idle to 12 to 1 under full engine load.

 

Thanks for your reply! I started thinking about this a bit more...

I found an old thread on this topic: https://www.fitfreak.net/forums/eco-...f-control.html

In that thread, there was a link to this page which seemed crucial: Fuel and Emissions System Description - Idle Control System/Fuel Supply System (K20Z)

Here it is stated that "During deceleration with the throttle valve closed, current to the injectors is cut off to improve fuel economy at engine speeds over 1,000 rpm (min-1)..."

My interpretation, from reading that thread and the article, is that when the RPM's are decreasing (at any rate) the "Deceleration Fuel Cutoff (DFC)" engages and fuel consumption is near, or at, 0. The engine essentially enters into a different "mode" where vacuum (and friction) is used to reduce RPMs. I'm guessing that when the car is running at low RPMs (but greater than 1,000) the effect of the engine braking is minimized (I assume, from experience, that higher RPMs cause greater deceleration experienced from engine braking), and the slight deceleration is offset by using no gas.

In summary, to gain MPG from leaving it in gear you must be at low RPMs; higher RPMs will result in greater deceleration which will offset the gains from DFC over shifting into neutral.

The way to test this would be to see how RPM's affect fuel economy when coasting in gear. Evidence that my theory is correct would be that shifting into a high gear (with lower RPM's) would offset gains from engine braking, where higher gears would not since 0 fuel usage would be offset by deceleration. In the above thread, one member "Goobers" tried this with the expected results.

 

I found it. http://www.autoshop101.com/forms/h25.pdf I was wrong.

 

Starting circuit has no relation to running program, emissions or otherwise.Short term, long term, where do you get this stuff?
the ECU is programmed to measure load, including a/c, lights, etc, and air flow and inject the correct amount of fuel.

 

I know that the starting circuit has no relation to decelleration. ST fuel trim is at 0 when you start the motor until it goes into close loop the same as when decellerating the ST goes to 0. I was just making a comparision that I should not have. Get a ultra gauge and you will see all of this. The only way to see fuel enrichment is by the instantaneous mpg. fuel trims usually pull more fuel due to not needing all the the fuel that was added.

I am still not sure the Fit uses no fuel during decelleration considering the mpg most people get.

 

That's correct; I think a check of the programming is in order but it sounds like a good way to save fuel. Probably by lots of cars.
The Fit is not aero shaped and does not have econ gearing so its mpg is good only by weight and smaller size. There are lots of cars, like Hyundai Elantra, that get better mpg whem measured precisely on a dyno controlled program as is the stated mpg numbers.

 

It says simply that any time the throttle is released and the rpm is greater than 1000 that stopped injector operation will reduce gas consumption.
Its a lot like saying if you drop a ball it will fall..

 

My point is that IF fuel savings are from DFC and DFC is engaged "during deceleration with the throttle valve closed," you must be decelerating AND you must have your foot off the throttle.

 

Geof, Try using higher octane gas. It makes the car more efficent and makes it easier to hypermiling. This way you can short shift with better throttle response at lower rpm. http://www.crcao.com/reports/recents...l%20Report.pdf

I have a new Honda and on the last tank I added premium so with a mix of midgrade and premium I got 1.4 better mpg than regular and midgrade. I just filled up with premium again and I already see a improvement in mpg and timing advancement with leaner fuel trims. It does take time for the ecu to change and with the cold coming mpg will fall to winter mpg which is about 20 percent less on summer average. Running premium will regain some of those losses. My Fit I got 42 summer and 32/35 running premium winter depending on the cold and snow.

 

People have always told me that using higher octane gas doesn't pay off for cost savings, but I just made a spreadsheet and your numbers seem to indicate that the trade off is at least really close... the article you link suggests the greatest gains are for mid to larger bore engines (85+, ours is 73), with lower compression (10 vs. 11, ours is 10.4).

Maybe you have done this, but one way to see if you are saving $$ would be to compute the cost per gallon (or tank) for each fuel type.

For me, since I'm getting about 34 mpg on regular (and prices here are 3.97, 4.11, 4.25 for reg. mid. and prem., respectively), I would have to get over 35.2 MPG to save $$ using mid. and 36.4 for premium--so an increase of 1.2 MPG for midgrade and 2.4 for premium.

If you are getting over +1.4MPG on premium over mid than the economics work out.

Do you find greater gains on the interstate/highway over city driving or vice versa?

 

There are a bunch of reasons I use premium. Mpg is one but power and consistency are the main. I have been getting 10 percent better mpg than the highway estimates with mixed driving on average. In automatics it shifts smoother and with manual you can short shift and get into 5th faster.

I have driven the Fit in Chicago and only lost 2 mpg driving in stop and go for a hour. My Fit was a 5 speed sport and had a scan gauge at that time.

Your fuel prices are similar but premium is 24 cents more that regular. I get better mpg driving the back roads that are 45 mph then the Interstate. I have a 26 mile one way commute with 20 miles of interstate driving. I slowed down to 60 mph compared to 75.

 

I have a 2009 fit sport with a manual gear box. According to my Scan gauge II and the built in MPG meter they show my best milage coasting, throttle off, and out of gear (about 400mpg) I tried in 5th coasting, throttle off and it showed worse milage (about 130mpg). I know the Honda gauge doesn't show over 80mpg so I've been following the SGII. Why isn't my car totally cutting fuel at coast in gear? My 2011 Tacoma and 2003 civic both go to straight 9999 on the SGII mpg readout and it reflects immediately on my avg milage. Does the manual gearbox model not allow for total fuel cut off?

 

Something is wrong with your car or your SGII. My SGII goes to 9999 after just a few seconds coasting with my foot off the throttle in gear. My car is an '09 Sport with MT also. I don't think I've ever seen my SGII register a mpg over 100 more than once or twice either. It jumps too fast for display to register, anything ~85mpg or over is just a blip on its way to infinity (9999) when zero gas usage achieved. Now coasting in neutral like you state above I bet SGII could register ~400mpg. My car idles using 0.18gph, so idling gas usage but with car coasting at ~55mph would calculate to 305mpg. Higher coasting speed then higher mpg.

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The Gauges are not set up right. In gear it will cutoff fuel to something like 30/1 fuel air. If your in neutral it is just idling at 14.2 with E10 gas. The most gas is from idle to 1500 and WOT but at loads over 70 percent it run as rich as 11.5 to 1 with regular gas.