To those who have accurate monitor of their fuel economy, i.e. instantaneous readouts, which is better when driving on level or slight downhill ground in town? The top gear of your car or neutral for coasting? My Cobalt's readout would peg at 99mpg and the result seemed to be the same, but there is a winner here between the two ideas and I'm not sure what it is. Or is it just plain different on different cars?
It's different on different cars, as well as different terrain. However, as a general rule it's better to coast in neutral than to be in gear. When in gear, your engine has to turn more times along your trip; the friction and reciprocating loss of those extra revolutions are wasted energy.
If you're planning to slow/stop and you know your car's DFCO (Deceleration Fuel CutOff) behavior, you should stay in gear and take advantage of DFCO. If you plan to maintain speed, accelerate, or only slow a little bit, neutral will save fuel.
Even if the instantaneous readouts were the same for coasting in and out of gear, you have to realize the limitations of those displays. It's a snapshot, not a longer average. Since neutral has less friction, you can coast for longer than you could in gear (ignore the case of needing to stop for the purposes of this observation). That means that you can sustain that high mpg value for longer and will in the end be more efficient. Since the benefit occurs on a time scale greater than that of the instant readout, it won't reflect it. I hope that was clear. When I was first doing hypermiler research (before finding this site), I saw a couple people who were confused by in-gear and neutral coasting reading pretty much the same.
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In gear on grades where you will go to fast coasting.
In gear when you have to slow down considerably, and would have to use the brakes otherwise.
DFCO means no fuel used, neutral uses fuel. If you can coast in neutral without having to apply the brakes then coasting is your best option.
Even better is to coast with the engine off, but that may not be practical depending on your dedication to high mileage and your traffic intensity., as well as the type of car you drive (usually not good for an automatic trans).
Rural deserted roads with hills that allow you to coast at a safe speed, would be your ideal engine off coasting scenario, as long as you do not have to deal with aggravating other drivers.
Your Cobalt with electric steering and manual transmission would be an ideal car for Engine-Off Coasting. As mentioned above, such a technique raises concerns with safety, other drivers, and traffic; also it raises legal questions. It should only be done by drivers who are 100% comfortable with it, and only where legal. Is it something you'd be interested in learning about?
Probably not. I'm actually trying to sell the Cobalt right now since I recently got a Honda VX. I used the Cobalt as an example since it's the only car I have right now with a mileage indicator. I fake the average MPG into instantaneous MPG by hitting the reset button.
The VX has more possibilities with engine off as it had manual steering, but the speedo shuts down, so I'm probably not going that route.
You can usually coast farther in neutral, because there's no engine braking. The engine will actually use a small amount of gas in neutral where it doesn't in 5th, but over all I think you'd do better in neutral.