Most of us who ordered the hybrid did so to maximize fuel efficiency. So here is a question for the math phobes among us. If a car is traveling from point A to point B, we know we will obtain better gas mileage at say 60 m.p.h vs 80 m.p.h. However, we also know if will take longer to get there, burning gas for that many more minutes or hours. So in the end, which is truly more efficient?
Fuel efficiency
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But there are so many external variables that will give you a different fuel burn at either speeds you mentioned. Ultimately, the onboard computer will give you the exact figures to tell you how efficient the car is based on per 100km over time.
Of course in the real world you are correct. But since this is purely theoretical let's assume all outside factors are equal for the one trip we are talking about. My thinking goes like this. An 80 mile trip at 80 mph will take 1 hour. At 60 mph it will take 1 hour and 20 minutes, or 30% longer. I doubt that the reduction in speed would result in a 30% improvement in fuel efficiency so to me there is no benefit in the lower speed. Someone else might prove me wrong.
Well, I have an Hons B. Math from Univ of Waterloo, so I guess I'm "qualified" to answer!
Seriously though, I would expect it comes down to one's situation, which BTW, can vary from one occasion to the next. Sometimes you're on a schedule (or even behind), so time is of the essence. Those times aside, driving slower to save money or save the planet might be a higher priority.
You've answered your own question in the third sentence.
Exactly. But to spell it out, you're making an invalid conflation of the RATE of gas consumption and the VOLUME of gas consumed. Miles per Gallon is the rate and Gallons is the volume.
Your second conversion of time and fuel efficiency to evaluate what is "worth it" is a question that can be quantitative if we assign a dollar value to the time as well as the fuel. The average salary in the US works out to about $25/hour and the average fuel price is $3.40/Gallon. For your example of driving for an extra 20 minutes to go 80 miles, that's 20minutes*1hr/60minutes*$25/hour = $8.33. So $8.33*1 Gallon/$3.40 = 2.45 Gallons of extra fuel you could buy with that time. Assuming 80mph gets you 35mpg (rough estimate from my car) you'd need to get to 80 miles on less fuel than: 80 miles - [saved miles] = [miles remaining] *1 gallon/35 miles = [Gallons to travel remaining miles]. Finally, you'd take 80 miles/[Gallons to travel remaining miles] to get the MPG you'd need to make it worth it. However, since [saved miles] = 35miles/1 gallon*2.45 gallons = 85.8 miles, you'd need to be creating fuel as you drive to beat the time savings.
TLDR: Time is money and this car is pretty damn efficient so speed to your hearts content.
Driving a given distance at 60mph burns less fuel than driving the same distance at 80mph even though we're driving less time at 80mph. The reason is that the forces that must be overcome to go the faster speed are exponential while the time saved is only a linear function. The chief force working against the higher speed is aerodynamic drag:
Notice that velocity is squared in the drag formula. That squared function will overwhelm the linear time saved.
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