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MPG plunges from 70 MPH to 80 MPH -- do other cars do this?

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MPG plunges from 70 MPH to 80 MPH -- do other cars do this?

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  #1  
Old 05-16-2018, 08:20 PM
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MPG plunges from 70 MPH to 80 MPH -- do other cars do this?

I drive to Los Angeles quite a bit. I generally set the cruise at 80 MPH and consistently get about 33-34 MPG. On a recent trip, I set the cruise to just 70 MPH and found I got 39-40 MPG. A jaw-dropping improvement. (My Fit is the 5MT).

Is the Fit just a weird shape (tall)? It seemed like the old 2000 Toyota Corolla I could cruise along at 90 MPH and get 40 MPG on the freeway.

Thoughts from your experience?
 
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Old 05-16-2018, 11:29 PM
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do other cars do this?
Absolutely.
Wind resistance increases at an exponential rate as your speed increases.


My daily is a 12 Civic.
At steady 55 MPH I can get over 40 MPG.
(on the original wheels and tires I managed to get like 53 MPG--strictly highway)

At 80-85+ on the interstates it can drop to less than 30 MPG.

Of course city stop and go traffic immediately kills those high numbers.
 
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Old 05-17-2018, 06:00 AM
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I have an Ultraguage monitor plugged into my OBDII and I can watch fuel consumption in Gallons per Hour (Gals/Hr). It "says" that it is in the +3 Gals/Hr range around 80 mph and up. In the +/- 60 mph range, it "says" it is less than 2 Gals/Hr.

There might be an interesting graph in my future.
 
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Old 05-17-2018, 11:07 AM
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Originally Posted by larrymcewin View Post
I drive to Los Angeles quite a bit. I generally set the cruise at 80 MPH and consistently get about 33-34 MPG. On a recent trip, I set the cruise to just 70 MPH and found I got 39-40 MPG. A jaw-dropping improvement. (My Fit is the 5MT).

Is the Fit just a weird shape (tall)? It seemed like the old 2000 Toyota Corolla I could cruise along at 90 MPH and get 40 MPG on the freeway.

Thoughts from your experience?
The little motor in the Fit just has to work really hard doing 80. I regularly take my Fit on road trips, and 80 just tanks the MPG's. I get about 33-34 to Santa Fe.

By contrast, my 2012 Focus (I think it's 2.0 litre) gets worse in town mileage, but substantially better highway mileage, because that motor isn't working quite so hard to do 80. It's got about 40 more horsepower and I'm not sure how much more torque. I get about 38-40 to Santa Fe.
 
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Old 05-17-2018, 01:37 PM
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Originally Posted by larrymcewin View Post
I drive to Los Angeles quite a bit. I generally set the cruise at 80 MPH and consistently get about 33-34 MPG. On a recent trip, I set the cruise to just 70 MPH and found I got 39-40 MPG. A jaw-dropping improvement. (My Fit is the 5MT).

Is the Fit just a weird shape (tall)? It seemed like the old 2000 Toyota Corolla I could cruise along at 90 MPH and get 40 MPG on the freeway.

Thoughts from your experience?
It's perfectly normal with the Fit and all other boxy shaped cars. The faster you go, the more you will notice this. Your 2000 Corolla was more aerodynamic than any generation of the Fits.

If you have the time & patience you should try once to set the cruise at 60-65 MPH and you can expect 45 MPG.

I have a SGII always plugged in and I like to monitor my instant and average fuel mileage most of the time I'm driving. I can tell you for sure that driving on cruise is worse for fuel economy than driving with your foot on the pedal, especially if there are any hills. I don't drive on completely flat roads, so I don't know if things would equal out more on flat lands.
 
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Old 05-18-2018, 10:37 AM
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Sometimes with coefficient of drag, it's not so much size as it is shape.
According to a Road and Track review of the 2015 Honda Fit, the coefficient of drag was improved on the Honda Fit, even though no numbers are offered.

With under plating, and an oval wedge shape, my guess would be the Honda Fit actually has a very good coefficient of drag.

I'd much more explain the dive in MPG in a Fit at 80 mph, as being caused by going 80 mph...in a 4 cylinder Honda Fit.
The more you tax the engine, the harder it works, the less fuel economy you will get.
 
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Old 05-18-2018, 12:15 PM
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Add the 4.62 final drive ratio for the GE Fit into the equation as well. Turns more rpm on the highway than most sport bikes and doesn't have a high revving motor.
 
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Old 05-18-2018, 12:31 PM
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the Fit does fantastic up to about 65mph. then drag and all just eats up the mpg.
this is why i say the Fit is a great surface road car.
 
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Old 05-18-2018, 12:44 PM
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MEGA off topic.... have any of you scan gauge, and ultra gauge users ..... had the honor of testing out BOTH products? I'd like to see if theres any benefit of going with the scangauge, as it is 2x more expensive than the ultra guage. I value your input gents/ladies
 
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Old 05-18-2018, 06:18 PM
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I think that having a relatively flat back, vs. a trunk, makes it less aerodynamic.
 
