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Maximizing fuel economy #581453 06/14/06 10:51 AM
Joined: Feb 2003
Posts: 529
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rob-the-oil-nut Offline OP
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Joined: Feb 2003
Posts: 529
Hey y'all, it's been a LONG time.

With gasoline prices being what they are, I wonder if anyone has tinkered with various speeds to see at what speeds would one get the best fuel economy (everything else being equal). I have a 1997 Bonneville with a 3800 Series II from an " '01 or '02 Buick LeSabre" and I assume the transmission is obvious & ubiquitous (that there are a million identical or nearly-identical engine / transmission combo's out there). Mods & such: Synthetic everywhere, by-pass filter by OilGuard, nitrogen tires to max safe pressure, $170 fuel system cleaning about 6 months ago, plugs are Iridiums about 6 months old, K&N fuel filter, steady diet of MBF (MoleKule Brew Fuel?? I just call it MoleKule Mix)
Yes I followed the rules and did a search first, best one I think was "I want better mileage!!"
by ck8788. Ejjefino said "Accellerate gingerly until the trans shifts into its highest gear, even if you have to speed a little (32 in a 25 zone). "
So is this a simple matter of lower RPM's are better? It gets into AOD gear at about 83 KM/h but that's darn near 20 under the limit!
I know it gets better gas mileage at 55 mph / 90 KM/h than at 62 or 63 mph (100 KM/h is 62.5 mph) but I really would rather not do 85 KM/H for 2 hours just to try it out: I'm hoping someone else has! [Wink] (Even a computer simulation)

Related, I hope: I have found best gas mileage after several (4-5) consecutive tanks of 92 Octane gasohol, and if you want to point out that makes no sense, save your breath / keystrokes, I already know that. But it works. 94 Octane gasohol with octane boost (NOS with nitromethane in it) and it goes down to about the same as 90 Octane gasohol. FWIW one suggestion was that this got it too high and the engine computers & knock sensor got sort of confused and went back to a safe default setting; anyone 'in the know' able to refute or echo that?
What's the state of those instant fuel-consumption meters? I'm thinking something more than a vacuum guage; I heard about those 20 years ago.
Is there a point to cranking up nominal timing? Having a "summer thermostat"

I want to drive to beaches LOTS in the next 2-3 months and I know going over 100 KM/h really guzzles.
Thankyou very much in advance
Rob, the oil nut

Re: Maximizing fuel economy #581454 06/14/06 11:17 AM
Joined: Dec 2004
Posts: 7,420
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Quest Offline
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Joined: Dec 2004
Posts: 7,420
LOL! YOu try a so-called "summer thermostat" in an EFI car and next thing you know: your computer will take on the wrong signal, your engine may never enter closed-loop thus causing more trouble down the road...

Old legends die hard and computer controlled vehicles require a proper thermostat in order to enter closed-loop operation safely yet quickly.

p.s. I do use a vacuum gauge to monitor my engine condition and when I coast on a regular street and hiway coasting. I have been able to consistently achieve 30% higher than that of the published EPA fuel ecomony rating of my car.

Re: Maximizing fuel economy #581455 06/14/06 06:13 PM
Joined: Nov 2003
Posts: 1,573
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DriveHard Offline
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Joined: Nov 2003
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I have heard a lot of good things about this tool. I belong to the VW TDI website as well, and everybody there is in search of the best mileage. Many people there use this tool. I am not affiliated, endorse, or even own one of my own. I just know a lot of people who do.

http://scangage.com/

Re: Maximizing fuel economy #581456 06/14/06 06:38 PM
Joined: May 2003
Posts: 1,863
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ALS Offline
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Joined: May 2003
Posts: 1,863
Scan gauge have one and love it.

Re: Maximizing fuel economy #581457 06/14/06 07:30 PM
Joined: Feb 2003
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unDummy Offline
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Bumping your timing will help as long as you don't have preignition/detonation. If you have a knock sensor, you can test for detonation by removing it(not disconnecting), insulating it, and listening for it.

Use a high flow air filter. Yep, everyone will scream concerning Si increase in a UOA but I've never seen death caused by cone airfilters. Stick with the AFE/AEM/Apexi which filter better then the other popular oil'd filter.
Or, use the stock OEM airfilter and improve the flow to/from it. Smooth hoses vs ripples hose and bigger openings into the air filter box can help.

What is maximum safe PSI? tire sidewall or owners manual? A good alignment, with your incorrect tire PSI, will help.

What fluids are you using? weights?
Use thinner oil like 0w20, thinner ATF like Dexron6, thinner PS fluid like redline or CHF11s...

$170 for fuel cleaning? did this include the TB/AFM/.... Use a PCV catch can to keep intake clean. Use a good FI cleaner 4 times a year.

Gap those plugs. Run the gap on the high side. Degree the plugs if you have that amount of mechanical skill.

If idle is adjustable, set it on the low side of spec.

Clean all engine wiring. This includes battery/alternator/chassis/engine/transmission power/ground wires and harnesses. I not saying you need to invest in stabilant, but I break out the brushes and contact cleaner. I know of several vehicles that had noticeable increases in MPG with running a few redundant power/ground wires.

Install a shift kit. Your tranny will spend less wasted time in between shifts.

Does anyone make underdrive pulleys for your application?

Relax and drive slow. This is the best way to save gas. I don't need a vacuum gauge or scangage to drive efficiently. Learn to keep your distance, and to coast as often as possible. This also will save on brake wear. There are way too many gas&brake&gas&brake drivers out there.

