Calling all engineers, aerodynamicists, gear heads, and know it alls

[cstratton04]

My initial, un-thought-through reaction would be to whip out my key and return the favor....... but that would only be gratifying untill i get the police called on me. what should you do in that situation?

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[Car vs. Driver]

Get creative.

[aschen]

Theoretically it is a hard question to answer. Most engines are actually quite inefficent at low throttle settings. This is one of the main limitations of a gasoline engine and why a dodge viper cruising along at 55 mph would take alote more gas than a similar car with a smaller engine cruising at the same speed. They are both putting the same power to the ground but the bigger engine is less thermally efficent due to pumping losses. Most other losses increase with speed: aerodynamic, friction, etc. You really need to test a specific vehicle to know what the efficency vs speed curve would look like. "an accurate measurement is worht 1000 expert opinions".

[infantsam]

I hear that - but look at the 25% increase in MPG you get from 'nerve -wracking' attention (thats huge btw). So just do the little things some of the time - like every stop light is not a 0-60 test , you don't have to be in front of everyone all the time, use the AC after you cooled the car w/ the windows down for a mile or so - that kind of thing and you can save some pretty big bucks

If you average 1000 miles a month and go from 20 to 22 mpg (10% increase) you would save about $12/month. Not huge but essentially free money..

Cut 100 miles out of your monthly average by saving a few trips here and there and you'll save another $50/month. The average car costs about $.50/mile to operate. I try to make them worth it.

[infantsam]

Originally Posted by cstratton04

My initial, un-thought-through reaction would be to whip out my key and return the favor....... but that would only be gratifying untill i get the police called on me. what should you do in that situation?

I think you have to try to get them to be honorable, or else get their plate number and go that route with the cops/insurance. (yeah - that'll work)

The courts never like people who try to take the law into their own hands -

But hey it's easy to trip and fall over an anthill and drop your sledgehammer on the windshield of a nearby vehicle

[cstratton04]

so it would be a good idea to call the police even if it is just a dent? would the cops just laugh at you and tell you to stop wasting their time?

[Guru Imakuni]

Originally Posted by Car vs. Driver

Everything you wanted to know:

http://www.fueleconomy.gov/feg/drive.shtml

And I was wrong about the speed in my post above according to this link

I remember when they claimed it would go down .10 instead of .15 every 5 mph over! I'm only 17 and I remember that...so we're getting it pretty bad with the gas now.

[infantsam]

If I had a DENT and saw who did it I would definitely call the cops - I'm sure there is a way to force someone to do something and get the name of their insurance company or something. I'm pretty sure the law says that you can't just go around effing peoples stuff up and then play dumb.

[bdcz2]

Originally Posted by 8Complex

Sorry, I was talking about an NA vehicle... if you've got an STI, you should be loaded enough not to worry about fuel effeciency.

I'm a 20 year old college student paying for the gas myself. I'm not loaded but I also bought the STi intending to drive it, and drive it hard. You don't hear me complaining about gas mileage, or asking if there is a way to shut off the turbo, remap the ECU, or otherwise squeeze a few extra mpg out of the car. All I wanted to know is what I can do to save a few bucks on long road trips during which I don't need the Sti's quick acceleration or turbo.

If you guys still want to criticize me, go ahead. I do wonder how long it will take for you to change your mind, perhaps when gas hits $3, $4, or maybe $5 dollars a gallon. Will you still be so critical of trying to save a buck or two???

Thanks to everyone for the answers so far

[twizzstyle]

There absolutely will be some optimal speed where you are travelling the fastest, loading the engine the least. As far as aerodynamics go (I have extensive education in aerodynamics) the best speed is standing still (duh). So basically the best speed is the slowest speed thats....uh...... I've lost my point.

Best gas mileage is going to be highest gear at highway speeds with the engine revving at a normal range. Drag will be high, but theres nothing you can do about that.

