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-   -   HUGE difference between 60mph and 70mph (https://www.irv2.com/forums/f45/huge-difference-between-60mph-and-70mph-530249.html)

Jerry Burks 03-26-2021 06:45 PM

HUGE difference between 60mph and 70mph
 
On my way to the RV dealer from Fort Collins, CO to Cheyenne, WY and back in the afternoon with my new F350 6.7 diesel and 35' 5th wheel I experimented with different cruise control settings. Conditions were about the same but maybe 10 degrees F warmer in the afternoon. The 60 miles are mostly flat with a few modest hills but there is about 1000 ft altitude increase to Cheyenne.

The good news: on the way back cruising at constant 60mph on Hwy85 the rig made 15.1 mpg diesel. I think that is pretty decent.

Bad news: on the way up to Cheyenne at constant 70 mph on I25 the truck made miserable 7.6 mpg.

Pavement on Hwy85 is smooth asphalt while I25 is not that smooth concrete. But I still struggle to explain doubling the fuel consumption for only 10 mph difference.

So, just an observation for some idle time discussion ;)

HiWay-OurWay 03-26-2021 06:55 PM

So, hurrying uphill eats up more fuel than driving slower downhill.:cool:
Good to know.

Ljwt330 03-26-2021 07:08 PM

That is a dramatic drop.

Though you mentioned the conditions were about the same, I cannot discount the wind direction. Prevailing wind is generally from west to east, the direction in which you got good mileage. If not a direct tailwind, it might still be an assist rather than a hinderance. Opposite would be true heading toward Cheyenne, plus the higher highway speed.

Bottom line, wind resistance plays a much larger role in mpg than speed. I cannot accept a 50% decrease in mpg simply by a 10 mph increase in speed.

777 Driver 03-26-2021 07:10 PM

That's a drag
 
Well, that's a drag.

There are many things that influence aerodynamic drag. Since the frontal area of your setup didn't change between trips, the "drag coefficient" remained constant. The drag increases by the square of the difference in speed.

So, from 60 to 70, the drag increases by about 36%.

70 60 = 1.1667. So your speed increased by a little more than 16%. However, square this (1.1667 squared =1.36), 1.36 or 36% more drag to overcome, 36% more energy burned. Any wind will have an effect, too. A 10 mph headwind going one way would be a 10 mph tailwind driving the opposite direction. But, that's a net difference of 20 mph for your drag computation.

Take care,
Stu

96 Wideglide 03-26-2021 07:12 PM

HA!

The 7.6 is about what those of us with large diesel pushers get on good days going under 65 :D!

DutchmenSpor 03-26-2021 07:17 PM

I found my best mileage is right at 55 mph. For every 5 miles faster, I decline 2-3 miles per gallon. If I drive a consistent 60 mph, I consistently get 10 miles per gallon. Any faster and it really declines.

I've always said, RVing is not about "speed." It's all about safety. Arriving safe, having fun once you are there, and returning home safe. (It never, never about speed).

(If you have to drive "that" fast, you are driving too far for the time you have.)

bross 03-26-2021 07:21 PM

I just don't understand why people with RVs are obsessed with mileage, we all get about the same: 7-12mpg pretty consistently. You're dragging your house down the road, if you want good mileage, buy a Prius and stay in hotels. :banghead:

As mentioned I think it's drag and winds that affect mileage more than anything.

Jerry Burks 03-26-2021 07:58 PM

Quote:

Originally Posted by Ljwt330 (Post 5688787)
That is a dramatic drop.

Though you mentioned the conditions were about the same, I cannot discount the wind direction. .........

Ah, I think you found it. Just looked up the weather report of today and there was a northerly wind of maybe 15 mph during the day. So the relative wind speed north was 85 mph while the relative wind speed south was 45 mph. Without much science I guess that can explain doubling the fuel consumption!


Quote:

Originally Posted by bross (Post 5688808)
I just don't understand why people with RVs are obsessed with mileage, we all get about the same: 7-12mpg pretty consistently. You're dragging your house down the road, if you want good mileage, buy a Prius and stay in hotels. :banghead:

As mentioned I think it's drag and winds that affect mileage more than anything.

Well, I guess I am not obsessed that I know of. But going on occasional trips of 5000+ miles, the fuel consumption does become somewhat interesting. It does make a certain difference if I spend $1000 or $2000 on diesel (or more depending on the prevailing price).

I just wanted to share an observation that I could not fully explain but the post with the wind direction is surely sufficient.

Anyway, I am not going to plan my travel dates and routes around optimizing the wind directions :rofl:

Ray,IN 03-26-2021 08:09 PM

Here ya go Jerry,https://www.windy.com/?39.119,-86.738,5

consolenut 03-26-2021 08:10 PM

if I get 6mpg I'm happy. sleeping in my bed and using my own toilet is well worth the cost of additional fuel.

Bobby F 03-26-2021 08:25 PM

Quote:

Originally Posted by bross (Post 5688808)
I just don't understand why people with RVs are obsessed with mileage, we all get about the same: 7-12mpg pretty consistently. You're dragging your house down the road, if you want good mileage, buy a Prius and stay in hotels.

I just don't understand why all air travelers don't pay the extra money for first class.



:)

tuffr2 03-26-2021 08:34 PM

I do agree going 55 mph on a nice back road is best and driving on higheay at 72mph is worse.

But driving 68 - 72 mph on the highway in the right lane is the way I prefer to travel.

Old-Biscuit 03-26-2021 09:03 PM

I don't know why this was such a surprise

Fort Collins to Cheyenne is going UP
Cheyenne to Fort Collins is going DOWN

Just look at a map
WY is above CO

Steve43 03-26-2021 09:12 PM

:facepalm::laugh:
Quote:

Originally Posted by Old-Biscuit (Post 5688926)
I don't know why this was such a surprise

Fort Collins to Cheyenne is going UP
Cheyenne to Fort Collins is going DOWN

Just look at a map
WY is above CO



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