How Speed Impacts Fuel Economy

Over the holidays we usually drive several hundred miles to see family and friends. Along the way back from Thanksgiving this year Alicia noticed that our average fuel economy (MPG – miles per gallon) was dropping as she drove.  Of course she started thinking about writing a blog post on what’s the sweet spot for fuel economy, so she asked her resident energy geek for more details.

Probably the most important factor in your fuel economy is the type of car or truck you drive, so when you are in the market for a new vehicle, check out our Greening Your Family Car posts for our green car buying advice.  Since most of us hold onto our cars for a while, the rest of the post will focus on what you can do with your existing car.

Speed

Speedometer and fuel economy gauge

Once you get up to highway speeds, fuel efficiency drops off the faster you go.  Depending on your car, that sweet spot with the best MPG may be around 40-50 MPH or if you drive a performance sports car it may be higher like 60-70 MPH.  The factors that contribute to this are engine size, how your transmission gears are configured and your vehicle’s wind resistance.

The sweet spot (best MPG) is basically when your car is in the highest gear, but with the lowest RPMs (revolutions per minute) to maintain that speed.  This can be harder to tell for automatic transmissions and a bit confusing with a CVT (continually variable transmission).  When you go faster than this, your wind resistance (aka drag) requires more energy to overcome and therefore more fuel is consumed.  A bigger engine often has it’s sweet spot at a higher speed because it has more power at lower RPMs, but that is not a hard and fast rule.  The bigger engine almost always consumes more fuel than a smaller engine too.

Speedometer
Where is your MPG sweet spot?

A Toyota Prius, which has one of the lowest drag coefficients (least wind resistance) based on shape and size, but also has a very small engine, has a sweet spot around 50 MPH.  A Chevy Corvette on the other hand has a decent drag coefficient, a bigger engine and a transmission configured for speed, so its sweet spot is probably over 60 MPH.  Many newer cars have instantaneous fuel economy gauges (usually instead of the RPM gauge of the past).  Using that you can experiment to find where you see the best fuel economy.  Using cruise control on a flat stretch of road should be the easiest way to check.  Try out various speeds and see what MPG you get, but be sure to be safe.

There are many other things that affect your fuel efficiency, and all of these may contribute to why you probably don’t get the same mileage as the car dealership or sticker said you would.   These factors include:

  • Driving Style
  • Tire pressure
  • Engine Maintenance
  • Weight
  • Stuff (trailers, roof bags, attachments)

Also, hybrids and alternate fuel cars may perform differently than conventional cars.  Our hybrid generates energy from regenerative breaking so we’re often frustrated in our minivan when braking just stops the car, and nothing else.

The important thing to remember is that going faster may get you there faster,  but you’re also releasing more carbon dioxide and spending more money on gasoline than if you just take your time. That car plodding along in the right lane isn’t just a roadblock, he’s doing his part for the planet.

Happy Greening,
Jon



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Comments

  1. John Allen says:

    Gloria drove the 2012 Prius home from the beach mostly at about 70 mph. According to the car’s computer, her fuel economy gradually increased during the 80 mile drive to end at about 52 mpg. for the trip’s average mpg. It looks like the sweet spot might be a good deal higher than 50 mph. (However, the average speed may well have been around 50 mph since there was a traffic jam early in the trip and we used the battery quite a bit then.)

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