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2017 2.5 Premium & Base CVT
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Discussion Starter · #1 ·
Gotta say, loving the Forester so far.

Very used to my past Hondas which were all relatively slow but were rev happy drivetrains and encouraged spirited driving.

I'm sure the 2.5 boxer is too, but the CVT just never entices me to do anything but cruise.

Which brings up MPG (hey, I gotta pay attention to something while driving).

I average hand calculated 29-30 mpg every time I fill up using the Forester for my daily commute (80% highway 65-70mph/ 20% city).

I have taken 2 road trips with just the wife and the MPG really drop big time once I exceed 70 MPH.

First trip was just over 500 miles round trip where the speed limit was 70mph. I set the Cruise Control to 75 MPH and averaged exactly 25MPG on the way there (first 250 miles) and 27MPG on the return home trip.

This was a straight drive (no city or stops).

6 weeks later, we took another road trip just over 240 miles round trip where the speed limit was 70mph. I set the Cruise Control to 75 MPH again and averaged exactly 27MPG on the way there (first 120 miles) and 27MPG on the return home trip.

This was a straight drive (with under 10% city).

Could the 5 MPH wind resistance really drop the MPG by so much on these road trips? Just my wife and I taking trips to visit family or beaches. No extra load (bikes, surf boards, etc yet).

I am thinking that the CVT, you know, the one that doesn't encourage me to ever floor it, is really tuned or set up in to be super efficient but only up to 70 MPH. Anything past that and the MPGs drop like a brick.

See the attachment below from my 20+ mile commute this AM. I often get 36,38, even 40+ MPG on my daily commutes at 70 MPH. I never pass my 70 on daily since the roads here are marked to 65 MPG.

It feels like bizzaro world when my MPG average is actually hurt as a result of road trips. LOL.:icon_biggrin:

 

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2015 Forester I-Premium 6MT
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Forester is somewhat a brick for wind resistance.

I know going 55 instead of 65 would often net me about 10-15% better fuel economy.. but I just cant drive that slow... would get run over.

around 45-50mph is the sweet spot for wind resistance vs fuel economy

so theoretically you can have MUCH better mileage on a slow 2 lane rd vs interstate.

FWIW.. I went on a fishing trip to Canada with my dad. When I was driving.. around 65-70.. averaged 25mpg with a 1500lb boat and car full.

When we switched drivers after toronto so I could nap and mr. lead foot started driving.. the average went down to... 21mpg.. that included 200miles on the tank with me driving(@25mpg). I woke up once and we were @80-82 or so.


Headwinds and tailwinds obviously affect mileage but cross winds actually have an huge impact also. Driving back to the states there was a remnant of a hurricane passing through the great lakes with 50mph crosswinds.. and I think we averaged about 15mpg.
 

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2014 Forester 2.5i CVT
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320 Posts
Do you see increase in RPMS as you reach 75mpg? (I have not driven my car that fast yet :) . Perhaps you're at the highest gear at 70mph and the only way to go faster is to increase RPMS which would affect fuel economy.

Here are calcualtions:
Axle gear ratio: 3.7:1
Highest gear ratio: 0.57:1
Tire diameter: 27.6

@75mph - engine RPM: 1926
@70mph - engine RPM: 1797

You would have to increase wheel diameter to 29.5 to get the same engine RPM at 75mph as you get at 70mph with standard wheel size.
 

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2014 2.5i Premium Auto
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13 Posts
Pretty rarely drive under 70mph on the highway. At a consistent 80-85, with the CVT, running heat/AC, a decent number of hill climbs, and a fairly leaden foot, I'm getting 28-29mpg. If I ease up a tad I can get to 30+. In city driving I take a massive hit to fuel economy.
 

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2014 Forester 6 spd manual
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41 Posts
I get 33mpg at average speed of 69 and 28mpg at 74. Consistant testing

Look up any graph showing MPG v speed and you'll see a nice plateau or peak at 40-50 mph then a decrease after that getting dramatic at 65mph+

manual trans.
 

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2014 Forester 2.5 Touring CVT
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111 Posts
Gotta say, loving the Forester so far.

Very used to my past Hondas which were all relatively slow but were rev happy drivetrains and encouraged spirited driving.

Could the 5 MPH wind resistance really drop the MPG by so much on these road trips?
Forester is somewhat a brick for wind resistance.
Hi Shora,

Congratulations on your new Forester! My wife and I are loving our '14 2.5i Touring as well.

As most everyone knows there are many factors that can affect gas mileage. One big factor is wind resistance which goes up according to the square of speed. More importantly, the power required to overcome drag goes up proportional to the cube of speed.

