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Discussion Starter #1
in the Highway MPG thread, someone mentioned that he got on his 2.0:
With cruise set at 65 mph, average mpg is 32.0 mpg.
With cruise set at 63 mph, average mpg is 38.1 mpg.

is 63mph the magical Holy Grail optimum point?

and why does going 2mph more kill the mpg soooooooooo much?
 

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I'm not sure where the sweet spot is, but it could depend a lot on the road surface and wind.

I will tell you that 89mph is *not* the sweet spot. First time driving through rural Texas, I noticed that there was about a 10mpg difference between 79 and 89 mph.
 

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I posted those average mpgs you reference to.

The 'sweet spot' is going to be variable, depending on your drive. On smooth flat highways, I can easilly get 35 mpg at 70 mph. In my particular case, I commute over a series of mountain ridges to and from work, so attaining quoted epa values just isn't realistic.

I would suggest running your own tests and determining what you particular sweet spot is.

At a certain point, aerodynamic drag is going to destroy your mpg..... And when it does it will do so rapidly. 55mph to 60mph may not see much change, but 75mph to 80mph May easilly cut your mpg in half.

Incidentely, I was as shocked as anyone to determine how close the falloff is. I am also curious what will happen when I switch to summer tires, and what will happen to mpg in the summer heat.

I will keep updating my results.
 

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Drag increases exponentially with speed.
This is for a Cessna 152 but the idea is the same (parasite drag, if your inducing drag your having a bad day)
Looking at the red dotted line, parasite drag you can see going from 60mph to 70mph adds 15lb/pf of drag .
The car has a smaller frontal area of course however it also has surface friction and road grade and friction from the transmission to account for.
The smaller the frontal area the less drag and power required. That's why putting a fat wing on your car will reduce performance.

 

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I'm not sure where the sweet spot is, but it could depend a lot on the road surface and wind.

I will tell you that 89mph is *not* the sweet spot. First time driving through rural Texas, I noticed that there was about a 10mpg difference between 79 and 89 mph.
It's not 85 either. ...Not that I would have experienced a drop off in MPGs at that speed...

I think our sweet spot is somewhere between 55-65, depending on the road, incline and wind. Depending on where I'm driving, I get great MPGs at 55 up through 65. If I'm going downhill from like Flagstaff to Phoenix, AZ, then at 75mph, I still get 36 or so MPGs.
 

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Most likely the absolute BEST sweet spot is under 60mph. Mid to lower 50s is what I would guess. Engine RPMs are at their lowest and you're not fighting the wind too much.

Of course it's not very practical to be doing say 55mph on the highway with a speed limit of 70mph just because you're trying to squeeze a few more MPGs out of your gas tank and I absolutely HATE those random idiots that do this on the highway because they are the ones that always cause near accidents because people coming up behind them at 75-80mph get surprised and suddenly start to dodge left and right. I see this way too often (i'm a truck driver).

It's very possible that the whole world might lower our speed limits in an effort to "save the planet" of our greenhouse gasses... that is if the auto manufacturers aren't able to meet the required MPG goal set by our government by 2020 or whatever the deadline is.

Anyway, I usually beat the hell out of our Mazda 3 with 2.0L and manual tranny but hypermill like crazy whenever possible (usually coasting as much as possible if I see a redlight or slow down up ahead) and I always average 29-30mpg mixed city driving.

On all highway trips, it's usually around 32-34mpg at best doing 76mph on cruise control. I don't think i've ever tried to see what my best MPG can be in this car, but if I avoid redlining it to enjoy my Corksport exhaust note, and drive more efficiently, I'm sure I can reach the 38-40mpg advertised claim. This engine and chassis can definitely do it. Mazda did an awesome job on it.
 

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@Must Turbo This 3 Teach me yours ways. I get around ~30ish mpg at 80, but mine is a 2.0 stick so its geared lower. City driving sub 20 if I am having fun or 24 max hyper milling. Of course this is fully tuned on e85 so that might drop my numbers a touch.
 

