My Sidekick, MPG (miles per "USA"gallon) , Kilometers/Liter is horrible:                        
My car sucks too much fuel  (gas) (over uses, or wastes) , my fuel  economy is horrible, etc.
What is a Gallon?, seems an easy question and is, inside confines of USA.
 
The USA EPA 1995 spec MPG , is 19 to 30 MPG]US], << see arrows! in linked photo?, with E0 fuel (gasoline pure) E10 a tad less, a 1/2 mpg?)
My 2 Dr.-5speed, does 28 MPG one E10 fuel. (open road !) 60MPH. (unloaded and flat ground)
The same car gets 34 Miles per Imperial gallon[UK]. 20% more due to the large gallon size.  Even a tad more on E0 fuel
E0 is pure gasoline (octane) E10 is 10% Ethanal.
Our engines (EFI) are not tuned to get good Ethanol mileage.
The metric fuel economy is  8.4Liter /100km 
Most of these cars with 15MPG[USA] are STUCK  in limphome mode. (most)

 An Example of REAL MPG DATA here are may owners, reporting , add yours. (these are mostly mixed, MPG, so are lower)

If   engine issues are the cause of low MPG, it us usually due to bad Combustion. (too rich tops list)
 Don't let MPG make you sad, like this:  (funny Geo Metro jokes,  a 50 MPG[US] car )

See Hitlers RANTS ON MPG FAILURE ! (the 3 speed joke, is the best...)  "why make a 50 MPG car, then wreck it, with that 3speed ?"

BAD MPG is: (pure physics and errors)
  1.  Measurement errors (vast and complex ) or not knowing what Gallon means.
  2.  Rolling resistance too high (brakes and every inch of driveline and bearings, end to end)
  3.  A BAD running engine, (common)  See flooding page.  Over 50 reasons, to get bad fuel economy.  (flooding or Misfiring [50 ways])
  4. Weak or wrong combustion.
  5.  Poor Driving Habits (get some driver feed back?, buy any MPG Gauge)
  6.  Too heavy a car, Got 1000 lbs of bricks loaded in the cargo area? or pulling a heavy trailer? 
  7. Poor body ballistic coefficient?  This car is a BOX and has huge air drag fast. Slow down. It also has a high CG, so slow down again.
  8. You drove to the top of the Rocky mountains and expected, High MPG on the way up? why?
  9. You drive over 3/4 or 75% throttle all the time?  (this causes EFI to go to enrich mode full time and waste fuel all the time)

 Top reasons for poor MPG: (common or macro reasons)
 Cockpit Errors or others.
  1.   Driving wrong  , fast or fast right foot, or driving with throttle over 75% causing ECU to drop closed loop mode.   
  2.   Thinking your MPG is the same, as your friend in Canada, that gets 34MPG (his are imperial gallons, 1.2 time larger that USA gallons !  34-36[IMP] is reported and posssible.
  3.  Comparing City driving with open road, driving, don't mix them, they are opposites of each other, there is no point to it. (end the EPA malarky )  MEASURE HIGHWAY MPG.
  4.  Average 3 tanks of fuel using Gallons used 1,2,3 total and then total miles, on the 3 tanks,  then divide total miles by gallons, Algebra:  MPG = miles /  gallons  .
  5.  Thinking the odermeter is accurate, even after 20 years old, or oversized tires,? {use GPS miles ! and check speed with GPS ! } do not use the crappy odometer !
  6. Vehicle errors .   Brakes bad (dragging brakes, or wheel bearings seized), Engine running  bad. etc (near endless) , or the ECU stuck in LIMPHOME
  7. In the city, the fuel used is at 12:1 AFR    (Air Fuel Ratio or A/F) add to this loss, constant stops and pray tell jack rabbit starts.
  8. There is no low end here, ever drive a "service truck" that almost never runs fast, it's mostly stopped, you get HORRID MPG.
In the city , the fuel is burned RICH.  (by DESIGN)
In the city you are accelerating over and over and wasting forward momentum at each stop. (both acts, waste fuel)
On the open road the MPG will be best at 14.7:1 AFR,  this is the point, that every molecule of fuel is burned, with oxygen.  every drop.
This is how you get good MPG good on the open road and  keep the AFR at STOICH,  14.7:1  and you will get 28mpg with E10.  I do. (2dr .5sp)
To get better city mileage it takes careful driving, and not stopping. No fast right foot actions.   
(mosly, impossible, and my point here)


Basics:
If your tire/wheels spin freely (jack up car, and spin it by hand, must be an easy spin, no drad) then your engine is running super bad.
The engine can only do 2 things,  make lots of power efficiently or blow heat and raw gas out the tail pipe.
The engine can burn all the fuel or waste it.  (incomplete combustion will caused bad MPG)  MISFIRE is the top reason, is thet EFI over fueling ?

