RPM and Fuel Consumption Rate?

Question

I've seen some answers related to the topic, but my quesiton is a little bit different.

Supposing you're running your engine on 3rd gear with 3000 rpm(doing 60km/h), and 5th gear with 3000 rpm(doing 90 km/h). thereotically you consume similiar amount of fuel.

my question is that if I skip the gear four and run the engine on 5th gear, (ignoring the loss of power), do I save some gas? The speed will be the same but the RPM would be less on gear 5.

Answer 1

You made something simple really complicated. Several things come into play. Engine rpm is the impacts the least and load-power requirement impact the most. My motorcycle has an onboard computer that give fuel consumption. On level roads at 1000 feet above sea level, of course fifth gear is going to require the least amount of fuel. In the mountains at 8000 feet going up 10% grade at 40 mph, 4th gear used less fuel. Every engine has a "torque peak", the spot where the engine makes the most amount of power with the fewest rpms. Most engines also use less fuel when producing 80% of the power they are capable of at any given rpm. Going up a really steep hill requires more power and may require 100% of the power produced at a certain rpm and therefore the fuel mileage goes down. This is often referred to as "lugging" an engine. How do you know when you're lugging an engine? Simple, if you step down on the gas pedal and the engine doesn't speed up or you don't go any faster, then you're lugging it. Lugging isn't conducive for long engine life and giving more gas should produce an increase in speed. There's a lot more to it, but I think this covers it adequately.

Answer 2

You blew it on the setup of your question. There's no way you're going to consume the same amount of fuel at 60 km/h and 90 km/h. You've totally misunderstood the importance of RPM. At 90 km/h, you'll consume twice the fuel per km, and 3 times the fuel per minute, as you would at 60.

By itself, RPM doesn't have any importance. The reason it becomes a factor is that when you change to a higher gear at the same speed, lowering rpm, you're able to generate the (unchanged) required power at higher pressure. The pressure in the cylinders will be higher when operating at a lower RPM and a more open throttle. Cylinder pressure effects fuel efficiency.