5 km flat road or 2.5 km uphill and downhill, which road is more fuel efficient? The conditions of this question are quite incomplete. First of all, at what speed is the car driving, is it just starting or has it been driving at a constant speed? Do 2.5km uphill and 2.5km downhill mean horizontal distance or road distance? What is the angle of this slope? What kind of road do you drive on? And discuss what kind of car this is. What is the characteristic curve of the engine, etc. It's really not that I'm stubborn, but it's true that every condition is different, which will have an impact on the answer to this question. Therefore, if this question really needs in-depth analysis, it will be more than enough as a doctoral thesis on vehicle engineering.

But if you have to answer today, you can only ignore all the conditions and details and try to restore to the most common scenes in reality for analysis. Assuming that there is a certain angle on the slope, the traffic conditions on the flat road and the slope road are better, and the flat road should be more fuel-efficient. You can also refer to the fact that the fuel consumption of high-speed driving is more fuel-efficient than that of country roads. Paradox of perpetual motion machine: in the first step, we make a simple analysis with the law of conservation of energy: the oil saved when going downhill must not exceed the oil consumed when going uphill, otherwise we can invent perpetual motion machine. Therefore, even in the most ideal and fair situation, the fuel consumption of uphill and downhill can only be tied at most.

Next, the contradictory focus of this question should be: Can the oil saved when going downhill just offset the oil consumed when going uphill? This is impossible in theory and reality. Inevitable friction: When driving on a slope, the friction will increase, resulting in inevitable energy loss, whether uphill or downhill: 1. In order to climb the hill, the car needs high speed and high torque. In order to maintain a high engine speed, the internal rotating parts need to generate more friction, which will consume extra energy and increase fuel consumption. 2. When going downhill, the engine is not stationary, it is still rotating and needs extra energy.

When going downhill, in order to ensure driving safety, drivers need to step on the brakes. When braking, the brake pad rubs the brake disc to convert kinetic energy into heat energy, which consumes extra heat energy and still increases fuel consumption. Of course, if we have to say that the slope is big enough, the car can slide down after the flameout and the fuel consumption is zero, then this problem need not be analyzed.

There are many factors that affect fuel consumption, and the ramp problem is only a small part of it. There are several people in the car and several things in the trunk. Whether to take the highway or the urban road, the driving speed and the driver's driving habits have great influence. For the same car, Zhang Sanping drives 6 gallons, while Li Si drives 10 gallon on average, which is not unusual at all. There is no need for the owner to be too entangled. If you want to save fuel, you should step on the gas pedal lightly and drive at a constant speed as far as possible. Don't put too many things in the car, which is almost enough.