Top Solutions: (Micro) (no order , order is dictated by FATE)
  1. Someone stealing the fuel,  or fuel  leaking down that curb gutter drain parked on the street?
  2. Measured gallons wrong, (not letting the Automatic shut off end, the island pumping , do not top off) (for best effect? use same station and pump)
  3. Miles measured wrong.  (not checking the odometer against the mile markers or GPS.) wrong size tires? wrong odometer in car or wrong odometer  gear sets.
  4. Engine is Frigid all the time or a psychotic BAD Thermostat or worse missing !.
  5. Drive 55 MPH on the freeway.  MPG will be NO GOOD at 80 MPH, for SURE.  never drive over 75% throttle.(that drops closed loop)
  6. Try  to reduce your driving below 50-60 MPH. (see the "sweet spot" ,  find roads that have less fast speeds. 45 mph was my sweet spot.)  varies by transmission type (gearing) and body weight.
  7. Avoid city driving. (  avoid clogged? freeways? , use back roads, avoid traffic/accidents with a navigator system,  travel outside rush hours)
  8. Refrain from , jack rabbit starts ,  and passing cars on the highway aggressively, or any form of a fast right foot.
  9. Remember to coast down hills and that every braking action turns Gasoline in to wasted heat Coast in the Top gear, if safe to do so. sure.
  10. Sitting at stop lights (0 mpg)  Amazing waste of fuel doing nothing,  I see folks here in Texas, go the gym and leave car running (illegal too) and come back 1 hr. later, with cold A/C running. !!!
  11. Don't buy a car with a old Hydra-Matic  transmission car for economy max... (they are ok on long trips , using the locked up clutch ("TCC") make sure TCC works !
  12. Make sure that A/T Auto tranny actually does, shift to the top gear 4th or 3rd +TCC lock up , as the case may be.
  13. Tune the engine  , a full tune ,up really does save gas , especially the 60,000 mile[US] or 100km ,  tune up or service points.
  14. Get rid of the CEL lamp glowing while running. Scan for the DTC codes and correct them.
  15. Get the EFI system in to Closed Loop (on 96+ cars a full scan tool can check this)  Over 75% throttle KILLS MPG due to OPEN LOOP.
  16. Get the fuel pressure near 30PSI , and never near 60 (shut rating of pump). Use a pressure gauge.
  17. Buy an MPG gauge and learn to drive your car for best economy (human feed back.)  OBD1  95 and older              OBD2  96 and newer.
  18. Get aand AFR  gauge  (0²) and keep it at STOICH as much as possible .  or  build my OBD2 MPG gauge.
  19. Lighten the load in car.  Buy a 2 door car.  Some stock car racers find 500 pounds of dead wasted weight. (they remove it)
  20. Replace your old over 80k mile,  front O² oxygen sensor !  . Many cost only $20 <see them all here.  aka.: Bank1, Sensor 1. B1S1 !
  21. Dragging brakes. or bad driveline bearings. "excess friction" Bad wheel bearings.
  22. Near flat tires or under-inflated tires,   Buy low rolling resistance tires.
  23. Shop on the way to work or after.
  24. Avoid the drive through , walk in..... See #10
  25. Anything  like the 50+ reasons listed under flooding .   or   Misfiring [50 ways])
YMMV !  Your Mileage May Vary.
What is, "Close Loop"? , is when the Oxygen sensor is holding the Engine at best MPG.   Open loop is when the oxygen sensor is shut down , for passing or wideopen throttle.
The purpose (besides SMOG) for closed loop is to BURN every drop of fuel. Not waste it.  Read this book from Mr. Banish, for the full story. (best of best)[mech, engineer]

Close loop failures, are caused by many things. (but the EFI is out of control , if the hot engine fails to enter Closed loop at idle or light steady cruise)

What does MPG look  like?, well , this plot was done on my 96 with a full OBD2 scan tool and Laptop.  See the instantaneous MPG?  See the complexity?
The above car is a 5speed  2door 4wd and has a 95hp 16v motor that produces max. torque at 4000 RPM.  I get 29MPG.

