In this myTake, I want to share with you folks the principles of maximizing fuel efficiency, or in other words getting the most "bang for your buck".
This myTake is intended for audiences driving automatic vehicles. I am not aware of how manual transmission vehicles operate, however I imagine that a lot of these general principles will still apply and you can follow them in a manner that is appropriate for driving a manual transmission vehicle.
Warning: Safety comes first and takes priority over any of the principles or tips presented in this myTake. Don't mindlessly get yourself into an accident or an unpleasant situation with these ideas.
Governing Principle: Minimize Braking
Each time you brake, you're reducing the amount of potential energy that gets converted into kinetic energy. The more you brake, the more that potential energy gets converted into heat energy instead, caused by the friction between the wheels and the brakes. If you think about it, the ideal scenario is if you press the gas pedal to accelerate and then never brake (disregarding friction and aerodynamics), since then you're allowing all of the potential energy from the gas to be converted into kinetic energy that makes your car move, and none of it is being wasted by conversion into heat energy.
Of course, you're still going to have to brake, but here are some tips to MINIMIZE the amount of braking you have to do.
If one drives naturally, they may find themselves naturally accelerating and braking a lot, especially if one has a hard time sticking close to the speed limit, which for as a side-benefit, this is a great feature for those who have a hard time sticking to the speed limit.
Cruise control is mainly excellent because it only allows the RPM (and thus the fuel consumption) to reach the level that is minimally required to maintain your desired velocity. When the road inclines, it will automatically go to a higher RPM, but only as high as it needs to in keeping you at your desired speed. The extra force that the car will need to overcome will be mgsinϴ, and with cruise control, it will only use as much RPM's (and thus fuel) to equalize with this opposing force. When the road declines, it will decrease the RPM's because it will detect the, in this case, assiting force, mgsinϴ.
The above image illustrates mgsinϴ, one of the two vector forces when the resultant vector force mg is broken into its components.
Cruise control is traditionally thought of as a feature to only be used on expressways and high-speed roads, but I personally use it anywhere I can, even in 25-mph zones (my car doesn't do cruise control under 25 mph however) because the principle is still the same. One thing you may find is that if you set it to the speed limit, especially on slower roads when there's a decline, that even if it stops applying force to compensate for the assisting force mgsinϴ, your car will still accelerate and may go over the speed limit. So there is a little bit of trial and error you may have to do when finding the ideal speed to set it at. I personally nowadays, even if it goes 5-10 mph over the speed limit, just let the car run naturally downhill, especially because in my area people are extremely notorious for speeding, so I'm still often one of the slower folks on the road. If speeding is not an option for you, see the (third) tip about canceling cruise control up hills.
Slowing Down by Canceling Cruise Control
This is applicable to stopping for redlights and stop signs. In light of minimizing braking, the ideal scenario is if one can come to a stop with their car without any braking at all and instead slowly get there by the natural deceleration that will be caused by the opposing force of friction and drag. Or rather, in most cases get down to 10 mph since even if you don't press the gas pedal, the car will naturally run about that speed from the RPM that it idles on.
That's ideal - if you can get down to 10 mph before you apply the brakes. However, that rarely is viable in a real life scenario, so instead I'd say a good goal is to see if you can slow down to 20-25 mph by no braking at all and just natural deceleration caused by friction and drag forces after you cancel cruise control.
It's a game of anticipating times you'll have to stop and canceling cruise control at the right moment. When you cancel cruise control, the car runs on its own with no extra applied force. This will take a bit of practice, because one one end you also have to consider the extra time you'll be driving and potentially irritaitng drivers behind you. That's why I say the sweet spot, which is a compromise between not irritating drivers behind you and not braking, is if you can naturally slow down to about 20-25 mph before you have to brake, in most cases. If it's a slower road, with a speed limit of 25, 30, or 35 mph, that you need to stop on, you may be able to achieve even lower like 15 mph before you have to apply the breaks.
