20-22mpg!

[slow6]

130mph passes on the freeway? you must have high gears(numerically low #)(ex: 2.73s) and manual.

if you cant get decent gas mileage, i'd check your driving style.


(me) honestly, why do factorys make cars with 2.73s? (only answer i hear): for better mpg!
(me) ok, but not all of us stay on the highway ALL DAY, some of us have to do a little accelerating from time to time...

[Ender2664]

I have my stock gears, everything i have is in my sig, but i also have a pcm, hence being able to break 118. My speed stops at 125 which i pull by with ease. I am going to have one of my friends with a really fast car pace me to get a better reading than i can by just calculating it out with the rpms.

[rhouse21]

Originally posted by Ender2664:
I have my stock gears, everything i have is in my sig, but i also have a pcm, hence being able to break 118. My speed stops at 125 which i pull by with ease. I am going to have one of my friends with a really fast car pace me to get a better reading than i can by just calculating it out with the rpms.

Borrow a handheld GPS from someone. Most have a feature that will log max speed as well as show instantanious speed. I'd loan you mine, but you're on the wrong coast. ;)

[ellik]

I consistantly get 18mpg all city. :( 31 or so on the highway.... unfortunately I hardly ever do any highway driving in the bird. typically I get about 250-260 miles per tank. fill-up is usually 14gallons.

[mwh27]

My highest highway was 36 one time, IDK how that happened.

Average is 31, city ranges from 19 - 22.

94 M5 CAI 3catback" no options
50k

[rhouse21]

Originally posted by mwh27:
My highest highway was 36 one time, IDK how that happened.

Average is 31, city ranges from 19 - 22.

94 M5 CAI 3catback" no options
50k

Downhill in a hurricane burning "supergas" maybe? [img]graemlins/naughty.gif[/img]

You just got really lucky most likely. I've been trying to beat my best 31 mph tank for 4 years.

[SpeedingFirebird]

Originally posted by rhouse21:
Math time again:

You missed two important parts to your equation. 1 - MOMENTUM. 'nuff said

2. you missed the gearing. The engine works harder at lower RPMs because of the gears.

My bimmer has an "instantaneous fuel consumption" gauge that agrees with me completly. High RPMS to maintain 55 uses less fuel then lower RPMS. Period.

Originally posted by rhouse21:
(Don't you love engineers. :D )

Yes, I am one. [img]graemlins/rock.gif[/img]

[rhouse21]

Ok, let's not make this into battle of the engineers. Lets just put it into simple, real-world terms.

Most decent cars can acheive 55-60 mph in 2nd or 3rd gear. If we get better mileage at higher rpms, why do automatic transmissions always shift up to their highest gear after accelerating? Why do they even have higher gears? Why are there so many 5 and 6 speed autos popping up on the market? And why does my Y87 with 3.42 gears have a lower EPA sticker than the exact same cars with regular gears?

As for momentum:

M = Mass * Velocity
At cruise, your car has constant velocity, so it has constant momentum. It is neither accelerating or decelerating. (An object in motion stays in motion...) Aero drag is trying to slow you dow (rob you of momentum), so your engine has to contribute the same amount of energy to maintain your speed. (For every action there is an equal and opposite reaction.) Newton's Laws don't care about gears. It takes the same amount of energy to maintain this cruise speed (and level of momentum) regardless of what gear you're in.

As I said in the previous post, for a given power level, an engine at higher revs has to produce less torque, but HP = Torque * revs!!! That's why a Honda 4-banger may need to spin to 7,000 rpm to make 200 hp, but ours engine do at at ~5,000. BUT, that Honda 4-banger would be more efficient at producing 30 road-horsepower because it has less internal drag. That's why the 4-bangers generally get better mileage than the same car with a bigger engine, and why ours generally get better mileage than the V8's, even with the SAME gearing.


As far as your 3 series... The rpm that you get peak efficiency can vary from engine to engine, especially when you take into acount things like variable intake geometries and VVT, tuning trick that your bimmer might have. There very well may be something interesting going on in you car's systems that's a little different than in out good ol' pushrod V6's. Maybe that engine just gets better combustion efficiency at a higher rpm due to some nifty BMW trick. In general this is not the case.

Like I said. I don't have to prove anything here. My brother designs engine management systems for GM's hybrid group and we talk about these issues all the time. I will pass him your thoughts and report back what he has to say.

[dz]

While we're on this topic, is it better to run air conditioning in your car or is it better to have the window down?

