Cal Club Autocross Forums

Let's see what kind of discussion we can generate from this topic - fuel economy (for the daily driver) :mrt:

I've seen some folks say pull away gently and others say full throttle to get up to speed asap. Can anybody provide data on what is the most economical way to drive a regular gas engine?

This is NOT a discussion about hybrids, just how to get the most mileage from driving style.

Seriously??? There are people the argue full throttle?

How stupid are these people?

Stephen Yeoh wrote:Let's see what kind of discussion we can generate from this topic - fuel economy (for the daily driver) :mrt:

I've seen some folks say pull away gently and others say full throttle to get up to speed asap. Can anybody provide data on what is the most economical way to drive a regular gas engine?

This is NOT a discussion about hybrids, just how to get the most mileage from driving style.

For best gas mileage, accelerate gently and coast when possible. This is a no brainer.

Steve Ekstrand wrote:Seriously??? There are people the argue full throttle?

How stupid are these people?

Actually, I just read recently that accelerating at wide open throttle, then cruising in high gear is more economical than trying to accelerate gradually. Obviously, this assumes that you won't be stopping and starting constantly, and that you'll actually have room to cruise once you reach the speed limit. I'll try and remember where I read it and post a link.

I've heard both methods, now I'm looking for data (besides the fun debating all of this)

Kurt Rahn wrote: Obviously, this assumes that you won't be stopping and starting constantly, and that you'll actually have room to cruise once you reach the speed limit.

Why? As long as the distance between starts and stops equals or exceeds the distance to reach cruising speed in a "utilitarian" manner, then the best way is the best way.

I've read maximum fuel efficiency is achieved by maintaining almost full throttle up to a very low engine speed, generally around 2,000 rpm.

In my M coupe I can exceed 21 mpg in the city doing this versus 17 mpg driving "normally" and shifting around 3500 rpm (or 15mpg shifting around 4500.) It just isn't any fun. In the truck, driving with nearly full throttle and letting it shift at 2100-2200 rpm (it's a small V8), I get at least three or four more mpg versus accelerating very slowly with revs below 2000.

Mike

Pffffffft! So there!

I have read the same thing in many of the mags that accelerating at almost full throttle up to desired speed and then cruising is the most economical.

My car is rated at 16/26 but I am getting 11

Maybe I am missing the short shift part...

Clearly, the middle ground is the place to be, but I found some benefit in skipping gears and shifting at low rpm. For example, I got 30.5 mpg by pressing the gas about halfway, shifting at 3000 rpm, and using the shift sequence one-three-six. The 0-to-45-mph time was in the reasonable 10-second range. By changing just the shift sequence to one-two-three-six, the fuel economy fell about 1.5 percent, to 30.1 mpg. In all, I experimented with a dozen different sequences, and we've posted the results at http://www.CARandDRIVER.com" onclick="window.open(this.href);return false;.

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My V6 4Runner disagrees with the full throttle argument.

3/4 to full throttle acceleration up to cruise speed, them maintaining a light foot will return the best gas mileage.
A short (low RPM) shift to the next gear will also improve MPG.

The rational is that the engine is pulling a vacuum against a mostly closed throttle plate at low openings (pumping loss).
Engine power is absorbed (primarily) internally by these pumping losses and friction.
Friction generally increases with RPM, thus the short shift requirement.

One of the reasons that diesels get better MPG is that they operate without a throttle plate restriction.
They also operate at lower RPM reducing friction. Of course diesel has a higher energy content than gasoline as well all of which helps MPG.

People are really getting confused these days because many vehicles have an instaneous MPG read out. If they watch while accelerating they think they should light foot it. What this doesn't take into consideration is the amount of time spent accelerating, which is obviously less with greater throttle openings.

This driving style also minimizes jerkiness from lean carburetors (what are they?) and jerky clutches / drive trains.

There is much more to it than this oversimplified explanation, but you get the idea.

When I want to get good gas mileage, I light foot it and shift early. As soon as I see there's nowhere to go up ahead, I shift to neutral and coast. Cars will race by me while I'm coasting to the traffic, but with proper timing, I'm usually passing them where they have had to stop. I just put it back into third or forth since I'm already rolling. I'm sure it pisses them off to see me getting ahead of them again. Unless it's stop and go traffic, which I wholeheartedly advised avoiding, I always use cruise control, not only to control speed, but also to accelerate and decelerate. By driving at a safe distance and looking ahead, I rarely have to brake and the cruise control insures smooth throttle inputs.

