A chip the gets best fuel economy at say 70-75 mph?

[Jason]

I'm just throwing this out there, i don't know if its possible or what the performance ramifications would be, so i defer to the experts (calling Mark D).
Just thinking that w/ the rising prices, it could be interesting if its feasible and nice to have it a higher cruising speed. just a thought...

[Matt P]

There is something that has bothered me for a number of years. When I was in high school, I remember being told by my physics teacher "A car gets its best mileage at 45 MPH".

Quotes like that were thrown around in the 70's as justification for the 55 MPH speed limit.

Recently, I heard some so-called expert saying the same thing (only @ 55MPH) on a talk-radio show.

They are all full of bull-sh*t. It was a generalization made perhaps 40-years ago when US car's had 3-speed automatics, weighted 2 tons, had engines that redlines at 4500 PRM and the aerodynamic properties of a brick.

That generalization has not been true for many decades now, and it is a generalization - the constant speed at which a particular vehicle gets its highest mileage varies quite a bit.

Anyway,

From extended cross-country road-tripping, I've found that my 1990 535i (5-speed, Dinan Chip) gets its best mileage around 75 MPH, about 22.5 MPG according to the OBC.

For most modern cars, with low CD's, efficient, high-revving engines, and 5 or 6 speed transmissions, I would bet that they get their highest fuel economy over 55 MPH.

Results are specific to that car, but in my case, the speed limits should be raised, not lowered like so many people are saying.




I'm just throwing this out there, i don't know if its possible or what the performance ramifications would be, so i defer to the experts (calling Mark D).
Just thinking that w/ the rising prices, it could be interesting if its feasible and nice to have it a higher cruising speed. just a thought...

[tim s]

i get 26mpg highway at around 80mph in my 540i/a.
i also noticed in my 325e i got much better efficiency at 75-80mph.
it all has to do with the gearing.
my 540i/a is not geared to well for around town & therefore i suffer with 12mpg. i have a theory that if i change my diff. from 2.93 to 3.46 or 3.64 my around town driving would become more efficient due to the engine not needing to work as hard, but i would suffer on the highway.
does this make sense?
tim s.

[Bill R.]

aerodynamic drag increases as a squared function with velocity.. the faster you go the more horsepower it requires and the worse mileage you'll get. Now modern day cars may be geared to get better mileage at 75 than in the past but they will still get better mileage at 55. If you look at some of the hybrid sites where they are really stretching for gas mileage this is clearly illustrated , where 10 mph difference will make a big difference for them in mileage. since 10% of 60 mpg is 6 versus 10% of 18 is only 1.8 , the point is much more clearly illustrated with hybrids... and they have the lowest cd going.






There is something that has bothered me for a number of years. When I was in high school, I remember being told by my physics teacher "A car gets its best mileage at 45 MPH".

Quotes like that were thrown around in the 70's as justification for the 55 MPH speed limit.

Recently, I heard some so-called expert saying the same thing (only @ 55MPH) on a talk-radio show.

They are all full of bull-sh*t. It was a generalization made perhaps 40-years ago when US car's had 3-speed automatics, weighted 2 tons, had engines that redlines at 4500 PRM and the aerodynamic properties of a brick.

That generalization has not been true for many decades now, and it is a generalization - the constant speed at which a particular vehicle gets its highest mileage varies quite a bit.

Anyway,

From extended cross-country road-tripping, I've found that my 1990 535i (5-speed, Dinan Chip) gets its best mileage around 75 MPH, about 22.5 MPG according to the OBC.

For most modern cars, with low CD's, efficient, high-revving engines, and 5 or 6 speed transmissions, I would bet that they get their highest fuel economy over 55 MPH.

Results are specific to that car, but in my case, the speed limits should be raised, not lowered like so many people are saying.

[DanDombrowski]

Your overall efficiency goes down (exponentially) the faster you go due to air drag – that one is pretty obvious. The faster you travel, the higher the drag force, the more force is required to keep you going a certain speed.

