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MetroMPG 09-09-2009 01:24 PM

Consumer Reports tests speed vs MPG & comes to false conclusion about Honda Insight 2
When I saw that Consumer Reports had recently done a comparison of the fuel consumption of various cars over a range of speeds (55, 65 & 75 mph), my first thought was: "good, more data to add to the pile."

Here's their data:

All well and good, right? You could make some strong, logical conclusions from those figures. Right?

Apparently not if you're Consumer Reports!

Here's what they concluded and decided to highlight about the 2nd gen Honda Insight:


The Honda Insight hybrid showed the largest drop in fuel economy — over 15 mpg going from 55 to 75 mph. [...] Vehicles with lower fuel efficiency had the smallest drop.
That statement is misleading at best, and outright wrong if you examine the numbers in a meaningful way.

If we accept that fuel economy is the relationship between fuel used and distance travelled, then the Insight's absolute increase in fuel consumption caused by increasing speed actually lies closer to the middle of the pack of the vehicles tested.

You can see this if you use litres/100 km (or you could use gallons/100 miles):

So the Insight may have had the biggest numerical MPG drop of the vehicles tested, but that's not the same as the largest change in fuel economy of the fleet.

It's pretty clear that in terms of energy consumption, emissions, and money, the absolute costs of increasing speed (ie. most meaninful in the real world) are largest in the vehicles we would expect (the bigger ones with the largest engines).

This is a glaring example of how MPG can be a misleading/confusing metric for consideration of fuel economy: I can only assume the writer saw the Insight's apparently "massive" 15 mpg drop and it drew her focus.

Whoops 09-09-2009 01:34 PM

It is very interesting how information can sometimes be presented, which is factual, but not necessarily correct in the respect of the right question, rather than a question. The data they presented is technically correct, but the way in which it is presented does make it appear that the insight is a "worse" performing vehicle and the way it is presented implies that it will give the user the worst value in mpg, which is completely not true.

Very interesting example of totally correct, misinformation.

MetroMPG 09-09-2009 01:43 PM

I would have said her statement was correct if she'd said the Insight experienced the "largest drop in MPG" rather than "fuel economy".

gone-ot 09-09-2009 02:17 PM

...they're all "...LIES, damn LIES, and STATISTICS..." , said Humorist Will Rogers.'d figure they'd look at the "rates-of-change" or something, NOT the absolute numbers.

tasdrouille 09-09-2009 03:06 PM

Well, if you ask me the conclusion is not necessarily wrong. The Insight did have the biggest relative percentage drop in fuel economy. This just means that as speed increases its fuel economy goes down at a faster rate than other cars tested.

IsaacCarlson 09-09-2009 03:26 PM

even at 75 mph the insight is still blowing the competition out of the water!!!
I don't like consumer reports anyway because they almost always vote for the ones that are crap.....

tjts1 09-09-2009 03:36 PM

Consumer reports already decided to the new Insight is crap in a previous issue. Now they are just trying to reinforce that point of view. You should email them on this.

solarguy 09-09-2009 03:40 PM

We could quibble about who had the biggest percentage drop, depending on how you compute it, blah blah blah.

But I did notice that the Insight still returned the best fuel economy of the vehicles in the test cohort, even at 75 mph.

I would also note that I'm pretty sure I could beat the Insight with my 04 VW diesel at 75 mph, and maybe the other speeds as well. Admittedly, that's using hypermiling techniques, while they are following some "normal" protocol.

Finest regards,


MetroMPG 09-09-2009 04:10 PM


Originally Posted by tasdrouille (Post 126831)
The Insight did have the biggest relative percentage drop in fuel economy.

You're right: relative to its first value (at 55 mph), the Insight had the steepest change in fuel economy at 75 mph (regardless of whether MPG, L/100 km, or gallons/100 miles is used).

But I still feel that absolute (vs. relative) fuel economy figures are more meaningful/useful to the average person in a comparison like this. The writer has decided that the slope of the graph is more significant than the actual fuel used in the change from 55 to 75:


...the most significant change in fuel economy comes from the most fuel-efficient vehicles we tested.
Is the slope of a graph more "significant" to the average consumer than the actual change in fuel consumption? Hmm...


tjts1: they have comments under this Consumer Reports piece. Since I posted this, someone has brought up the same topic. (Not me!)

Commenter says:


This is one place where the difference of the real cost is much better understood in gallons per mile, rather than miles per gallon. The insight's cost for speeding is far less significant than the Mountaineer's loss of "only 6 MPG."

Frank Lee 09-09-2009 07:26 PM

You should go fix her wagon.

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