What's the Best Driving Method for Improved Fuel Economy?

You took the words right out of my mouth....be smooth and anticipate.

I am driving a 1994 Camry 3litre (Only 360000km - barley run in) and found that by driving calmly I am getting the best consumption.

I am using cruise control and mostly drive within the speed limit (the last ticket I had was in 1965) 120 in SA or depending on the road down to 100 or 80. Now and then I do 160 and once in a blue moon 220. What I like about a big engine is that I can accelerate fast when I need to or want to to leave a 1600 far behind.

I use a Tazz for city travel. My repair bill on the 1995 Tazz is much higher than the Camry. (my wife is not that calm a driver)

my average on the Camry is 8l / 100 km which I pay with a smile for that comfort.

Another complication with an automatic transmission is the torque converter (TC). If you have a tachometer, look at it when you press the gas pedal. The engine speeds up immediately, producing high friction loss in the TC due to slippage. Later, the car speeds up but still runs slightly slower than the gear ratio would suggest due to slippage. Most transmission have a lockable TC which eliminates this at higher speeds.(My old Dakota had an overdrive which locked up the TC above 50 MPH at light/moderate throttle.)

The best milage with an automatic comes from accelerating "briskly" through first and second gear, minimizing the total slippage.

Using Public Transport system gives you best (national) fuel economy

Or a bicycle - even better.

Can't remember the last time I had to put gasoline in the bike!

You need to find out the RPMs for max torque and max BHP. Always keep the engine between those two, preferably in the area nearer to max torque, wich is generally the lower of the two. You will find that info usually in the car manual.

Get info the higher gears fairly quickly, low gears waste fuel. Discover the lowest RPM where the engine is still smooth in acceleration for each gear, not lumpy!

Anticipate slowing down, use the brakes as little as possible, do not drive close to other vehicles, never closer than 4 seconds, more is better.

Keep within the legal limits, or better on them for single lane roads, as it reduces the numbers of overtaking cars.....less danger.

After a while, some weeks, you will get a "feel" for the speed in each gear, especially top, that is the best for economy. It will be around the same RPM each time. If having to drive slowly, you will find a lower gear will bring more economy at that RPM than a higher gear working the engine too slow.

If towing something heavy, it is possible that top gear will be "too high", take the next lower.....

Think ahead, think smooth. Do nothing abruptly, except where needed for safety's sake of course!!

I have an example that contradicts your class. However I don't have physical data to back this up, you might find good information in boating magazines and online boat and engine reviews.

I've spent much time on non-commercial Mercruiser I/O powered boats. I've always found the 220HP 5.0L to use significantly more gas than the 310HP 7.4L when used in the same application traveling at the same speed. I'd imagine it's related to the energy lost due to the engines reciprocating action. The 5.0L needs to turn much faster to generate a similar power output to the 7.4L. I'm a big fan of the now obsolete 7.4L simply because it can push a 3 ton vessel at cruising speed with a minimum of around 5.5MPG. That sounds low, but in the world of boating it's quite excellent.

Also, boats only use 1 forward speed, there are no selectable gears in the transmission.

In your point of accelerating quicker, you'd be pushing the engine to higher RPM's then if you chose to accelerate slower. With a manual transmission you can select the gears yourself, and I find even with the petal on the floor I can maintain good MPG according to the dash gauge by shifting short around 3500-4000RPM opposed to the redline at 6200RPM.

I also run full size diesel trucks. Some people use gear splitters on the highway (I don't) and claim to get better MPG's by slowing their engines down by creating a taller top gear.

You are trying to compare apples and oranges.

Under test condition, IC engines have a "sweet spot" near the maximum HP and max torque points where the engine will achieve the best BSFC. This is a steady-state condition, and if the application is steady-state this is the area you want to operate at.

Real world driving is anything but steady-state. the torque and HP demands vary as the vehicle goes uphill, downhill, around curves, stopping and starting, etc.

The compromise to be made when selecting a vehicle engine is how much power is required for normal driving conditions, and how much excess capacity is needed for acceleration, uphill, towing, etc. If you decide you do not need to race, climb mountains, or haul big loads, a smaller engine loaded closer to the best BSFC will get better economy than a large engine operated further from its BSFC sweet spot.

As previously posted place an imaginary egg between your "lead foot" and the accelerator and do not "break the egg".

I seem to remember some research that SAAB did a number of years ago where they received maximum fuel economy by accelerating at full throttle, but only used 1st, 3rd, and 5th gears to get to the cruising speed. Of course, other smart things like anticipating stops, tire pressures, prudent maintenance, removing unnecessary clutter (junk in the trunk) all help too.

This makes some sense if you consider that accelerating the mass of the vehicle to a given speed takes a defined quantity of energy regardless of how long it takes for you to get to the speed ( 1/2*m*v^2). Therefore if you operate the engine at its most efficient operating point to get to speed, you could save some infinitesimally small amount of fuel and then cruise by operating the engine/drivetrain/aerodynamic profile system at it's most efficient mileage point.

As some have pointed out, maximum engine efficiency, defined by the energy output (horsepower*time or kW*time) of the engine divided by the energy input (gasoline in Btu or kcal) is not necessarily synonymous with mileage efficiency defined in miles per gallon or liters per 100k.

Telling the highway patrol that you were just trying to drive your engine at its most efficient operating point probably won't help you in court.

I drive manual transmission from 1st to 3rd to 5th when casually driving. Stretches fuel quite a bit. I use 2nd and 4th for fun!!

As mu transmission bearings have continued to wear, I have had to stop using 4th gear also. I have experienced a great savings by not replacing my transmission.

Why don't you look into replacing your tires with a "taller" tire. The additional roll out will drop your engine speed at the same road speed. There may be some errors on the indicated road speeds, But there are remedies for that as well as the anti lock brake system if they develop. A web site devoted to your vehicle would know what is required if anything. Good luck.

Both are correct.

An engine will operate most efficiently with a carburetor, or fuel injection, when the throttle opening is at wide open position, limiting unwanted air turbulence.

But there are problems with that. Your car needs drivability. For that reason, there are helpers added to it. Ignition timing is load sensitive. Fuel mixture is load sensitive. And now valve timing and even the number of cylinders operating are load sensitive.

Go back in time with me to the days of carburetors. You are driving at a steady 50 mph. The throttle has not moved for the last 3 minutes. Your car is now at it's most fuel efficient setting. As you start to climb a hill, the load on the engine increases because you are in effect lifting the weight of your car. This causes the manifold vacuum to become less. Your carburetor has a device designed to compensate for this. That is the power valve. It is a fuel enrichment system that is held in the lean position by manifold vacuum. When your manifold vacuum became lass, the power valve allowed your mixture to become richer to make more power to compensate for the hill.

There is a thing called vacuum advance that causes your car to have more power as you open the throttle by advancing the ignition timing. This is accomplished by using the vacuum again. If the vacuum lowers the vacuum advance stops advancing the ignition, causing a loss of power.

With current computer controls, these things are now regulated by the computer, but they are still happening. Only now there are also valve timing, and possibly displacement changes also happening. Industrial engines that operate under a narrower set of operating conditions, commonly use smaller carburetors that are required to open closer to wide open to achieve better efficiency.

In short, your book is theory. Your car is reality.