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Old 05-19-2018, 12:35 PM
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i dont see the point of those obd2 monitors. might be a fun toy for a few months but unless ure using it to read dtc's and wat not, they are honestly meaningless. ull forget to even look at it after a while.
 
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Old 05-19-2018, 12:49 PM
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I kind of agree. Without the ability to alter the ECU programming, they are mostly novelty unless you're using them to absolutely maximize fuel mileage.

Back to the OP. I would say that all cars do it, but the speed at which it happens varies greatly. I've noticed very little economy difference between cruise set at 70mph and cruise set at much higher speeds in my Lincoln. At 100mph it's still turning under 2500 rpm and isn't in boost. Push it much past that and it starts needing boost to overcome the wind resistance.

It gets between 25 and 27mpg on the highway. Neither are great mileage, but it's about the same either way.

The pick up trucks I've owned have been the most drastic for speed vs economy. They have to move so much air that they just become incrementally less efficient with every 1mph increase in speed it seems.
 
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Old 05-19-2018, 01:15 PM
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i dont see the point of those obd2 monitors. might be a fun toy for a few months but
Many of us don't have a temp gauge on the dash. That was the main reason I got an ultragauge for my car.



Plus, I'm a tech and I like to be able to see live data sometimes, even if the UG can't display all parameters that I would like to see.
 
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Old 05-19-2018, 01:19 PM
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Originally Posted by ezone View Post
Many of us don't have a temp gauge on the dash. That was the main reason I got an ultragauge for my car.
I did the same thing, but to his point, once I found what the car runs at over the course of a few drives, I never looked at it again.

If the high temp light ever comes on, I'll plug it back in to check.

That being said, it's really just a fault of our cars that we don't have adequate gauges. Most cars show you all vitals with the factory gauges.
 
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Old 05-19-2018, 02:34 PM
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Originally Posted by Brain Champagne View Post
I think that having a relatively flat back, vs. a trunk, makes it less aerodynamic.
I honestly don't know. Because Honda isn't readily making the the drag coefficient numbers available.

But I do know, The Toyota Prius has a low coefficient of drag by design.

That oval wedge shape, does great in a wind tunnel.
And The Fit really embraces a similar shape.

There aren't a lot of squared edges on The Fit.
To me?
Non-Aerodynamic "penalty" comes in with vehicles like the KIA Soul. Now that is a non-aerodynamic box. And still it get's pretty good MPG.

I'm convinced the loss of MPG in this case has much more to do with driving at 80 mph than anything else.
 
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Old 05-20-2018, 11:57 AM
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Originally Posted by kenchan View Post
i dont see the point of those obd2 monitors. might be a fun toy for a few months but unless ure using it to read dtc's and wat not, they are honestly meaningless. ull forget to even look at it after a while.
I'll keep this reply simple, in an effort to not clutter the thread

there's more to a scangauge than novelty. it provides live data, its damn useful. example... when I go to work and climb a steep hill.... should I keep it in 3rd gear? should I keep it in 4th gear? how fast should I go? the gauge tells me (live), my mpg is X when I'm going X mph in 3rd gear. and then, I would test it again in 4th gear instead and see what the impact is on mpg. for those who care about mpg, its useful. cleanmpg, ecomodders, and a few other forums may give better insight as I havent owned my for long. and hence, why I asked if the UltraGauge is any better... cause its like 2x cheaper

I will say tho ... most of us can get by with the oem "0 - 45 mpg - 90 mpg" gauge just fine....
 
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Old 05-24-2018, 01:48 PM
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The flat back of the Fit is the biggest un-aerodynamic feature.
Sure the back part of the car narrows down a bit into a kammback feature (similar to a Prius), but it's simply not long & narrow enough to be all that effective, so it's still very much a flat back car.

Sure that making all corners smooth & round helps a bit, but the overall shape plays a much larger role than minor items like bumpers, headlights, corners, etc

Here's the most aerodynamic car shape:

As you can see from the 'rain drop' design, the Fit is not aerodynamic at all.
Unless you drive it in reverse!!!

I've been using my ScanGuage2 for 3-4 years (since my old car) and love it. But I'm usually trying to maximize my MPG and it helps a lot.
The parameters I always look at are:
* instantaneous MPG
* average MPG per trip (the car's computer is optimistic by 5-10% most of the times, while the SGII can be calibrated to match exactly with pump results)
* oil/coolant temperature (especially since I use lower grille block in the winter)
* engine load (since it's MT and don't want to ever lug the engine by mistake)
I generally like seeing all the different parameters in real time under various driving conditions.

I've never used the UltraGuage but picked the SGII instead because it looks more inconspicuous and a lot less desirable than the UG, so it's easier to 'hide' in plain sight and never worry about thieves.
Also the SGII has more features and reads more parameters than the UG, which is why you basically pay more.
 

Last edited by Andrei_ierdnA; 05-29-2018 at 12:56 PM.
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