Fuel economy

A friend and I were debating whether driving downhill with the foot off of the gas pedal in say 2nd gear (steep, slow-speed dirt road) with engine running at 2200 rpm would use more gas than coasting in neutral with engine running at 900-1000 rpm. Is the gas consumption dictated by engine rpm or throttle opening (fuel injected vehicle)?

First of all, DO NOT coast down a steep road (especially a dirt road) in neutral.
This will not save gas and it is dangerous. Additionally, it is illegal in most jurisdictions.

On modern fuel-injected engines, when your foot is off of the gas pedal the engine is using the bare minimum amount of fuel. Coasting in neutral will not save gas, and operating the car in this manner might get you into an accident.

I support VDC’s warnings.

Gas consumption isn;t that simple anymore. The computer takes signals that measure engine RPM, the engine temp, the throttle position, the manifold absolute pressure, the amount of air flowing into the engine (mass airflow), and the percentage of oxygen in the air out the enhaust. It runs all those numbers into a computer program and THAT controls the amount of fuel being fed the engine. I call them “engine demand” inputs.

If the engine does not need much fuel, as when coasting, the computer will reduce the amount of fuel (the injector pulsewidth, the amount of time the injectors are kept open) to almost nothing.

Short version: whether you’re in neutral (not recommended) or 2nd gear, the fuel used will be minimal. You’ll need to find something else to worry about than this.

I appreciate you’re concern for my safety ( I would NEVER coast down this hill in neutral!). I was just trying to establish ( for sake of friendly wager) wether the vehicle would use more gas going down hill using engine compression (ie. higher rpm than idle).

Do you think the difference would be of any pratical value? how much difference does there have to be to be a winner 1cc?

When descending a hill, fuel consumption is determined by throttle opening, not engine RPM, regardless of which gear you choose. Fuel injected vehicles will deliver only enough fuel to maintain idle, regardless of engine RPM.

Vehicles only a few years old (most, anyway) will shut off the fuel injectors under such conditions, so you actually use ZERO gas if your foot is off the throttle and the vehicle is in gear. Under these circumstances, it would use more fuel to coast in neutral, because the computer would have to inject enough fuel to maintain idle.

As others have pointed out, coasting in neutral is dangerous, and does not save fuel. Leave it in gear and don’t worry about it. The amount of fuel saved, or wasted, under such conditions is miniscule.

If the hill’s suficiently steep that you can go w/o throttle in 2nd gear, then you’d be using a fair amount of brake in neutral. I’d expect the increased maintenance costs of brake jobs would eat up any minimal savings on fuel.

I’d be a different story if the question was a gradual downgrade where legal speed could be just barely maintained in neutral. Then you’d definitely be using gas in gear that you wouldn’t in neutral.

Assuming the car doesn’t shut down the engine in such situations, gas consumed would be proportional to: displacement X rpm X density of air to engine (approximated by local air pressure–corrected for temp–less manifold vacuum). At idle, fuel use would be low due to low absolute pressure of air in manifold.

Remember that an idling engine in neutral uses a small amount of fuel, and that shutting down the engine to get 0 fuel use will result in the loss of power steering and brakes.

We have two late model cars with gas mileage computers. One has a manual transmission and the other has an automatic. I have done enough with these to know that yes, your car will use less gas idling in neutral while coasting down a hill as compared to leaving it in gear. The difference in fuel usage, although there, should be minimal.

The meter on one maxes out at 70 mpg and the other at 99 mpg. The rate of change to maximum is faster when coasting out of gear. Also, if a downhill slope is moderate, the max may not be reached in gear but will do so while out of gear.

I’ve done this myself, coming down the moutain on PA22, headed east towards I-99. (Anyone who’s driven Pittsburgh --> State College will know what I mean.)

Athough not illegal in PA, it’s generally not a good idea: not so much for braking in a passenger vehicle, but more if an emergency situation would require acceleration…might take too long to engage transmission, then step on gas.

All the fuel economy nuts will say that you get better gas mileage by coasting. Thay do it a lot, so they would know. You can decide which risks to take. Some of their methods are labor-intensive and distracting. Coasting isn’t the worst one unless it’s done in a city.

Wouldn’t have thought that having a brake light out would “take out” the cruise control

Only those who don’t know about modern engines that totally cut fuel delivery when the engine is running faster than idle RPM. In my 2002 car at 950 or above RPM with my foot off the accelerator I use zero fuel. I also save brakes.

Most all modern cars completely shut off the fuel if your foot is off the gas and the engine speed is more than 1200 rpm or so. I believe the throttle or the idle air bypass also completely closes while in deceleration fuel cut off mode so your engine isn’t pumping a lot of unburned air through the catylitic converter which helps the catylitic converter stay hot. That is my guess anyway, I’m not 100% certain but on my car when coasting down in 5th gear, there is a sudden decrease in engine braking at about 17 mph ac off and 25 mph ac on. I believe that is the speed where the ECU starts to deliver fuel.

Anyway, if you are going down a hill that is steep enough to need braking to keep the speed safe, leave it in gear so the engine does the braking while using zero fuel.

If it is a really mild grade where gravity only just maintains speed, then you will use less fuel with the engine idling in neutral or with the clutch pushed in. If you have it in gear, you will need to give it just enough throttle for the engine to match the car’s speed, a very fast idle. If you are worried about suddenly needing power, coast in 4th or 3rd gear with the clutch disengaged, you can let that clutch out and accelerate much faster than someone not coasting in fifth gear can downshift to 4th or 3rd in order to maximize the available acceleration.