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Drove my V, which I bought new and has 38,XXX miles on it, yesterday up north on the interstate for about 300 miles. As the outside temperature dropped from around 32 Fahrenheit to 10 Fahrenheit, so did my mileage, even though I was going a solid 70 mph the whole way. One would think the engine would be at its peak operating efficiency at that speed. My mileage (hand calculated and according to the MID) dropped from around 30 mpg to 25 mpg on the trip in accordance with the temperature.

On the return trip, from 10 Fahrenheit to about 32 Fahrenheit and again at 70 mph the whole way, I didn't have to fill up, so I can't say what the hand-calculated mileage figure would have been, but the mpg figure on the MID rose back up to about 31 mpg as I got farther south.

For both legs of the trip, the needle on the engine-temperature gauge never wavered from dead center.

In seven years of ownership, I don't think I've ever seen the mileage fluctuate that much in accordance with the outside temperature at highway speeds. Is that normal? I've seen it fluctuate that much with the temperature at around-town speeds (25 mph to 40 mph), but from this non-mechanic's perspective, that seems more logical given that the engine often doesn't have as much time to warm up to optimal operating temperature and there is a lot of stop-and-go driving on local roads.

The car has no mechanical problems that I'm aware of (aside from a possible replacement of the heater box to resolve a groaning sound). I use Mobil gas and two tanks ago had added a bottle of Chevron Techron Complete Fuel System Cleaner to the tank after reading about it here at the forums. My oil life is at 50 percent (it's been just over 5,000 miles since my last oil change).

Just wondering if highway mileage should be dropping that much in synchronization with the outside temperature.
 

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Just my opinion, but at 70 MPH would be far from the "peak operating efficiency". I would also think any differences, even in slight ones such as hills, overall elevation changes, wind direction, how many cars/trucks you accelerated to be able to pass will all have an impact in MPG. I assume the return trip was the following day, so conditions could be dramatically different.

Interesting post though.
 

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Discussion Starter #3
Thanks, BigD. Oh, so 70 mph is far from "peak operating efficiency"? I was going on the assumption that the engine is most efficient when it's in fifth (the topmost) gear, but again, I'm not a mechanic. At what speed is the engine operating most efficiently, or is there such a speed?

Good point about the terrain, but it was pretty flat and straight, and I was able to keep the cruise control on and not have to pass anyone for the most part on either leg of the trip. And the pavement was dry and clear in both directions.

Yes, the return trip was the following day.

I also thought of another factor after I posted the question: tire pressure, which I hadn't checked before or after either leg but which I assumed was within the normal range because the TPMS light wasn't on (and it has come on in the past when one or more of the tires has been low, so I know it works).
 

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Tire pressure would almost certainly be the most important variable in your case. I think you have the answer there.
As for speed, I even see significant, measurable differences between 65mph and 70mph. While the engine may or may not be operating at peak efficiency, the increased drag (wind resistance) much over 50mph outweighs efficiency gains.
 

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Discussion Starter #5
Interesting, aluminated, that you have also seen measurable, significant differences within a 5 mph range.

Oh, OK, so the benchmark is around 50 mph for efficient operation. Thanks.

Yeah, I guess tire pressure could be a factor, but then the mileage (on the MID) went back up when all other conditions remained the same once the outside temperature rose. Maybe the warmer temperatures (even considering 32 Fahrenheit is still feeezing) also affected the tire pressure?

Tire pressure would almost certainly be the most important variable in your case. I think you have the answer there.
As for speed, I even see significant, measurable differences between 65mph and 70mph. While the engine may or may not be operating at peak efficiency, the increased drag (wind resistance) much over 50mph outweighs efficiency gains.
 

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Googling speed vs MPG produces a lot of charts, but none of them are specific to a 2012 Honda CR-V. In general, it looks like around 50 to 55 MPH is a good rule of thumb. If someone had Honda year and model specific graph I would love to see it. But since you kept your speed steady at 70 mph in both directions it doesn't really figure into your question, right?

Also in general, as the roadway temperature increases tire pressure will also increase. Several articles indicate 1 psi for every 10 degrees temperature change. The higher the pressure the less the rolling resistance. So it could be a factor. But in your case 10 degree to 32 degree would only be a 3 psi difference. So going North you lose pressure and increased rolling resistance, but heading South you gained pressure and lessened rolling resistance.

Assuming you made the same number of "pit stops" in both directions, I would say, wind resistance and total elevation change from start to finish would have the greatest affect in MPG.

BTW, the MPG average in our 2014 Honda CR-V EX-L is 25.6 mpg for the last 30,000+ miles. The best MPG we ever got was 50,000 MPG. Of course we were towing the Honda behind the motorhome!:banana:

Thanks for the question.
 

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Discussion Starter #7
Googling speed vs MPG produces a lot of charts, but none of them are specific to a 2012 Honda CR-V. In general, it looks like around 50 to 55 MPH is a good rule of thumb. If someone had Honda year and model specific graph I would love to see it. But since you kept your speed steady at 70 mph in both directions it doesn't really figure into your question, right?

Also in general, as the roadway temperature increases tire pressure will also increase. Several articles indicate 1 psi for every 10 degrees temperature change. The higher the pressure the less the rolling resistance. So it could be a factor. But in your case 10 degree to 32 degree would only be a 3 psi difference. So going North you lose pressure and increased rolling resistance, but heading South you gained pressure and lessened rolling resistance.

Assuming you made the same number of "pit stops" in both directions, I would say, wind resistance and total elevation change from start to finish would have the greatest affect in MPG.

BTW, the MPG average in our 2014 Honda CR-V EX-L is 25.6 mpg for the last 30,000+ miles. The best MPG we ever got was 50,000 MPG. Of course we were towing the Honda behind the motorhome!:banana:

Thanks for the question.
Hah! 50,000 MPG. If only every vehicle could get that!
 
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