This also applies to turns because you don't want to take a turn too quickly, so you may need to slow down there as well. A lot of the time though, I've found that isn't really necessary to slow down on turns despite what the advisory speed limits may tell you, as long as you're careful and in control. But if you want to slow down for a turn, don't brake. Reference the governing principle of minimizing braking. Instead cancel cruise control a little bit before the turn, and then accelerate once the turn is completed and reapply cruise control. Then you will convert the potential energy of the fuel as ideally as possible, only doing it enough to meet your needs for velocity instead of more than necessary and then having to brake. This should be much easier than stopping as in most cases you'll only be looking to slow down by around 5 mph or 10 at most.
Another benefit of this overall is it reduces the odds that you'll have to come to a complete stop, as you increase the time it takes to get to that position where you need to be stopped, so in turn it becomes likelier that the traffic ahead will get going on its own anyway before you reach that point, and you won't have braked at all! So really, you shouldn't lose much time since you'll be spending less time being stopped and more time just moving slowly which should be about an even tradeoff in most cases.
Canceling Cruise Control Up Hills
I recommended earlier to let the cruise control stay in effect and not break while going down hills, and if you're okay with going 5 or 10 mph over the speed limit, that's a fine method. If it's really critical for you to stick to the speed limit, another alternative is to cancel cruise control at some point uphill and thus end the maintenance of your current speed and let the mgsinϴ force which will be working against your car (in addition to the friction and drag force) slow you down about 5-10 mph by the time you get to the top of the hill (depending on the grade). The reason for this is as you go uphill, you're gaining potential energy, due to the tendency of the vehicle to roll downhill. Remember, fuel is your source of potential energy here, so in a sense, you're replenishing some of it. The idea is that say the speed limit is 45 and you go uphill, cancel cruise control mid-way through and your speed drops down to somewhere between 35 and 40 mph. Now when you go downhill, at the end of it that potential energy you will have gained from climbing the hill will be reconverted back to kinetic, and you'll be back up to around 45 (or at least higher than the speed you were at at the top of the hill). So essentially instead of using the fuel as a source of potential energy and converting that, you'll be using the potential energy from the increase in altitude, thus saving on fuel.
Minimize Gas-Pedal Acceleration in Start-and-Stop Traffic
The reason for this is often you'll think that the traffic ahead of you has gotten going when really it's just a short-lived moment before you find yourself having to brake and stop again. The best thing you can do in these scenarios is simply rely on releasing the brake and letting the vehicle run on the same RPM that it sits at while you're idle, and playing with those range of speeds by altering how much you release the brake pedal. This also takes a little practice and not something you'll do perfectly all the time, but generally, unless the traffic really gets going above 10 or 15 mph, you shouldn't be hitting the gas pedal much.
This is also a game of anticipation, not with the cancellation of cruise control (as of course that's not applicable in this situation) but with when to release and apply the brakes, and how much.
Drive 55 Miles Per Hour
The National Maximum Speed Law of 1974 was passed in response to the oil crisis in 1973. This federally set a limit on speed limits nationwide at 55 mph. This idea was based on the fact that 55 mph is on average the most fuel efficient speed. This may vary by a few mph by car, but generally going above 60 mph, fuel efficiency starts to decrease due to larger drag force the car has to overcome, and this is where you'll notice that your base RPM will start to increase, which means higher fuel consumption. Under 50 mph or so is usually not as fuel efficient because then you're typically not in your highest gear and thus aren't getting the most out of your fuel. At around 55 mph, most automatic vehicles will be in their highet gears at around the lowest RPM, usually a little under 2,000.
So on the highway, just sit in the center/right lane, put on your favorite music, and set your cruise control to 55 mph and just chill. Leave a few minutes early if you need to. If you drive a lot on expressways, this will probably the most beneficial thing you can do for your fuel efficiency.
Aside from the 55 mph guideline, all of these tips go back to the fundamental governing principle of maximizing fuel efficiency: minimize braking. Again, the reason for this is each time you brake, you're reducing the amount of potential energy that gets converted into kinetic energy which is what causes your vehicle to move, and instead converting that into heat energy, which is of course for us a waste. What a lot of these tips aim to do is minimize the amount of excess fuel you're using that gets converted into heat energy, in addition to the kinetic requirements for your vehicle in order to meet the desired "change in distance".