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How To Save Gas

Posted by pneumo, Feb 22, 2007

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  1. pneumo

    pneumo Proven Member

    3,765
    56
    Joined Oct 19, 2002
    Bay Area, California
    Saving gas has been a hobby of mine for several years and I've gotten quite good at it. My best so far is 53.11 MPG. The most miles I've driven on one tank of gas is 702.9 miles. This is on a 1990 Laser FWD turbo with mods. See www.GasSavers.org/garage/view/207 for more details on the car and it's mods. Along the way I've picked up some tips that I've verified to work, so I though I'd pass it on. Some of these tips are specific and may not apply to everyone, so be aware of the mods on your car, the terrain you drive on and traffic conditions you drive in so you can make the most of it.

    1. Starting the car
    The factory settings for fuel enrichment when starting a cold engine are very rich, so don't sit and idle after starting. Just start it and go. A stock motor does not need a warmup. Oil is already present in the bearings, oil pump, and oil lines when the motor is off, so once the motor is started it's getting full lubrication at full pressure. Even in cold climates you can just start and go, as long as you're using light oil. OTOH do not drive it hard when the motor is cold either.

    A built motor with forged pistons and looser clearances might need some warmup, use your discretion here.

    2. Accellerating.
    Keep the revs low and shift early. It's OK to dip into the throttle so the car still accellerates quickly, but delaying the shift and holding higher rpms just burns too much gas. I typically shift at 2k rpm when driving around town. If I'm feeling especially stingy I'll try to shift at 1500rpm, but it's not easy to shift that early. Try it!

    Keep out of boost. When the turbo makes boost the ecu uses open loop which uses very rich fuel tables.

    3. Cruising.
    Shift into the highest gear without lugging the engine. I can cruise as low as 1200rpm at light throttle. If more power/ throttle is needed to hold a steady speed I like to see around 1500rpm.

    Try to hold your speed as steady as possible. Constant on/off throttle burns up more gas for a given speed than holding the throttle steady. Especially try to avoid adding so much speed that you have to brake for slower traffic.

    Try to look ahead and adjust your speed as needed. For example, if you're driving in town with stoplights, stop signs, and other traffic it's wasteful to stay on the gas and hit the brakes at the last second. Instead, if you anticipate any slowdowns you can get off the gas sooner and save gas. You can't look too far ahead. Most of the time I keep track of the next two stoplights and hardly need to use the brakes at all. At a minimun, look ahead to the next stoplight and take in the traffic situation. Is the next light red with other traffic stopped in front of it? Is it just turning green with traffic just beginning to move? Is it a stale green about to turn red? Like I said, plan ahead and go with the flow.

    Drive slower! Aerodynamic drag is the biggest factor affecting fuel efficiency at freeway speeds. Air drag increases as the square of vehicle speed. If you go twice as fast you encounter 4 times the drag. Reducing speed from 70mph to 65mph typically gives me an extra 3 mpg.

    Cruising around 75mph and faster wastes a ton of gas because that's approximately where the ecu goes into open loop where it relies on fuel tables that are set very rich.

    Advanced Technique: Pulse and Glide. This is done by first accellerating at a moderate pace. On a turbo DSM aim for 5 vacuum. Once you reach your target speed, shift to neutral and coast. Let your speed drop by 10 to 15 mph, then accellerate again. Obviously this shouldn't be done in traffic, you'll piss off a lot of drivers behind you, and the frequent speed changes will make you unpredictable to other drivers, which is dangerous. Drivers in remote areas are lucky here.

    4. Braking/Slowing.
    When it's time to come to a stop there are a few different methods that can save gas when used correctly for the situation. The most wasteful way to brake is to put the tranny in neutral and let the engine idle while braking. An idling engine uses gas, and using the brakes wastes momentum that could've been used better.

