True or false: The lower the RPMs of my car, the less gas I am using.
I drive a manual obviously, so am I saving gas when I am going 20 mph on a flat road and shift my car into 5th gear so that my engine is at about 1500 RPM? I feel that I must be saving gas this way, but what if I come to a slight incline and press the gas pedal in a lot but don’t really accelerate very quickly and my RPMs don’t go up that much either? Before my engine starts to lug but before I downshift, am I wasting a lot of gas or is that bad for my car?
This seems like a simple straightforward question, but being a girl and not really understanding anything about cars, I wanted to ask and make sure I wasn’t operating under false assumptions!
False, you can have low RPMs and floor it, when you do you’ll be dumping a lot more gas in the cylinder which will make the engine go faster. To get the best gas mileage you need to find out at which RPMs your car is most efficient and try to stay there in your top gear. This will usually be kinda slow but it won’t be lugging it or high RPMs. The best way to figure out if you’re saving gas, is basically how far down you press the gas pedal. It is the closest indication you’ll get of how much gas you’re using.
Just to add, in older cars, the gas pedal was a literal valve that controlled everything, so how often you had the pedal pushed in and for how long was a good indicator of gas usage. However now it’s all controlled by fuel injectors. This allows, for example, you to go down a hill in gear and use no gas at all.
Well, to be honest Balthisar, I thought I would be using less gas at 75 mph with only 2500 RPM. Because of my momentum I guess? Wouldn’t I be pressing the gas pedal less at 75/2500 then I would at 35/3500 also?
Also, Merkwurdigliebe, I have been pushing my clutch in and coasting down steep hills thinking that my car at 800 RPM was using less gas then if I coasted in 5th gear at 2500 RPM but you are saying that my car uses the same amount of gas either way?
Well this is a point of contention. There was an argument about this earlier, so I won’t pretend to know, but I’ll simply say that there are positives and negatives to this. If leave the car in gear, you burn less gas but you slow down and lose speed meaning that you’ll have to give it some gas at some point. On the other hand, if you put it in neutral, you’ll coast faster and further but you’ll have to burn gas to keep the engine turning. The surest way to get the best gas mileage is to inflate your tires properly (makes a big difference) carry no extra weight in your car (like a bag of soil, for example), and go easy on the gas. Try to predict traffic lights and try to slow down to half speed well before you need to stop so you can hopefully catch it when you get there. Acceleration and is what kills you in city driving so do as little of it as possible. If you were to drive like you had an unprotected newborn lying on the back seat you’ll do well! As far as highway driving is concerned, there’s not a whole lot you can do besides slowing down. If you double your speed, the wind resistance increases 4 times. I imagine you’d probably get great gas mileage at about 50 miles an hour, but then you can’t really go that slow.
Also, just to add that RPMs don’t do what you think they do. RPM isn’t a good measure of how much gas you’re burning. If you’ve got high RPMs from accelerating and you let off the gas, you are now burning none at all, while your RPMs are very high. The best indication of how much gas you’re burning is how far down you’re pushing your foot.
[nitpick]The fuel injectors don’t control shit. All they do is spray fuel in response to commands from the engine control module.[/nitpick]
Other than that, I agree with your post. Balthisar I think it is impossible to answer your question without an idea of what vehicle we are talking about. in a vehicle with a super low CD the 2500 at 75 might be right. If on the other hand, we are discussing a Hummer that has the CD of a fucking barn, I vote for 3500 RPM at 35 MPH. You can never outrun aerodynamics.
Getting back to the OP. It will vary from car to car and engine to engine. I would say as a general rule, most vehicles will get their best fuel mileage at somewhere between 200-2500 RPM in top gear. In this thread truly YMMV
ETA:Merkwurdigliebe As I have said in previous threads on this the difference in the amount of gas burned between coasting in and out of gear is very very small, a few CC, not gallons. The fuel cut on deceleration was not put into the computers as a fuel saving measure, it is for emissions. IT makes a very large difference in emissions, and a very small difference in fuel usage.
In the first example, it depends. If you have an engine that is more efficient at 2500 rpm than it is a 3500 rpm, 2500 at 75 might actually be more efficient than 3500 at 35. In both of my diesel vehicles, this would be true, as 3500 is way above its most efficient operating speed, and near the redline. To give another example, the most efficient range of the TDI Jetta is about 2000 rpm. So, when driving around 45 miles an hour, I use fourth gear (2000 rpm) rather than fifth (1500 rpm) for maximum fuel efficiency. Whether it is true in your case will depend on the most efficient operating rpm of your engine, and how slippery your car’s bearings and aerodynamics are.
In the second example, 800 rpm is going to be more efficient. This is because the injectors are going to put out the same amount of fuel each time the engine revolves no matter what the speed, because you are off the throttle. So, in that situation, fewer revs=less fuel.
You can never outrun aerodynamics.