November 9, 2011

Does Driving over 55 MPH Still Reduce MPG in Newer Cars?

If you were around in the 70's you know that the freeway mileage used to be 55 MPH.  This was done to save oil.  It was well proven at the time that higher speeds reduced the fuel efficiency in cars.   But years have gone by and cars have changed.  SO one might wonder if 55MPH is still more efficient than higher speeds or if newer cars wouldn't be able to hit a higher MPG at a higher speed. 

Can newer cars get higher MPG at faster speeds?  Is 55MPH still more efficient when driving a newer car?

Its not hard to find the answer.  Doing a quick google search reveals multiple tests people have done with recent model cars.  All the results are the same and confirm that faster highway speeds result in lower fuel economy for newer cars as well.

Here's several examples :

Consumer Reports tested several cars back in 2009 and found a drop off in MPG for every model as you increased speeds from 55MPH to 65 MPH or 70MPH

An article on MetroMPG site shows several of the charts in one place.     They also did testing of a 1998 Metro and found that MPG dropped off proportionally as speed increased above 55MPH.  

A user in the Hyundai forum did testing on a new Hyundai and they found that going from 60MPH to 70MPH differed as much as 6.5 MPG.

In the TDIclub site a user there tested new VW's and found that MPG dropped off steadily at freeway speeds above 50MPH.   They show results for 2009, 2010, 2011 Golfs and Jettas cars.

You may be thinking.  OK but what about a high efficiency super sports car?   Wouldn't a big engine in a sports car have a peak efficiency with a higher speed?   Someone tested a 2008 Corvette and found that MPG peaked around 35-40MPH.      Again, high speeds are lower efficiency. 

Hybrids are no different.    This page on the Priuschat website has data on a Prius Hybrid.   More data from another site shows similar data.  The MPG efficiency of the Prius drops off sharply after 40-45 MPH.   The most efficient speed for a Prius is then around 40 MPH.  

Bottom Line : Newer cars still have worse fuel efficiency at higher speeds.   Keeping your speed down will improve your mileage.


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