[Lexington]

One of my buddies took a '91 civic hatch, did some light modding and engine management (all geared towards fuel economy, not speed), a wind-tunnel-proven aero kit.. and a peice of thin sheet metal covering his entire underbody. He sealed up every single hole/crack except for intake/radiator (both had funnels so that no air could get inside the "bubble").

this combo netted him 80 mpg on a gasoline civic

[fireball_jones]

Originally Posted by bdcz2

If you guys still want to criticize me, go ahead. I do wonder how long it will take for you to change your mind, perhaps when gas hits $3, $4, or maybe $5 dollars a gallon.

$4 a gallon and I'll hook my PS2 up to my windshield and use the car as a giant simulator.

I can't believe this conversation has gone on so long about gas mileage. You're best mileage comes when you've overcome most rolling resistance and are experiencing the least amount of wind drag. That's about 50 mph, or for almost every road going car, the very bottom end of their top gear. I believe this was actually covered in an issue of that Subaru "informational" magazine owners get recently

Although one thing I did notice is that cruise control usually nets me a good 1-2 mpg better than the same speed if I use my foot for the throttle. I guess I vary the pressure more than I realize, but it's good to know since there isn't a highway in CT people do 55 mph on.

[Car vs. Driver]

Yeah, if 55 is optimal, I'm screwed. THe road I take to work is at least 80 in the slow lane or you'll be run off the road and killed.

[toke513]

I read somewhere that 55mph is the best speed. This is part of the reason why that was the speed limit on the highways. Also if you were worried about gas the WRX is not a good choice. It is not aerodynamic overall not to mention that big thing sticking out of the hood probably doesnt help things at all.

[cstratton04]

Originally Posted by toke513

I read somewhere that 55mph is the best speed. This is part of the reason why that was the speed limit on the highways. Also if you were worried about gas the WRX is not a good choice. It is not aerodynamic overall not to mention that big thing sticking out of the hood probably doesnt help things at all.

It helps to cool your intercooler








Yeah im a smartass

[Vlad]

There is stuff you can do to save gas:
-never decelerate (unless there's ice and you have to), always bring it to neutral and coast down, if there's not much traffic around you. The same if you're crawling in stop and go, try to avoid to accelerate and slam on the brakes. There is a disadvantage in doing an emergency stop while the gearbox is in neutral however.
-staying at the top of the torque curve gets you a bit lower gas mileage than just under-revving a bit. But the savings are more consistent between various traffic situations (town-interstate mix)
The climbing into the torque curve should be done by feathering the gas pedal.
The torque curve that I speak of is not the maximum torque curve.
For a turbo car there should be 2 types of curves. The on-boost curve, with the factory quoted max torque, then lower curves for less than floored accelerator and an off boost, where you open the trottle without putting load on the engine and stay N/A.
What this means is when in light traffic on the parkway, use a gear so that you're just under max torque quoted RPM. When a hill comes up, let the car slow a bit, or use extra speed acquired from the previous downhill.
-Dont put cruise on, A/C on or open windows, use the fan
-Check tire pressures and slightly over-inflate maybe 1-2 PSI.

In Europe, where you usually read gas mileage on every brochure distributed for any car, they give an in-town value, Out-of-town value and a mixed situation.
The out-of-town value was usually quoted for 80-90 Km/h. I remember many friends and neighbours testing what was best, for gas mileage, with all different small displacement engines (including boxers) and they all confirmed 80-90. Something like a 2.5 used to be considered a bit on the larger side and might not exactly fit. But I would doubt it differs by more than 5-10 Km/h, therefore 6 MPH. I say it's probably 60 MPH for an Sti.
Vlad

[hwy61]

Since the cruise control won't engage until the vehicle speed reaches 25 mph anything under that is ruled out. The faster you go the more resistance in everything, tires, wind, engine friction, transmission friction, etc. The more resistance, the less mpg. So I suggest 25 mph in the lowest numerical gear ratio the engine will smoothly move the car. Lugging along with the engine struggling to keep up a 25 mph speed probably isn't as good as a gear where the engine can move the car steadily and w/out extreme effort.