Now, your engine has an efficiency curve, I would draw it but don’t have the time (I’m supposed to be working). It will have RPM on the x axis and efficiency on the Y and looks something like a bell curve (less efficient at idle and peak RPM, more somewhere in the middle) and will have multiple curves for the different load percentages (inertia of the car felt by the engine). Now, if you take your “drag efficiency” and multiply it by the engine efficiency, you will have your overall efficiency, which is what you want to maximize.

If you fix your speed (say 70mph), your drag loss is a constant (depending only on the drag coefficient and speed), the only thing you can play with is the engine operating point, which is controlled through the gearing. At a certain speed, the closer you can get the engine to its BEP (best efficiency point (point in this case is RPM and load)), the better your overall fuel economy (at that speed). This is the whole point of hybrids – to use the electric motor at idle (no engine) and charge the batteries at the BEP (all engine).

This however does not give the absolute best efficiency. That comes from fixing the engine at its best efficiency point (say X,XXX rpm and XX%load) and gearing to give the lowest drag force. The problem with that is is that if you change the gearing to give you a different speed and drag force, your engine load changes. If you try to figure that one out in your head (ie, which diff should I put in?), you’ll really get wrapped around the axle (pun intended) in a “which came first, the chicken or the egg?” type problem. The only way to solve it is to fix the gearing (low), run it through the whole range of RPMs, find the BEP of that system. Increase gearing, go through the whole process again. Increase gearing again, and again through the whole range of RPMs. Go through all that data and find the most efficient out of all that. That’s what the engineers do at car companies to determine the best gearing for your car, its not just “gear low for highway, gear high for city”.

Now, if the engineer that designed the car said “75% of the time, the customers using the car are going 55mph”, so I’ll use the first method and design for that speed. Guess what? 55mph is the most efficient speed, period. If the engineer uses the second method, it could be anything, but I guarantee you that the car was not designed to be most efficient at 75 mph (at least the US models) because we’re not technically allowed to go that fast. Why on earth would you design for a BEP at a speed you can’t attain? If you’re building a total highway cruiser (and they’re not) the best you can hope for is to design for efficiency at 65, 70 tops. In reality, engines, transmissions, and drag have not changed drastically since the 70s. Engine efficiency is based upon completeness of combustion, intake temperatures and exhaust temperatures. While engines may be able to rev higher now than in the 70s, that doesn’t make the general operation of them any different, and they will still have similar efficiency curves based on RPM and load, albeit somewhat better, and therefore the you will still get your best gas mileage at lower rpms in high gear, usually near 55mph.

[Bill R.]

engine at the rpm which produces maximum efficiency almost all the time , thats one of the reasons they seem weird to people if because if you give it alot of gas the engine runs at a constant rpm and the gearing changes accordingly as you accelerate, you don't get the sensation you get as each gear shifts on a regular automatic. And typically the torque peak rpms is also the point of highest efficiency. Say around 4500 or so for the prius , thats the rpm it usually runs at constantly as your accelerating.







This however does not give the absolute best efficiency. That comes from fixing the engine at its best efficiency point (say X,XXX rpm and XX%load) and gearing to give the lowest drag force. The problem with that is is that if you change the gearing to give you a different speed and drag force, your engine load changes. If you try to figure that one out in your head (ie, which diff should I put in?), you’ll really get wrapped around the axle (pun intended) in a “which came first, the chicken or the egg?” type problem. The only way to solve it is to fix the gearing (low), run it through the whole range of RPMs, find the BEP of that system. Increase gearing, go through the whole process again. Increase gearing again, and again through the whole range of RPMs. Go through all that data and find the most efficient out of all that. That’s what the engineers do at car companies to determine the best gearing for your car, its not just “gear low for highway, gear high for city”.

