How to Get the Most Fuel Economy

Astro14

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13,104
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Virginia Beach
The cool thing about buying a cool car, a Lamborghini, or Rolls, or, even a lowly V-12 Mercedes, is that you get to drive it the way you choose. You get to experiment. To try what you like and share the results.

Personally, I think the Doc’s observations and some of the posts are technically interesting.

Sadly, not all of the posts in this thread meet that standard.

Let me be clear: thoughtful discussion is welcome on BITOG.

Personal criticism is contrary to BITOG standards and will not be tolerated.
 
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9,745
Location
Ontario, Canada
I think there is a law against applying "fuel saving" procedures to a rental car! No? 🚀
Yep, its the law you have to drive a rental car like an idiot... ;) and 3 neutral drops are required by law.
I've never actually read a rental car agreement but I think they mean stuff like shutting off the engine while coasting down hills. Not recommended in an unfamiliar car for sure.
 
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249
Location
Toronto
So the man wants the most out of a tank for his cars whether it be an Enzo or expedition, so what? He's just sharing and is experimenting. Not every person can be painted with the same brush. Hell my brothers girlfriends father has an aventador and he never drives like a nutcase. Drives the speed limit, coasts to stops, checks tire pressure etc. Very conservative driver.

It's their money their rules. I just laugh when people tell others how to live when they're not the ones paying the persons bills or taxes.
 
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2,363
Location
South Carolina
How does your engine-braking feel now? Similar or is it gone now?
Engine braking never changed. Engine braking in my truck is about the transmission programming (automatic). The computer will hold the torque convertor in lockup until about 15 mph where it releases the convertor lockup (it's quite noticeable). As long as the convertor is locked upon deceleration, engine braking is quite effective.
 
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1,981
Location
FL
All great points, OP!

Some I wouldn’t do, such as over inflation of tires. I’d rather take a hit on fuel economy than riding a bouncy Ferris wheel. Usually, I inflate to recommended PSI in passenger cars. In trucks, perhaps a bit more if tow/haul is part of the routine. A small decrease in mpg is compensated by a nicer ride :)
 
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795
Location
98245
Improving fuel economy ... My definition of a good driver is one who rarely uses the brakes. This is done by constantly looking as far ahead as you can, in some cases a mile or so. In areas where you know the pattern of red lights and traffic patterns you can improve on fuel economy by 5 or 10 MPG assuming all else is correct with your vehicle. You anticipate what will happen in front of you. You will maybe slow down a little as you approach a traffic light but rarely stop. Take turns a little faster. Coast as much as possible.
...
That right there is the single most important factor, swamping all others. It also means higher situational awareness and of your momentum. This makes you a safer driver, seeing things sooner and avoiding collisions or other hazardous conditions. It will also make your brakes last longer.

The other one is driving slower on the freeway. Fuel consumption is roughly proportional to power output, and power increases with the cube of speed. That is, twice the speed requires 8 times the power, and 10% faster requires 1.1^3 = 33% more power. Of course, at higher speed you travel further in the same time, so fuel economy relates to the square (not the cube) of speed.

For an simplified example: suppose you go twice as fast. Air resistance (drag) is proportional to the square of speed, so you have 4 times the drag. That means 4 times as much thrust (or force) is required. Over the same distance, 4 times the force is 4 times the work. But you travel that distance in half the time. Power is work / time, so this means 8 times the power. That means you burn fuel at 8 times the rate. But you did this for half the time, so you've burned 4 times as much fuel to travel the same distance in half the time.

Now 75 mph is about 15% faster than 65 mph, so you'd burn 1.15^2 = 32% more fuel traveling the same distance. Or, slowing down: 65 is 13% slower than 75, so you'd burn 0.87^2 = 76% of the fuel going the same distance. That's about 1/.76 = 31% better fuel economy.

All else equal of course, that's basic theory and real world figures will vary.
 
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515
Location
KY, USA
Everyone says that, but do you have any evidence? Other people saying it somewhere else online? Even the engineering explained guy assumed this is true without actually testing his cars. My scangauge reads what the ECU is doing in real time through the OBD port, and in my cars true fuel cut off is only at rpms way above what you use in top gear. At 60 mph rolling down a hill in top gear every car I've owned uses about 30% more gas coasting in gear than idling in neutral, plus you are using engine braking to loose speed. It seems crazy that's what they do for emissions, but that's what they do.:(
I believe SUV's and trucks have slacker emissions standards and they do cut fuel completely more often, I haven't tried the scanguage in the Outback yet, but our 2003 Tracker would completely cut fuel when coasting down to near idle speed. I don't recall if it would do that before the engine was at normal temps though.
I too use a Scan Gauge. It shows 9999 MPG on my Ford Escort from about any speed until it's down to a specific RPM around 1100 RPM if I recall correctly.
 
