The title does not give the criteria for choosing a gas station. I think if the driver needs to leave the driving route to refuel, then the fuel consumption from the driving route to the gas station needs to be converted into oil price. According to this, we can know the actual oil price of each driver (different vehicles) and each gas station, and then choose the cheapest one.

Gas station a is on the driving route, and the oil price does not need to be converted.

Gas station B is 5km away from the driving route, so the round-trip distance is 10km.

20km round trip to gas station C.

Gas station d is 4km round trip.

The first car, the fuel consumption per 100 kilometers is 6 liters. So the amount of gasoline needed to drive to each gas station and come back is respectively

A: 0 liters

B:6* 10/ 100=0.6 liter.

C: 6 * 20/ 100 = 1.2L。

D:6 * 4/ 100 = 0.24 liter.

In other words, for the first car with a 40-liter fuel tank, it is assumed that it will be filled as much as possible at one time.

If 40 liters of oil is added at Station A, the total cost is 6*40=240 yuan, and the actual unit price is 6 yuan/liter.

In Mile Mile, it costs 5.55 yuan/liter to add 40 liters of oil, and the total cost is 5.55*40=222 yuan, but the distance to and from the gas station consumes 0.6 liter, which is actually equivalent to only adding 40-0.6=39.4 liters of oil, so the actual unit price is 222/39.4=5.63 yuan/liter.

The cost of Station C is 5.4*40=2 16 yuan, the actual fuel consumption is 40- 1.2=38.8 liters, and the actual unit price is 2 16/38.8=5.57 yuan/liter.

The cost of Station D is 5.85*40=234 yuan, the actual fuel consumption is 40-0.24=39.76 liters, and the actual unit price is 234/39.76=5.88 yuan/liter.

In this way, station C is the cheapest.

But the reality is that it is impossible to fill it completely every time. A more reasonable way is to assume that there is still X liters of oil in the car and fill it up at the gas station. You can use the same idea to convert oil prices.

Station A still needs no conversion, 6 yuan/liter.

When you refuel at Mile Mile, the fuel consumption of driving to Mile Mile is 6*5/ 100=0.3 liter, so the remaining fuel is x-0.3 liter, you need to refuel 40-(x-0.3)=40.3-x liter, and the cost is 5.55 * (40.3-X) = 223.66.

Similarly, the actual oil price at Station C is 5.4 * (40+6 *1100-x)/(40-6 *10/00) = (219.24).

Actual oil price at Station D (234.70-5.85x)/39.88

Then, according to the different values of x, the actual oil price of the corresponding gas station can be calculated and selected.

The other two cars can be handled in the same way. A basic conclusion is that the lower the fuel consumption of a car, the more fuel you need to add, and the more worthwhile it is to walk a long way to find a cheap gas station.