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My best tank this summer was 41mpg and was 99% highway, probably on a trip to northern DC. I think my best tank between Hartford & home in NY (over a mountain) was 38 or 39. This was, of course, before I put the roof rack on.

Winter mileage has dropped to a consistent 32-36, most of those tanks have been 60%+ highway.

I set my cruise control at 72mph always.
 

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Discussion Starter #9 (Edited)
I posted those average mpgs you reference to.

The 'sweet spot' is going to be variable, depending on your drive. On smooth flat highways, I can easilly get 35 mpg at 70 mph. In my particular case, I commute over a series of mountain ridges to and from work, so attaining quoted epa values just isn't realistic.

I would suggest running your own tests and determining what you particular sweet spot is.

At a certain point, aerodynamic drag is going to destroy your mpg..... And when it does it will do so rapidly. 55mph to 60mph may not see much change, but 75mph to 80mph May easilly cut your mpg in half.

Incidentely, I was as shocked as anyone to determine how close the falloff is. I am also curious what will happen when I switch to summer tires, and what will happen to mpg in the summer heat.

I will keep updating my results.
thx.
just drove 120 miles (60miles each way) on a new tank of gas.
speed limit was 65mph. I usually drive 77. car avg= 37.5mpg at that speed.
this time I drove 72mph. car avg= 40.2mpg at this speed.

5mph slower over 120miles = 5 min slower or 2 1/2 min each way.
I think it's worth it for an almost +10% increase in mpg.

yeah it's a sample size of 1 but I expect to be doing this same route a few times times next year.
 

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I do a lot of highway driving and I stick to 70 mph 90% of the time (Bay Area/Sacramento) and I average between 39-45 mpg. City driving of course kills the mileage -- 30 mpg if lucky, but other than that, my highway mpg's have been consistent over the past 8000 total miles. Even when I do cruise control at 75 mph, my average mileage at the end of a long drive is still in the high 40's.
 

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@Must Turbo This 3 Teach me yours ways. I get around ~30ish mpg at 80, but mine is a 2.0 stick so its geared lower. City driving sub 20 if I am having fun or 24 max hyper milling. Of course this is fully tuned on e85 so that might drop my numbers a touch.

LOL. I don't know, I floor it constantly but coast to a stop slowly in 6th gear, hitting the clutch around 1,000rpm or so and blip the throttle into a higher gear if there's still plenty of distance before I come to a complete stop. When in gear and off the throttle, you're getting 99.9mpg, which is more efficiently then idling to a stop.

I don't know, me and the wife are constantly redlining and driving this thing like a stolen rental and average 29mpg at every fill up. I have receipts going back to 2014 when we bought it, 29mpg is written on practically every fill up.
 

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I got heavy winds with roof rack and 4 snowboards .... I was getting 20 mpg going 80....
 

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A general rule is that the slower you go in the highest available gear, the better the fuel economy is going to be.

So, the "sweet spot" is the "best" combination a driver chooses between a self-defined "best" speed and "best" economy.

Any dramatic change in fuel consumption between a two or three miles-per-hour change in speed is a misrepresentation of what actually happened during the test. Once in high gear, the mileage difference is much more linear except when outside influences set in, such as wind, rain and elevation changes.

Speed costs, but time is considered valuable too. What's the best compromise for one may not be for the other. The "sweet spot" is a psychological choice similar to what one goes through in choosing a mate. One person can't choose it for the other.
 

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I have a 2015 iGT hatchback. For school commute I get average 40-41 mpg on cruise 62-64 for a 1 hour and half round trip but I really never touch the brakes either if that makes a difference. And when I go the other way to see my girl I drive between 40-60 mph depending on lights 30- 40minutes drive time I would get 45-47 mpg which is crazy. I'm very light footed and rarely press the brakes like I wont touch the gas if I see red lights coming up. I'm always staring at that fuel app lol, and I'm sure being in Texas the people think I'm a old person driving. I just did a trip to Missouri from texas which is roughly 1200 miles round trip, averaged around 33-34 mpg at 80 cruise. Definitely didn't like that number but I didn't want to drive for 11 hours one way lol. Either way this car has saved me so much in gas compared to my old daily 91 Camaro I was getting 12-14 mpg lol.
 

