Fuel Used Vs Speed & Time Taken

Question

There is a lot of debate about MPG ratings of cars but I have not seen anything which looks at the actual fuel used that takes into account the reduced time it takes if you go faster.

If I drove a car at 60mph for 60 miles it would take 60 minutes to complete the distance.

If I drove the same car at 90mph for 60 miles it would only take 40 minutes to complete.

Does the saving of 20 minutes driving time mean that driving at 90mph actually uses less fuel overall (despite the higher MPG usage) or does driving at 60mph use less fuel despite it taking longer?

Answer 1

Typically, MPG decreases as speed increases, so you couldn't go too much faster without burning more fuel per hour rather than less. If your car has a trip computer that shows instantaneous MPG, you could create a chart that would help you determine how fast is optimal. I had a car that got 26 MPG at 73MPH, but only 22 MPG at 75 MPH, so on long trips I set the cruise at 73 MPH.

This isn't really a useful comparison, however; you probably get paid every two weeks or so, and you also drive pretty much the same amount during that time. Whether your cumulative driving time per pay period is 10 hours or 7, you're still using the same amount of gas, so driving faster doesn't save you any money.

Answer 2

Driving at a slower speed contantly uses less fuel. I did this experiement with my truck. At a steady 65 i average about 18 mpg. At 70 to 75 i drop to 15 to 16 mpg.

Yea the time is less but the distance is the same and the fuel usage greater.

Answer 3

The rating your talking about varies between every car. I don't know the American statistic, but when I researched this, I found that on the average sedan, the optimum speed to get the best fuel economy was around 64km/h (that's about 40mph).

Answer 4

The reason its called MPG is because time has nothing to do with it. The fuel efficiency will change with speed, not with time.

Although, if you need to drive faster to get somewhere on time, I'd say it was worth it.

Answer 5

Going slower definitely SAVES gas.  I've actually done this experiment myself.  I'm actually saving at least $15 dollars a week by going 60 MPH as opposed to 80 MPH (which I used to drive)

Once your RPM meter goes over 2.5 your gas usage dramatically increases.

And besides using less gas, you also get the added benefit of not worrying about getting pulled over for speeding  =0)

I've found this very beneficial to my stress level