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9,745
Location
Ontario, Canada
I too use a Scan Gauge. It shows 9999 MPG on my Ford Escort from about any speed until it's down to a specific RPM around 1100 RPM if I recall correctly.
What year is that car? My 1995 Neon automatic wouldn't cut fuel except at high high rpms. I guess it sure depends on the vehicle.
 
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811
Location
sw ohio
That right there is the single most important factor, swamping all others. It also means higher situational awareness and of your momentum. This makes you a safer driver, seeing things sooner and avoiding collisions or other hazardous conditions. It will also make your brakes last longer.

The other one is driving slower on the freeway. Fuel consumption is roughly proportional to power output, and power increases with the cube of speed. That is, twice the speed requires 8 times the power, and 10% faster requires 1.1^3 = 33% more power. Of course, at higher speed you travel further in the same time, so fuel economy relates to the square (not the cube) of speed.

For an simplified example: suppose you go twice as fast. Air resistance (drag) is proportional to the square of speed, so you have 4 times the drag. That means 4 times as much thrust (or force) is required. Over the same distance, 4 times the force is 4 times the work. But you travel that distance in half the time. Power is work / time, so this means 8 times the power. That means you burn fuel at 8 times the rate. But you did this for half the time, so you've burned 4 times as much fuel to travel the same distance in half the time.

Now 75 mph is about 15% faster than 65 mph, so you'd burn 1.15^2 = 32% more fuel traveling the same distance. Or, slowing down: 65 is 13% slower than 75, so you'd burn 0.87^2 = 76% of the fuel going the same distance. That's about 1/.76 = 31% better fuel economy.

All else equal of course, that's basic theory and real world figures will vary.
Good analysis but need to figure in lower pumping loses with the more open throttle, too. Can be as much as 20%. Your analysis treats aerodynamics as the only consideration in fuel economy. Just one of several.
 
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1,060
Location
New York
....

A while ago I tried to drive the limit in my car; end result was the fabled 10% bump in mpg. That proven I went back, 10-20% decrease in driving time was fine by me in losing 10% in fuel economy. Considering I typically spend 2 hours a day in the car, it was measurable.

This is true for long distance driving, but for the average commuter there is almost no difference commute time no matter how fast you drive. I drive faster than the speed limit as well and regularly pass NY State Troopers doing 70-75 mph without much worry. I still am getting about 40-41 MPG in my 16' Focus with the 2.0L VCT engine in extended highway and rural route jaunts, about four over the window sticker hwy avg of 36. Other than swapping out the air filter I haven't even tuned it up yet but am unsure if the previous owner already changed the plugs or not...

I tend to think people that drive under the limit or way over it are complete morons that are not achieving anything. They may save maybe 30 mins a long distance drive, but if you get pulled over your probably losing time and possibly adding a gas stop...
 
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795
Location
98245
Good analysis but need to figure in lower pumping loses with the more open throttle, too. Can be as much as 20%. Your analysis treats aerodynamics as the only consideration in fuel economy. Just one of several.
True, which relates to my "all else equal" comment at the end. All else is never truly equal in the real world.

However, if in top gear, the engine revs are so low at 60 mph that the engine would actually run more efficiently at 70 mph, that alone isn't sufficient to get better MPG at 70 mph. The difference in efficiency would have to be so great as to overcome the higher drag. That's unlikely, as drag increases with the square of speed. What is more likely is MPG still drops at 70, but the drop is less than predicted, as it's partially offset by slightly improved efficiency.
 
Messages
811
Location
sw ohio
True, which relates to my "all else equal" comment at the end. All else is never truly equal in the real world.

However, if in top gear, the engine revs are so low at 60 mph that the engine would actually run more efficiently at 70 mph, that alone isn't sufficient to get better MPG at 70 mph. The difference in efficiency would have to be so great as to overcome the higher drag. That's unlikely, as drag increases with the square of speed. What is more likely is MPG still drops at 70, but the drop is less than predicted, as it's partially offset by slightly improved efficiency.
Conditions and safety permitting, rpm @45-50% of red line is the most efficient engine speed to run at (or torque peak).
 
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1,051
Location
Wash, DC
I tend to think people that drive under the limit or way over it are complete morons that are not achieving anything.
I aim for 5 mph over the limit and go no higher than 9 mph except in the rare occasion I'm in a cluster going faster.
My area has speed cameras set to trigger 11 mph over and I suspect LEOs around here use about the same threshold.
Haven't received a speeding ticket in the 10+ years I've followed this rule.
 
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