    There is an easy way to use less gas when braking- simply leave the trans in gear and downshift normally. The ecu has a feature called 'rolling fuel cut' or 'decel fuel cutoff' that cuts off fuel when the throttle is closed and rpms are above 1200. Again, the fuel is cut completely off if both of the folowing conditions are met: the throttle is closed (foot off the gas) AND the engine rpms are turning faster than 1200 rpm. Below 1200rpms the ecu turns the fuel on so the engine can idle normally. When slowing I usually downshift from 5 to 4 to 3, then let the car slow until the revs hit about 1200 rpm, then push in the clutch. 1200rpm in 3rd is about 15mph, which is slow enough.

    An even better way to save gas while slowing is to plan ahead far enough in advance that you can put the tranny in neutral and coast to a stop. Yes, the engine will still be idling and using gas, but coasting in neutral will let the car travel farther than downshifting and using engine braking as I described in the above paragraph. So overall it uses less gas than downshifting because you can get off the gas much earlier.

    It's also possible to combine the two techniques. For example, when I get close to my exit on the freeway I'll coast in neutral until I get to the next stoplight. If the light is red I'll need to scrub off more speed, so I'll put the tranny in gear and use engine braking to decellerate quicker. If the light is green then I can continue coasting until the speed drops down to the speed limit, then put it in gear and cruise. I like it when that happens. :)

    Advanced technique: Engine Off Coasting (EOC) while in neutral uses no fuel and lets the car coast far distances. To shut the engine off, just push in the clutch and turn the key one click to the off position. Wait a couple seconds until the engine stops turning, then turn the key back to the 'on' position. Make sure you don't turn the key beyond the 'off' position since the steering lock will engage! Warning! EOC defeats power steering and limits braking power. It can be hazardous on long, twisty descents. Learn to use it in a safe, empty area. Be smart and only use it on familiar roads. Remember to switch the key to the 'On' position so the turn signals still work.

    5. Turning.
    When approaching a turn, begin by using the best braking technique for the situation, but keep as much speed as possible. Yes, that's right, high speed cornering saves gas! It's important to stay smooth since jerky motions will scrub off speed and precious momentum. Unwind the wheel coming out of the turn to reduce tire scrub while accellerating up to cruising speed.

    6. Shutting off.
    Essentially the motor should be shut off as soon as it's not needed to propel the car anymore. This can be tricky since most DSM's have turbos which can run hot if driven hard. The stock turbo and upgrade turbos from Mitsubishi (16G, 20G) have water lines which help keep the turbo cool during hot shutdowns to a certain extent. Garrett ball bearing turbos also have water lines. Garrett plain bearing turbos only have oil lines and need more cooldown before the motor is shut off, which is why turbo timers were invented. Here's a trick to cool off the turbo without wasting gas: if you unplug the A/C wire that goes to the compressor you can activate both radiator fans by turning on the A/C switch on the dash, then just drive slowly with the fans on for the last mile before arriving at your destination. It's like using a turbo timer while driving.

    7. Mods.
    Here's a few mods that can improve fuel efficiency.
    Pump up your tires to reduce rolling resistance. I recomend a minimum of 38psi. More is better, so read the label on the sidewall of your tires and look for the max pressure rating. If you can handle the hard ride, go for it!
    Lower the car to reduce frontal area and air drag.
    Make sure you have good alignment so the tires are not scrubbing as you drive.
    Use lighter weight synthetic oils in the motor, tranny, and diffs.
    Don't overfill the motor, tranny, and diffs with oil.
    Reduce weight.
    A lightweight flywheel lets the car accellerate faster with less gas, especially in lower gears.
    Use a good engine management system that gives you full control of the ecu so big injectors run like stock and non-stock airflow sensors are fully compensated for. This keeps the fuel trims centered and happy instead of constantly searching high and low for stoich.
    Install a taller 5th gear, it'll reduce engine rpm when cruising on the freeway, saving gas.
     
    Last edited: Dec 11, 2009

    3K  0

    1992 Eagle Talon TSi AWD
    11.514 @ 118.130 · 1G DSM
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