Other motors have different sweet spots, due to different motors and transmissions ,etc.

Best practice, is to buy or make an MPG gauge (#11) and discover your cars MPG sweet spot in each gear, and learn to drive all over again,  For Sure.
A cheaper way is to make a closed loop LED, for the dash, my MPG gauges has one. Drive and keep this simple light glowing and MPG will happen.

Commentary: It's called a heat engine.
In the Otto Cycle engine, where does all my fuel get lost (ICE= internal combustion engine) . See it here, it is lost as HEAT !
Automatic transmissions: A/T
These old school , A/T gear boxes are a big waste of fuel  , newer cars have 9 speeds. or CVS or dual shifted auto , M/T boxes (Porsche)
They (A/T) slips every time you accelerate and turns fuel to pure wasted heat.
The A/T does quite will, out  on the open road, with the TCC locked up; sure does. but wastes fuel ever time you down shift or take off from low speeds.
The new A/T has 9 speeds and a 2 speed tranny oil pump, that does not waste huge energy (@ a2-kw) at every stop signal, at a dead stop, while  in "Drive", selected.

 
Pure Physics!:  ( Saving money is the deal, but saving fuel and saving money are NOT the same thing. but Cost of ownership, is the topic)
  • The engine wastes fuel all the time, yes, they all do, but increasing CR, helps greatly but needs, higher Octane fuel, defeating, saving money.
  • The Otto cycle engine normally wastes at least 70% of the fuel,  as waste heat  (keep the engine tuned, means running right)
  • The Cars weight is a big factor on economy , the heavier the vehicle the more fuel it will use.  Pure physics,  all other things the same.
  • Driving fast, increases the wasted fuel by the Cube of speed. Slow down. Not only Live longer, but save fuel.  Buy a car shaped like a  bullet. Not a brick, like me.
  • Friction, All Drive-line; tires, bearings and dragging brakes all add up to total drive line friction.  If you can't push car by hand, with the car, on flat pavement, fix that first.
  • The Driver of the car , is one of the largest factors , effecting fuel usage, due to aggressive driving.   ( physics of the driver)
  • Running any engine at 0 mph is hopelessly wasteful. (if not charging up some usable energy storage sink device (battery, compressed air,etc)
  • Stopping a car with friction brakes, is very wasteful,  you need to store that energy., but all cars must stop , sooner or leater.  Some folks (and cars) kill the engine, after 2 mins stopped.
  • One of the best ways to make and engine more efficient, is to use  Variable engine displacement (VED) and automatic shutdown.  THE PDE has the best promise (dream).
  • Newer cars, and Hybrids, do use some or all these advanced fuel saving ways,VED, brake energy storage, and better transmissions, and better tires. But, can be very expensive for than FUEL.
  • Energy in, and energy out,  make them equal and become the richest person on earth.  Personally , I'm working on anti-gravity. (a warp shield, per Einstein)
CR = Compression ratio.

COASTING, etc: IMO:
Only fools, hypermil ,  that is, tail gate other cars,etc. IMO ! or drafting big trucks.   (gas is cheap , death ,$$$$ and inconvenient big time)
Coasting in neutral gear is dangerous and illegal in most States, for good cause.
Coasting in gear, is the way to go, down any hill, in top gear, if you can keep legal speeds, if not down shift and use the engine braking to slow car to safe legal speeds. (saves fuel)
Coasting in gear , and lets the EFI use cut fuel mode.  zero fuel flows, when coasting in gear , down hill.  I'm a life time coaster.