IIRC Rand's descriptive brick example has a drag coefficient of ~1 and I found the spec. cd= 0.33 for our new Foresters which is respectable. So, the following brick example is not strictly applicable however bricks can be useful for simplifying math. If I should screw this up all our math and physics majors please feel free to jump in here:

For cd=1
75 mph is 7% faster than 70 mph,
drag goes up by the square of the speed or about 15%,
energy required to overcome drag goes up by the cube, or about 22%

Other possible factors are elevation changes and wind conditions. Years ago I was living on the north side of Ft Wayne (~ 900' el.) and taking classes in Detroit (~ 500' el.). The highway between is sorta flat so with the slight downhill and prevailing westerly winds my Mazda consistently got about 4-5 mpg better going up than coming back.
 

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2019 Crosstrek 2018 XT
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14,713 Posts
You should get about 5% less mpg at 75 vs 70 mph

I made this post some years ago and combined speed vs fuel consumption from several cars and used the curves to adapt to mine. I think these are pretty good numbers:

I'm gonna try to refine these numbers. And this mileage is for my forester 2.5 X .
For the Engineering Types. :D:
I used 3 data points and 62, 72, and 82 mph and that would correspond to a "true" speed of 60, 70, 80 mph. I then "fit" in two other data points at 65 and 75 mph. Rounded off to two places on the last 2 columns.

MPH......MPG......Gal/Mi......Gal/Mi vs. 60MPH.....MPG vs 60 MPH

60........31.6.... .0316...........1.00 .................... 1.00

65........30.8.... .0325...........1.03 .................... .97

70........29.8.... .0336...........1.06 .................... .94

75........28.4.... .0352...........1.11 .................... .90

80........26.8.... .0373...........1.18 .................... .85

Reader's Digest:

Compared to 60 mph fuel increase/cost is:

3% higher at 65 mph

6% higher at 70 mph

11% higher at 75 mph

18% higher at 80 mph

http://www.subaruforester.org/vbulletin/f107/2200-mile-trip-25103/
 

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2014 Forester 2.5 Touring CVT
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You should get about 5% less mpg at 75 vs 70 mph

I made this post some years ago and combined speed vs fuel consumption from several cars and used the curves to adapt to mine. I think these are pretty good numbers:

I'm gonna try to refine these numbers. And this mileage is for my forester 2.5 X .
For the Engineering Types. :D:
I used 3 data points and 62, 72, and 82 mph and that would correspond to a "true" speed of 60, 70, 80 mph. I then "fit" in two other data points at 65 and 75 mph. Rounded off to two places on the last 2 columns.

MPH......MPG......Gal/Mi......Gal/Mi vs. 60MPH.....MPG vs 60 MPH

60........31.6.... .0316...........1.00 .................... 1.00

65........30.8.... .0325...........1.03 .................... .97

70........29.8.... .0336...........1.06 .................... .94

75........28.4.... .0352...........1.11 .................... .90

80........26.8.... .0373...........1.18 .................... .85

Reader's Digest:

Compared to 60 mph fuel increase/cost is:

3% higher at 65 mph

6% higher at 70 mph

11% higher at 75 mph

18% higher at 80 mph

http://www.subaruforester.org/vbulletin/f107/2200-mile-trip-25103/
Hi ADC,

Without seeing your data and computations I will venture to say yours is a very good empirical effort.

Edit: Whoops! adc- I just took another gander at your figures and they do seem to follow an exponential curve. I have no excuse for my earlier thinking it was linear. Sorry for that

Everyone else can just skip to the next post.

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If I take the last result in my above post and multiply by the Forester's cd - .33, I get 7% which is in the ballpark with your numbers. Yes, I know that is not entirely correct but it should be acceptable for the sake of an approximation. Forgetting calculus (as I did decades ago) I'd say we agree!

We may be even closer though... In looking at your "fit" numbers it looks like you may have used a linear fit in place of an exponential decay. Am I correct? To see what I am getting at, look at the curves in your post of 12/01/2007 in the same thread. All of the cars show curves declining toward zero as opposed to straight lines.

Straight lines will intersect with zero mpg whereas the curves approach but never quite reach zero. If my supposition is correct, compensating should raise your percentages for 65 and 75 mph a bit making our results more closely aligned.
 

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2012 Forester 4 speed auto
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Torque converter lock up

Have the dealer take a ride in the Forester at 70-75 mph with his computer hooked into your car's computer and check that the torque converter in being locked up. If for some reason it isn't, that is where your gas mileage is slipping away. Pun intended.:cool:
 

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2017 2.5 Premium & Base CVT
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Discussion Starter · #10 ·
Great info ya'll.

I guess your stating it's normal for such a large drop in mpg just between 70 and 75 mph.

All my past cars got much better mpg averages than my on my regular commutes on these road trips.

This is the first car where I get worse mpg on road trips with no stop n go, many cold starts like the average daily commute.
 
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