Re: Maximizing fuel economy #581458 06/15/06 08:09 AM
Joined: Jul 2002
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Dominic Offline
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It's difficult to make the 3800 get better fuel economy without PCM reprogramming. The best thing you can do is never go past 40% throttle on acceleration, and keep the RPMs at 2400 or below so that the EGR stays running to keep the cyls cooler. It works.

I've owned a 97 Pontiac Grand Prix GTP, 00 Firebird V6 Auto, and now 02 Camaro V6 Manual - I know the 3800 too durn well [Smile]

Re: Maximizing fuel economy #581459 06/15/06 08:36 AM
Joined: Oct 2005
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Master ACiD Offline
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when i had a rental car with an instant fuel economy gauge i found 55 to be very roughly the precise speed at which you maximise fuel economy while still traveling at a decent speed so you get there on time. it isnt quite as efficent as 45 but its 90% there.

maybe thay had it right when they adopted a national 55mph speed limit back in the day. seems like 55 is the magic ticket.

Re: Maximizing fuel economy #581460 06/14/06 11:15 PM
Joined: Oct 2002
Posts: 1,904
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TheTanSedan Offline
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Fuel economy really is all in the head.

Eliminating all trips that are less than fifteen miles, for example, by planning ahead.

Average speed over a short elapsed time for a complete tank.

Limits to driving style. Speeds at which one neither passes or is passed, with favor to the former in all situations. Go with the flow, and then ease back some.

I consider maps, weather, traffic conditions, even to run some errands around Dallas. The truck hasn't moved since Sunday, and I can get all my errands done today starting at a particular time, holding to direct routes, and at a speed that gets me through most stop lights without ever coming to a full stop.

On the Interstate, the fact that one is traveling a limited-access roadway makes much driving skill redundant . . except to those who pay no attention to what they'll have to do up ahead.

A successful trip is one in which one accelerates to just below posted speed limits and uses the brakes not at all until the actual ending.

And so forth.

Re: Maximizing fuel economy #581461 06/15/06 12:17 AM
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ikeepmychevytoo Offline
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It is possible to cheat the PCM with 20 cent resistors on the sensors. You may strategically install a resistor and remove it in the winter. PCM will think it is hotter. However you'll need the sensor voltage specs, some calculations and the operating logic of the PCM. Poor guys chip tuning.

Mine 2.8 GM does respond to the basic advance crank-up. It never knocks (really wonder why?) it is the starting that gets difficult. Take-offs gets livelier and past 100km/h gets more dull. Better mpg than the EPA spec. in one of the worlds craziest city traffics. Formerly I was able to get better than spec hwy. mpg but never in the cities. However I fear of having un-audible knock for the long term.

Re: Maximizing fuel economy #581462 06/15/06 03:16 AM
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carock Offline
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For a normal passenger car, you will get your best fuel economy driving in top gear at near 2400 rpm, if the car's speed is under about 70 mph at that rpm. The biggest reason for this is that the rate of pressure increase in the cylinder with unleaded gas corresponds to the optimum crank shaft angle for producing torque (whew! What a mouthful)at about 2400 rpm if you are using "normal" valve timing.There are a lot of engine engineers who never learned this idea that was fully developed in WWII to get the best fuel economy for aircraft.

Re: Maximizing fuel economy #581463 06/15/06 03:23 AM
Joined: Dec 2003
Posts: 12,385
XS650 Offline
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quote:
Originally posted by carock:
For a normal passenger car, you will get your best fuel economy driving in top gear at near 2400 rpm, if the car's speed is under about 70 mph at that rpm. The biggest reason for this is that the rate of pressure increase in the cylinder with unleaded gas corresponds to the optimum crank shaft angle for producing torque (whew! What a mouthful)at about 2400 rpm if you are using "normal" valve timing.There are a lot of engine engineers who never learned this idea that was fully developed in WWII to get the best fuel economy for aircraft.

Best fuel economy in most modern cars is at well under 2400 rpm. That's why they are geared the way they are.

Re: Maximizing fuel economy #581464 06/15/06 07:10 AM
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Gary Allan Offline
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I'm sure that best economy is below that. It's just that most don't drive under 2400. They don't know what they're missing. Best for them is how much they got at 3000. [Big Grin]

Re: Maximizing fuel economy #581465 06/15/06 07:15 AM
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mechtech Offline
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Air resistance becomes a big drag after about 40-50 mph, and it goes up by the square of the speed.
Drivetrain friction and rolling resistance are linear.
Around 40 mph will get you the best mileage, and will go down after that.

Re: Maximizing fuel economy #581466 06/15/06 07:29 AM
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55Test Offline
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Heh, 2400 rpm in top gear in my Crown Vic would have me doing nearly 100 mph. As much as I can figure, my engine turns over at approximately 1500-1700 rpm at speeds from 62 mph to 70 mph.

Re: Maximizing fuel economy #581467 06/15/06 07:39 AM
Joined: Dec 2003
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XS650 Offline
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quote:
Originally posted by mechtech:
Air resistance becomes a big drag after about 40-50 mph, and it goes up by the square of the speed.
Drivetrain friction and rolling resistance are linear.
Around 40 mph will get you the best mileage, and will go down after that.

I've seen some curves that illustrate that but can't find them now. Gas mileage was usually best and pretty flat from about 40mph to 60 mph depending somewhat on the car. It was worse at faster or slower speeds.

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