The first automatic transmissions using hydraulic fluid were developed by General Motors during the 1930s and introduced in the 1940 Oldsmobile as the "Hydra-Matic" transmission.
This helps to understand why old A/T waste fuel.
Books to read, just one, the best by Jeff Hartman. ISBN-13: 978-0760315828
Better or just as good is the Mr Banish book.
Read this book from Mr. Banish, for the full story. (best of best)[mech, engineer]
Top reasons, for bad MPG not the 50 listed under misfire and flooding for gross fuel waste are: (just engine, not cockpit reasons, not bad fuel)
  • Timing belt slipped. (no 60k miles service done)
  • Engine Never tuned up {service points}, this decade or in the last 60,000 miles [US]  Spark parts, and all filters. (fresh fluids help)  Don't forget a fresh front  (0²) sensor
  • Missing or bad engine coolant thermostat. (wrong stat, use a 190 to 195F not installed backwards)
  • Ignoring the CEL lamp in the dash, glowing away , every day, driving.  It's crying, I'm sick, fix me.!...  The ECU may be in Limphome mode , which sucks fuel like crazy.
  • ANY Exhaust cracks near the Front (0²)  sensor rendering it useless and stuck lean (causes max rich....)
  • Too high fuel pressure, 30's good, 60's bad. ( the regulator is bad (lost vacuuum to it?) or return pipe crushed)
  • Leaking injector(s) (need to be cleaned or are bad, or the signals that drive them is bad (ECU))
  • 1989 to 1995 only, Bad ECU,  the caps have never been replaced (a $5 fix)
  • Bad EGR,  the EGR will actually increase MPG working right, the ECU (PCM) is tuned to do that. and assumes it works.  (it helps MPG by reducing pumping losses, at cruise and cools exhaust valves)
No more guessing ,above list is just common failures. Not guesses. There are simple tests, for each failure mode.
See real MPG live and instant MPG, below, the true MPG is the average of all those spikes. Test car: 1996 2dr, 5speed, 4wd in 2wd mode.  a 2000 lb car.
The below is driving on country roads, below 45 MPH, and those spikes are cut fuel mode, coasting.
To find you cars maximum open road MPG, just log this data and find what speed on flat ground gives you the highest MPG. 
My newer Jeep JK 2008, I have a factory MPG gauge. and can see instant and average mpg driving.
What happens , in real time , is every time you Accelerate  or drive too fast, the average mpg drops.
The below data was pulled using a cheap ELM327 scan tool, then plotted.
The less air you engine sucks with the MAF, the more MPG you get,  and the longer the coasting and less braking....too.  The script is from "Palmer Performance Engineering"


My next graph, is trying to get 40mpg.  (just at 1 speed, and gear) the average will always be less.
The spikes are shift points, and cut fuel mode.
This is Not flat ground it's rolling  hills an stops.




Example 1:
Lets say , I'm getting 15 MPG (USA gallons)  15 Miles per gallon, that is 150 miles on 10 gallons of fuel    MPG = Miles divided by gallons.
Not only that, the fuel was not stolen, and my GPS shows, my mileage  really  was 150 miles driven.
I do not have dragging brakes, I check that carefully, by hand (spun tires by hand, I did.)

I suspect my engine is running badly. (I did the full 60,000 mile tuneup per the operator guide.)
My check engine light (CEL) is on, telling me , it has problems.  DTC P0171  In all cases, I . find the cause of this first.  This DTC,  means the fuel trim is way off the pass/fail rails.
.
I next connect my scan tool and see if the hot engine holds closed loop status at idle and that idle is near 800 RPM (± 50 RPM) HOT !  If not?,  I find out why , now.
I next drive the car while my friend scans the ECU/PCM,  and to see if on flat ground, at a steady easy cruise, say 45 MPH, the PCM holds closed loop status, if not? ,fix that next.
The early cars are very hard to check for closed loop status, due to Suzuki (inanely) not publishing their stinky OBD1 protocol.
In all cases,I check my ECU/PCM long term fuel trim is not over 10% or out of control.

OBD1, 1989 to 1995 (USA)
The closed loop can be seen with a scope, connected to the oxygen sensor CELL pins. (not the heater pins)
If the sensor swings fast, say 3 to 6 times a second, it is , in closed loop.
One other way is to watch spark (1991 to 1995) advance as you race the engine fast in neutral or park, and see timing move way out to near 40 Degree's before top dead center BTDC. If not?, it is in limphome mode.
The OBD1 DTC code 13 , is very very poor monitor and shows 13s only if the fueling is HORRID.  It is not reliable at all, this monitor 13.  I pretend it don't even exist.
Trouble shooting all non OBD2 cars, is a pain, all tests must be done by hand and with a DMM and scope. (all sensors. and check spark timing very carefully)
Finding a scan tool is not easy. ( there are some crude hacking tools about)


On any 1989/90 Sidekick, with the crude , manual distributor, make sure the thing, advances spark properly first, then check ECU LAST.  Both the fly weight spark and vacuum ! BOTH!
The engine will NOT burn all the fuel , unless spark is advancing proper.

end example1:







rev 15:  5-11-2014 ( started in 2007) added graphics.
The cheapest gas you will ever find,  is the gas you didn't burn.   (move closer to work?)