xrotaryguy wrote:An easy way to monitor your fuel economy is to install a vacuum gauge. A cheap vacuum gauge will only set you back about $15... maybe $25. The gauge will not actually tell you what mileage you're getting, but it will have a green, yellow, and a red zone. Keep the needle in the green as much as you can to optimize fuel efficiency.
This is not correct.
Both diesels and petrol engines have their lovest specific fuel consumtion at around the rpm where the highest torque is AND at full throttle. In order to acheive the best mileage you have to try to drive at a speed where your drag and rolling resistance is lowest possible, and at the same time the specific fuelconsumtion is lowest possible.
The drag is aproximately proportional with the square of your speed. Ie if you double the speed from lets say 40 to 80 mile/h then the drag vill be 2x2 = 4 time as high. The necessary hp to overcom the drag vill be 8 times as high as you are traveling twice as fast. Driving from point A to B at twice the speed requires 4 times the energy to overcom the drag (8 time as much power in half the time).
An increase in speed from 50 mile/h to 75 mile/h increase the drag by 125% (1,5x1,5=2,25). Even a modest increase from lets say 50 mile/h to 60 mile/h result in a 44% increase in drag (1,2x1,2=1,44)
Increasing the speed DRAMATICALLY increases the required energy to overcome the drag!
The rolling resistance of the tires is not very dependant on the speed. Driving from point A to B requires aproximately the same energy to overcome the rolling resistance regardless of the speed.
At very low speeds, lets say below 25 mile/h, the drag is very low and lower than the rolling resistance of the tires. Thus increasing the speed by 40%, lets say from 20 mile/h to 28 mile/h only slightly increases the required energy to propell the car from point A to B.
As the speed increases, the drag eventually become much higher than the rolling resistance of the tires. Thus an increase in speed when the speed already is high requires a much higher percentage increase in energy to propell the car from point A to B. This is because the drag increases proportionally to the square of the speed as mentioned above.
All this means that the speed shall be as low as possible in order to minimize the required energy necessary to propell the car from point A to B. But an increase in speed to a modest level, lets say 40 mile/h only requires slightly more energy for the same distance. An increase in speed to a higher level, lets say 60 mile/h, requires a considerable increase in energy for the same distance. Further, even relatively small increases in speed requires even larger increases in energy for the same distance.
Over to the engine.
First an explanation on specific fuel consumtion.
Specific fuel consumtion is a measurement on how much fuel the engine consume to produce a certain amount of energy, ie 1 kWh or 1 hph.
Modern large two-stroke ship diesels can have a specific fuel consumtion as low as 155 gram/kWh, 115 gram/hph. Diesels in large trucks can have a specific fuel consumtion of 180 gram/kWh, 133 gram/hph. Light modern diesels in cars have slightly higher spesific fuelconsumtion, as low as 200 gram/kWh, 150 gram/hph.
Petrol engines have considerably higher spesific fuelconsumtion not much lover than 300 gram/kWh, 220 gram/hph.
All this data is valid at rpm coresponding roughly to max torque AND at full throttle. The specific fuelconsumtion increases only slightly at different rpm, but increases significantly at reduced throttle.
There is mainly two variables determining the specific fuelconsimtion to an ICE. Namely the RPM and the load (full throttle or 1/4 throttle for instance). As stated abov the single most important variable is the throttle or the actual load of the engine at any given rpm.
When we combine all this informaton together we see that we wish to drive at a modest speed in order to minimize the required energy necessary to propell the car from point A to B, and at the same time load the engine heavily (full throttle) in order to acheive minimum specific fuelconsumtion. Due to the fact that practically every american (and european) automobile is heavily overmotoriced, we quickly would find ourselves without driver licence and awarded with a severe fine or penalty if we followed this aproach to the end.
If the engine was very small, lets say 20 hp, and we only acheived a modest speed at lets say 50 mile/h at a medium rpm and full throttle, that would have brought us the highest possible mpg.
In order to come as close as possible to this mode of operation with ours overmotoriced car we have to choose the highest gear to get the lowest possible rpm vhere we can load the engine as much as possible without the speed be to high.
In an upphill we shall continue in top gear as long as the engine runs smoth, and apply more throttle to maintain speed. If we had a vacuum gauge we would maybe see the needle entering the red zone. That would be very fine, the engine running at high load and consequently at minimum specific fuelconsumtion. If we in stead of continue in top gear selected a lower gear with higher rpm, we had to reduce the throttle compared to top gear to maintain desired speed, We would se the wacuum gauge indicating higher vacuum (maybe yellow instead of red). In this case the fuel consumtion INCREACES due to the reduced load on the engine and the engine running at conditions where the specific fuel consumtion is higher and at the same time has to output the same hp to maintain the desired speed.
I will write more tomorrow on this subject if i can find spare time .
), so thats about as bad as it gets.
I remember my dad doing this all the time in his (now my) corvair. Modern cars have the disadvantate (on this topic anyway) of having power brakes and power steering. These things will not work while the engine is off. If you were driving a Corvair, you wouldn't have to worry about power anything 