BMW did a test back in the early 70s and found that full-throttle and short shifting produced better mileage. Of course, that was mechanical fuel injection ...

Bob Beamesderfer wrote:BMW did a test back in the early 70s and found that full-throttle and short shifting produced better mileage. Of course, that was mechanical fuel injection ...

Based on what I've read here, I'm going to try the full throttle/short shift method and see what gives. Certainly more fun than starting slow.

George Schilling wrote:

Bob Beamesderfer wrote:BMW did a test back in the early 70s and found that full-throttle and short shifting produced better mileage. Of course, that was mechanical fuel injection ...

Based on what I've read here, I'm going to try the full throttle/short shift method and see what gives. Certainly more fun than starting slow.

It's a little harder with an automatic, but i've been known to practice it.

George Schilling wrote:Based on what I've read here, I'm going to try the full throttle/short shift method and see what gives. Certainly more fun than starting slow.

I was thinking what this would be like in my WRX. As fun as it sounds to be going full throttle, the short shifting would cause the turbo to barely be used and acceleration would still be slow.

The plan for the Barracuda was engage transbrake, full throttle, press button flash to 4500 rpm, massive torque multiplication, wheels up, shift at 7200, and pull third early to cruise at 65 holding 4600 rpm. I'm anticipating great gas mileage since I get to cruising speed in 2.8 seconds.

I tried the full throttle & cruise approach on the way home today, all surface street with lots of lights and traffic...
and found that doing full throttle in 1st quickly after getting the car rolling is difficult to do when you want short shift, and undoable with cars in front...
but what I was able to do was to be easy on the gas in 1st and short shift to 3rd and give full throttle to about 2000 rpm and then grab 5th and cruise and sometimes go to 6th...
I reset the average mpg when I started and it started out around 17, dropped to 16 when I hit a lot of long red lights in a row, and climbed back up and was at 18.1 mpg as I pulled in to my driveway!

I am not really sure if the full throttle thing made a difference... and my car may not be the best for this approach... also I think that the full throttle approach is harder on the engine...
Overall I think a more moderate approach to quickly getting up to cruising speed is better...

Whatever, I will probably be down to 11 mpg again by the time I get to work tomorrow... }:)

This is a fun topic.
I would advise against FULL throttle and use something slightly less. 7/8ths??. Many fuel injection systems have an enrichment at full throttle.
I think George made a good point about coasting. I try to anticipate what energy the vehicle will need to get to the next stop or close on traffic. If I get it right there is almost no need for braking.
My last truck (Ford F-350) really benefited from coasting due to mass. Unfortunately, accelerating it was painfull.

The results of BMWs 1970s research into fuel economy was the Eta engine family. Low friction, low rpm, more torque, designed to be used with large throttle openings and low rpm shift points. Small throttle openings create pumping losses which reduce fuel economy. Think of a diesel engine - no throttle butterfly, low end torque, low rpm. BMW fairly successfully duplicated these ideas with the 3 and 5 series Eta engines in the early 1980s.

So, large throttle openings, low rpm shift points (3,000 rpm or less), and get into top gear as quickly as possible where you back off the throttle and cruise.

Is it bad to shift to neutral, and then back to drive while moving in an automatic trans equipped vehicle?

Sebastian Rios wrote:Is it bad to shift to neutral, and then back to drive while moving in an automatic trans equipped vehicle?

no

Sebastian Rios wrote:Is it bad to shift to neutral, and then back to drive while moving in an automatic trans equipped vehicle?

I think there's a trade off Seb. It's another cycle on the system whether it's the automatic clutches reengaging or another cycle depressing the clutch. In general, I agree with Marshall. The amount of addition wear is so insignificant as to not be a factor.

Sebastian Rios wrote:Is it bad to shift to neutral, and then back to drive while moving in an automatic trans equipped vehicle?

As long as the vehicle is under light or no load it doesn't really have any negative affect. Think of the minor tug you get when you put it in gear from start up. If it's never more than that there should be no probs.

BTW I have seen some of the high miler groups advocate putting the car in gear and driving away as soon as possible after start up.
This is bad news for engine longevity. There is very little oil protection available at start up. Adding load to the engine will cause increased wear.
I always start, then put my seat belt on before even putting an auto in gear.