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LOL. I don't know, I floor it constantly but coast to a stop slowly in 6th gear, hitting the clutch around 1,000rpm or so and blip the throttle into a higher gear if there's still plenty of distance before I come to a complete stop. When in gear and off the throttle, you're getting 99.9mpg, which is more efficiently then idling to a stop.

I don't know, me and the wife are constantly redlining and driving this thing like a stolen rental and average 29mpg at every fill up. I have receipts going back to 2014 when we bought it, 29mpg is written on practically every fill up.
This is exactly how I drive my 2.0L however I drive about 90% Hwy and have been getting 6.0-6.3L/100km (39 - 37 MPG) based on the computer for the last 6 months.

Also regarding in gear and off the throttle, the injectors actually shut off and no gas is being fed into the engine. in Imperial mode the car's lowest gas usage to show is 99.9mpg whereas in metric it actually shows 0.0L/100km.
 

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This is exactly how I drive my 2.0L however I drive about 90% Hwy and have been getting 6.0-6.3L/100km (39 - 37 MPG) based on the computer for the last 6 months.
Whenever I see l/100k values I feel the need to point out 2 things, as I assume they are Canadian.

1. Often the conversion from L/100k is into Canadian or UK gallons, which are significantly different than US gallons (the above poster converted correctly)

2. I know Canadian fuel standards are much higher than the U.S. It is pretty common across the world for fuel quality to get better where winters are harsher. Equatorial countries can toss complete garbage in their engines and they still run fine. Also Canada doesn't have the ethanol requirement the states does, (which slightly damages MPG).

Of course L/100k could be anywhere else in the world except the U.S., Liberia or Myanmar, but the fact is I only know Canadian fuel information.
 

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Whenever I see l/100k values I feel the need to point out 2 things, as I assume they are Canadian.

1. Often the conversion from L/100k is into Canadian or UK gallons, which are significantly different than US gallons (the above poster converted correctly)

2. I know Canadian fuel standards are much higher than the U.S. It is pretty common across the world for fuel quality to get better where winters are harsher. Equatorial countries can toss complete garbage in their engines and they still run fine. Also Canada doesn't have the ethanol requirement the states does, (which slightly damages MPG).

Of course L/100k could be anywhere else in the world except the U.S., Liberia or Myanmar, but the fact is I only know Canadian fuel information.
Majority of people in Ontario talk about MPG in the US gallons whereas some advertisements post imperial MPG to make their cars sound more fuel efficient:|.

However recently every gas station (in ontario at least) will include up to 10% ethanol except for a select few stations offering zero ethanol in their 91 and 94 octane fuels.
 

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i guess we should remove situations with a lot of coasting such as going down mountain roads for hours.

so what ive seen so far for me 2liter 6mt
Conditions: driving along a flat, no wind, repaved surface.
Cruise control (or light foot) in 5th gear at 33-35mph, i can maintain 60-65mpg.
i dont see much improvement when i have the car in 6th, actually it might use more gas if i tried to accellerate a little bit since theres no torque in 6th at 35mph.
 

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Cruise control set on 55-60mph on flat country roads gives me instantaneous numbers fluctuating around 53mpg (45-62 actually). I've never gone a full tank in those conditions so I can't confirm the carputer's accuracy. It's been overstating by a couple mpg lately, compared to hand calculations at the pump.
 

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2016 2.0l sedan with manual transmission.
I frequently drive up and down from LA to SD. my one way mileage is around 110 miles. Making the trip at 55-60mph i received 45mpg. At 65-70 mph i get about 39-41mpg. At 75-80mph i get about 37mpg.
 
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