Just stick me in the gear head, know it all section, my wife does. hwy61

[John C]

Originally Posted by Vlad

There is stuff you can do to save gas:
-never decelerate (unless there's ice and you have to), always bring it to neutral and coast down, if there's not much traffic around you.Vlad

Isn't that the wrong way? In neutral, your engine uses fuel to keep idle speed, but decelerating in gear uses no fuel at all. It's definitely this way on Audis, as it in the '800miles on a tank' episode of Top Gear mentioned earlier.

[REX8]

Originally Posted by bdcz2

Ok- you can make this question as complicated or simple as possible, some will decide to immediately flame me for posting a gas related question (I bought a sports car yada yada) and some people will actually offer an answer...

Is there a single speed that, taking into account aerodynamics, engine rpms, and turbo use, will give you the best possible highway mileage...

In other words, if I wanted to go as far as I could on cruise control, where would I set it?

YES!!!! If you do a little searching, you'll see thats how they set the speed limit at 55 back in the day. It wasn't just a number, it was to conserve fuel. Around 52 MPH for an average car is when it is operating at its peak efficiency. So yes, anywhere between 50-56 MPH most cars are at their best...Kind of funny that after all the speeding tickets given, the real reason for that speed limit was fuel mileage....

[REX8]

Originally Posted by fireball_jones

Lowest possible speed in your highest possible gear.

Not true...but you'd think it would be...

[Vlad]

Isn't that the wrong way? In neutral, your engine uses fuel to keep idle speed, but decelerating in gear uses no fuel at all. It's definitely this way on Audis, as it in the '800miles on a tank' episode of Top Gear mentioned earlier.

I don't know what the episode mentioned, but have seen some stuff wrong before.
Let's consider a normally aspirated car for a minute. A lot of times, the increased momentary fuel consumption is associated with Lower (deeper) vacuum. If you look at the vacuum gauge that some normally aspirated cars have, when you suddainly floor it, the vacuum will really go deep. That's how theose "economy" gauges some cars have are hooked up too.
My point is that one of the deepest vaccums is when you use the engine to decelerate.
Even with a turbo car, the same applies. It will not be as bad of a consumption as in wot but right up there.
As far as consumption in idle, that's actually the minimum consumption. With carbureted cars there used to be a separate jet, just for idle. The main emulsion tubes used for normal driving are not even in the same category size wise.

[Murf_68]

Originally Posted by Car vs. Driver

That reminds me of something that happened to me a number a years ago when I was much more likely to freak out on someone ...

I parked my car in a lot and was about to get out when I noticed the woman next to me (in the passenger side) was waiting to get out. So, being a gentlemen, I motioned her to go aheand and get out of her car before I got out of mine.

Well, at that point, she flings the door open and it slams into the side of my car. Instantly, I was pissed. But, what pissed me off even more is that she started to walk away like nothing had happened. So, I jumped out of my car and was basically said "w-t-f?" (Really, I just wanted an apology). And her reply, "What do you expect? You're in a parking lot." and walks away.

I'll not get into what went down AFTER they went into the store ... but it just goes to show that people basically suck.


/hijack

Couldn't resist this one - did you see the Malcolm in the Middle episode where some woman dings Malcolm's mum's car? Basically she dinged her back and it kept going back and forth until they were at the point of ramming each other. Funny, but I could almost imagine doing that to some inconsiderate SOB's!

Mark

[cstratton04]

Originally Posted by Murf_68

Couldn't resist this one - did you see the Malcolm in the Middle episode where some woman dings Malcolm's mum's car? Basically she dinged her back and it kept going back and forth until they were at the point of ramming each other. Funny, but I could almost imagine doing that to some inconsiderate SOB's!

Mark

I saw a clip of that... not the whole episode. They should show that in driver's ed as what not to do.

[WimpWgn]

65mph in top gear.

[sdogg3kgt]

Originally Posted by REX8

Not true...but you'd think it would be...

I was just going to post that. Too low RPM requires more gas than a little higher RPMs in a lower gear. i.e 25mph in 5th gear instead of 3rd.