I'm imagine having the window down increases drag by letting air into the car, but having the air condition on taxes the engine.

I'm not sure which one would be more efficient though :-/

[Turbo V6 Camaro]

acucly all the matters is vaccum.... get a vac guage on your car more vac = more fuel = less mpg, so find the rpm that has the least vac and log your car to find the lowest inector pulse width......

you can do all the rocket stuff but al lthat matters is VE% and how much fuel the car is adding.....

better the VE% the more power you are getting from what fuel your are putting in...... so pulling number in extream off my head if your at 1200 RPMs and 40% VE get worse milage then 1500 and 80% VE

buy the way all city with the turbo and boost A LOT i got 16MPG last fill up

[slow6]

option #3: neither ac, nor windows down

rhouse1, we have gears to help the engine

Newton's Laws don't care about gears. It takes the same amount of energy to maintain this cruise speed (and level of momentum) regardless of what gear you're in.

newtons laws do care.
and sometimes its easier for the engine to stay at same speed by being in a higher gear than lower gear.

[Turbo V6 Camaro]

Originally posted by slow6:
option #3: neither ac, nor windows down

= death trap.

best would be hacth up a little and widows down that way the air can escape out the back lol

[dz]

I think I'd melt if I tried option 3.

[rhouse21]

Ok. This is from my brother who did his masters degree at the University of Michigan specializing in engine controls:

Emissions controlled vehicles have to operate within a very tightly controlled combustion window in order burn fuel cleanly and efficiently. In other words, every combustion cycle (or pulse as he called it) once the car is in closed loop mode uses very close to the ideal 14.6:1 fuel air ratio in order to best burn the fuel. They have to. If the engine were to run too lean, exhaust temps would get to high and produce to many NOx emissions. If it burned too rich, there would be too many unburned hydrocarbons and you'd fail emmisions.

So, if every pulse is burned at the optimum fuel/air ratio, the more pulses you have per second, the more fuel you have to pump into the engine to maintain emissions.

To use his words, “more revs = more pulses = more fuel.”

As far as the gearing thing,.. It is true that the engine has more power at its disposal at higher revs, so it does feel like the car has an easier time dealing with hills, headwinds and the like. What I’m saying is that from the outside reference frame (if you remember from high school physics), all the little events that arise during the course of an afternoon drive just feel like an additional external load to the engine. If I’m cruising at 3,000 rpm, it takes fewer additional revs to generate the power you need to deal with this extra load than it would at 2,000 revs. BUT from the outside reference frame, both situations produce the SAME amount of extra energy or else the car would be accelerating or decelerating... And yes, sometimes it does end up decelerating because the car cannot produce enough extra power in that particular rev band to maintain speed, which is why our cruise systems do these monster downshifts to maintain speed.

It’s all basic physics, be we need to be careful not to confuse all the different terms. Force is different than torque which is different than energy which is different than momentum. You can solve this problem a variety of ways using any of these terms, but you have to keep the terms consistent in order for the math to work. If you use the wrong term inappropriately, you can be led to some misleading conclusions. Intuitively, we all know what we feel in the seat-of-our-pants, but sometimes generating the equations to accurately represent those impressions is a little tougher than it seems. I studied Aerospace engineering at MIT (really, I did) and we actually solved a lot of these problems in our coursework. (A lot of airplanes have piston engines, remember.) Sometimes the truth seems a little counterintuitive. If you don’t agree with my conclusions, sit down and work through the equations. If you like, I’d be happy to walk you through them.

I’m not trying to tick anybody off or challenge anybody. It’s just that I’ve had to solve all or these problems in bloody detail before and I know how easy it is to get led down the wrong path.

But back to the original question about mileage:
Many schools describe combustion as a 3 part process: Suck, burn, blow. So to help your car get better mileage start at the beginning.

Suck: Freeing up your intake will help your car to breathe easier, which will yield better combustion and more power for the same work.

Burn: Good quality plugs and wires, clean injectors, and good spark will help your cylinders burn the fuel most efficiently.

Blow: A better flowing exhaust will also help you produce more power for the same amount of work.

Hit these 3 areas and keep the revs low and you’ll be good.

[SpeedingFirebird]

Originally posted by dz:
While we're on this topic, is it better to run air conditioning in your car or is it better to have the window down?

I'm imagine having the window down increases drag by letting air into the car, but having the air condition on taxes the engine.

I'm not sure which one would be more efficient though :-/

Mythbusters did a show on this last year. They found the windows down to be more fuel efficient.