Now, if the engineer that designed the car said “75% of the time, the customers using the car are going 55mph”, so I’ll use the first method and design for that speed. Guess what? 55mph is the most efficient speed, period. If the engineer uses the second method, it could be anything, but I guarantee you that the car was not designed to be most efficient at 75 mph (at least the US models) because we’re not technically allowed to go that fast. Why on earth would you design for a BEP at a speed you can’t attain? If you’re building a total highway cruiser (and they’re not) the best you can hope for is to design for efficiency at 65, 70 tops. In reality, engines, transmissions, and drag have not changed drastically since the 70s. Engine efficiency is based upon completeness of combustion, intake temperatures and exhaust temperatures. While engines may be able to rev higher now than in the 70s, that doesn’t make the general operation of them any different, and they will still have similar efficiency curves based on RPM and load, albeit somewhat better, and therefore the you will still get your best gas mileage at lower rpms in high gear, usually near 55mph.

[infinity5]

nifty!

[DanDombrowski]

I was thinking of the civic hybrid (which I got a $50 gift card to test drive, BTW), which alternates constantly between charging and assisting with the electric motor. That was all the technical engineering description they could give me about it (it flips back and forth), but when I noticed that it was always charging at 3500 rpm and always assisting at higher than that and at idle, I assumed thats how it worked.

When I asked about how the toyota hybrid worked, all he said was that Honda's technology was better, with nothing to back it up. In his defense, I don't think he was particularly prepared to sell the hybrid, all he wanted to do was put me in a 4 cylinder automatic accord (I cringe at the dullness of the thought) like the other millions of zombies each year that buy that car.

As far as the peak torque being the point of highest efficiency, that makes sense because at peak torque you can run the highest gear ratio and have your best shot at overcoming drag. I heard that somewhere else, but again, they didn't back it up with any references so I put it in the back of my head and forgot.

[mzarifkar]

ive concluded i can do about 26mpg, avereging 75-80mph, on a level interstate i can maintain 80 at the 25mpg mark

[Matt P]

Interesting. Thanks for the education.

[Matt P]

Bill. I've checked my mileage according to the OBC, and at a constant 55 I'm seeing lower mileage (by about 1.5 MPG) than at 75-ish. Any ideas why?

And as for the guy on the radio - he was cleary pusing his own adjenda for loweing speed limints on already overloaded urban highways.

[stx133]

if you want economy stare lookin at the losses. narrow tyres offer less rolling restance, high pressure the same. Not using the a/c will also save juice. but if it was all about saving money we wouldnt be driving such refined and well engineered driving machines, we would be on a bus or car pooling...

[genphreak]

Wasn't the topic was about the merit of which ECU chip was more economical?

I'd really like to know cos running high octane in an M30 is not what it was designed for I imagine. Here in Oz all M30s run on 91RON, but new cars are 95 and even 98 RON. The cleaner, higher octane fuels cost 10-20% more... so that has an effect too. Maybe by installing an EAT chip I get 10% more power, but at 10-20% more at the bowser... well that's not worth it unless the car is 10-20% more frugal when I'm not nailing the accelerator... I wonder where the truth lies...

For sometime I've been running 98RON with a strock chip. Its fine, but I'm paying the extra and barely get any performance increase... :) GP

[Gayle]

Second that. Dan your fabulous education with that freshly minted degree is showing. Thanks for the clear explanation.

[Bill R.]

your most efficient rpm range matched to the gear, If your pulling too low an rpm at 55 in fifth then your not in the range your engine is most efficient. What rpm are you running at 75 ? Select a lower gear that will give you the same rpm at 55 and see what kind of mileage you get then.




Bill. I've checked my mileage according to the OBC, and at a constant 55 I'm seeing lower mileage (by about 1.5 MPG) than at 75-ish. Any ideas why?

And as for the guy on the radio - he was cleary pusing his own adjenda for loweing speed limints on already overloaded urban highways.

[Zeuk in Oz]

As far as the peak torque being the point of highest efficiency, that makes sense because at peak torque you can run the highest gear ratio and have your best shot at overcoming drag.

Would this then partly explain the higher efficiency of diesel engines (apart from other issues like higher compression etc) as they have a wonderfully flat torque curve from usually 1500 - 3500 rpm ? This would be even more enhanced with a modern 5 or 6 speed auto which keeps the car in its optimum torque range ?
Or have I got this all wrong ?

