Never Brake on the Highway! (Cost Calculator)

The energy it takes to accelerate or decelerate is dependent on how fast you are going. It takes a lot more energy to change your speed by say, 10 mph, when you are going fast rather than when you are going slow. A 70-80 mph acceleration takes a lot more energy than a 40-50 mph acceleration, which takes a lot more energy than accelerating from 0-10 mph. The energy needed is proportional to the difference in the SQUARES of the speeds, not the difference in the speeds themselves. Hence, it takes FIFTEEN TIMES more energy to accelerate from 70-80 than from 0-10!

This has implications for brake wear and engine wear as well as fuel usage!

The less energy absorbed by brake pads, and the less gas burned, the slower your engine gets worn out.

How to reduce highway braking

Keep a lot of space between you and the cars in front of you. If you are too close, you will probably have to brake more often to re-adjust your following distance. I anticipate when there might be a slow driver in front of me and change lanes ahead of time to be able to pass them. I also get dangerously close to cars in front of me when they slow down, rather than braking, but I obviously can’t recommend this, since it is not only not safe, but also, if your experience is anything like mine, will drive your significant other crazy. You also don’t want to get a ticket for following too close or break your windshield on debris kicked up by or falling off of the vehicle in front of you. That will wipe out your fuel savings REALLY quickly.

Too much info on the physics behind the calculation:

30-35% of the gas that your car burns does work to accelerate the car (the rest of the gas goes to useless thermal energy (your engine gets really hot!)). According to the work-energy principle, when neglecting losses to friction, the work done is equal to the change in kinetic energy (KE) of the car. We calculate the change in KE as 0.5*mass*(velocity22 – velocity12). The calculator converts pounds to kg and speed to m/s so that KE comes out in units of Joules.

To figure out how many gallons of gas is needed to provide those Joules, we must find out how much energy a gallon of gas can provide. I took an average value between those given by two different websites: about 123,400,000 J per gallon. But remember, most of this energy is lost to heat! I assumed that 32.5% of this amount could actually go toward providing kinetic energy. That would be about 40,000,000 J. To find gallons of gas used, the calculator just divides gallons used by 40 million.  

Won’t somebody please think of the brakes!??!

Remember too that you are saving your brakes from having to absorb just as much energy! My brake pads are almost due to be changed, but they still have 50 or 60% percent of their life left.

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