[Randell]

Here in Oz all M30s run on 91RON, but new cars are 95 and even 98 RON.

am i the only australian with a big silver 'PREMIUM UNLEADED ONLY' sticker in the engine bay??

i put regular in once and it sounded bloody awful, i'll never do it again

[MarkD]

I'm just throwing this out there, i don't know if its possible or what the performance ramifications would be, so i defer to the experts (calling Mark D).
Just thinking that w/ the rising prices, it could be interesting if its feasible and nice to have it a higher cruising speed. just a thought...


Hi JAson,

I'm going to Mosport Racetrack to see the American Lemans and some other series or races in a few minutes. I'll also meet the Bimmerworld Racing team (as I'm a guest of theirs) I'll answer on Tuesday as I'll be there all weekend. I've been thinking of advancing the part throttle timing slightly to squeeze out a bit more economy. I may also lean out the closed loop fuelling a tad to help this also.

Keep in mid that you will already get better mileage when using one of my chips.

Anyone interested in a Group buy?

Mark

[Jason]

I do notice the improved fuel economy, but i was just speculating. Have a great weekend.

[632 Regal]

.
Hi JAson,

I'm going to Mosport Racetrack to see the American Lemans and some other series or races in a few minutes. I'll also meet the Bimmerworld Racing team (as I'm a guest of theirs) I'll answer on Tuesday as I'll be there all weekend. I've been thinking of advancing the part throttle timing slightly to squeeze out a bit more economy. I may also lean out the closed loop fuelling a tad to help this also.

Keep in mid that you will already get better mileage when using one of my chips.

Anyone interested in a Group buy?

Mark

[Jason]

drag and therefore increases consumption and that using the AC was actually more efficient. Some one please correct me if i've got this all mixed up.

[Jason]

.

[dacoyote]

drag and therefore increases consumption and that using the AC was actually more efficient. Some one please correct me if i've got this all mixed up.

Yes and No... at freeway speeds it burns less gas to run the windows up and the AC on... in stop and go traffic you burn less gas with windows down and AC off.

If I was truly inspired I could look for the needed numbers to run the math and tell you at what frontal wind speed the even point is.

-Charles

[dacoyote]

I am ....


Hi JAson,

I'm going to Mosport Racetrack to see the American Lemans and some other series or races in a few minutes. I'll also meet the Bimmerworld Racing team (as I'm a guest of theirs) I'll answer on Tuesday as I'll be there all weekend. I've been thinking of advancing the part throttle timing slightly to squeeze out a bit more economy. I may also lean out the closed loop fuelling a tad to help this also.

Keep in mid that you will already get better mileage when using one of my chips.

Anyone interested in a Group buy?

Mark

[Jason]

.

[Interceptor]

engine at the rpm which produces maximum efficiency almost all the time , thats one of the reasons they seem weird to people if because if you give it alot of gas the engine runs at a constant rpm and the gearing changes accordingly as you accelerate, you don't get the sensation you get as each gear shifts on a regular automatic. And typically the torque peak rpms is also the point of highest efficiency. Say around 4500 or so for the prius , thats the rpm it usually runs at constantly as your accelerating.
Do constant rpms lesult in less engine wear?

[philbyil]

Around town, obviously, I get around 21ish but I recently ran 1000 miles in a day and my OBC showed exactly those figures.....

This in a Jim C. chipped 525i 5spd.

P

[ryan roopnarine]

my best mileage around 70 mph on mixed hills/flat is about 28-29mpg. my best mileage PERIOD was about 35.1 mpg at 56 mph on the same mixed hills/flat with a/c on about 30% of the time. this is with an EAT, in a 92 525i, automatic, during a specific 107 mile trip. i was doing this back when gas was 1.80 as to try to make the entire 214 mile trip under $14 or $15 USD (i can't remember the exact amount i was aiming for at the time, today's gas prices make that seem like a frigging bargain). the eat chip, while a wee bit aggressive on ignition advance in this heat, enables wonderful fuel economy, at least in my specific case. since i don't drive a lower compression motor like an m20 or 30, using regular octane was never an option, really, so i save money all the time by leaving the chip in all of the time (as opposed to removing it to reduce octane when gas gets expensive).