MPG is very sensitive to our driving habits

[dlee1001]

I have a 2014 Sonic that is a 1.8L AT, and I have some things to share about its fuel economy. I often glance at my trip odometer and the fuel gauge, noting how many miles it takes for the fuel gauge to drop 1 bar. I just topped off my fuel tank today and reset both the trip odometer and average MPG to manually calculate fuel economy the next time I fill up (last time I did, I got 34 mpg, which is okay).

Below are my observations after having the car for about 6 months.

If I drive very conservatively (by accelerating slowly and gradually, maintaining my speed to conform to speed limits, etc.), I can go between 27-34 miles per fuel bar. However, if I drive like a stereotypical teenager would (accelerating fast, making the engine rev unnecessarily in the process), I've seen the fuel bar drop one bar after only about 15-20 miles since the last bar drop. I only use 89 octane gasoline in my Sonic.

Since the Sonic holds 12.2 gallons of fuel and the fuel gauge displays 12 bars, it makes sense that each bar roughly represents 1 gallon. It's worth noting, however, that my average MPG always reads in the mid- to upper-30s, no matter how I drive it. Sometimes I drive one way that is 150 miles long (90% hilly highway hovering around 60 mph), and I find that no matter how I drive it during that trip, it always uses 4 bars of gas between departure and arrival.

The scenario in the third paragraph leads me to believe that the Sonic is extremely sensitive to the way we drive it. Drive it like you baby it, and you'll get great MPGs. But drive it like a race car, and watch your fuel bars plummet.

Has anyone noticed the same thing?

 

[Papasonic]

I agree with your observations. I've noticed that the smaller the engine, the wider the fluctuation in fuel consumption, due to load, speed, and temperature.
The vehicles with larger displacement engines typically have more consistent fuel consumption under similar conditions.

 

[dlee1001]

This was partly why I chose the 1.8 over the 1.4, even though it was a turbo and gets better gas mileage. (Actually, Consumer Reports did a study on this and found that the turbos didn't have as big an advantage in fuel economy as we like to think. See http://www.consumerreports.org/cro/...on-t-deliver-on-fuel-economy-claims/index.htm for details.)

In the area I live, the terrain is very hilly, and I thought nothing can beat a larger displacement engine, even if it's not a significant difference. And it's why I need 89 octane. But I digress.

I suppose then the 1.4 turbo has even more fluctuations in gas mileage than the 1.8 due to the smaller displacement?

 

[Papasonic]

My 1.4 turbo manual trans has huge variations in fuel consumption. In warm weather, no wind, no hills, lower speeds; I frequently can achieve in excess of 40 mpg. If cold temperatures, wind, hills, or higher speeds are factored in; Fuel consumption can decrease by 10 mpg or more. Winter driving conditions have the most detrimental effect on fuel consumption.
My wife's 1.8 auto has less variation in fuel consumption under similar driving conditions.

 

[RMinNJ]

Have to concur...my 1.4 Auto MPG goes down with the cold weather. Of course we get winter blended gas which cannot help.

 

[dlee1001]

Why do smaller engines fluctuate so much in gas mileage? I know there was mention of engine load, speed, and temperature as a cause, but I'm one of those people who likes to know why things happen the way they do.

I'm probably answering my own question with this analogy, but I'm going to give it a shot anyway:

A person with a small lung capacity runs out of breath much sooner than someone with a larger lung capacity when working strenuously, but if the person with the smaller lung capacity relaxes a bit, breath support is conserved. The person with the larger lung capacity has more air to work with, so the rate of consumption can be more consistent.

Would this be the explanation why smaller engines fluctuate in fuel economy more than larger engines, or does it work in a different way?

If appropriate, cue the Captain Obvious looks.

 

[Larrymer]

You don't need 89 octane and are wasting your money. Calculate actual MPG and don't try to correlate it to "bars."

 

[sonic21]

I've been putting 91 octane in my tank. I don't have a tune. Is that gunna mess up my car in the long run


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[Papasonic]

I've been putting 91 octane in my tank. I don't have a tune. Is that gunna mess up my car in the long run


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No. The manufacturing engineers specify fuel requirements based on the design and performance characteristics of the individual engines and the application they are used in. If a lower octane fuel is specified, using a higher octane is neither harmful nor beneficial; only more costly. There may be some performance gains using higher octane fuels in some engine designs, but definitely not required or harmful.
Fuel quality is unrelated to octane levels, and that is why Top Tier fuels are recommended.

 

[dlee1001]

You don't need 89 octane and are wasting your money. Calculate actual MPG and don't try to correlate it to "bars."

We know that the Sonic holds 12.2 gallons of fuel in the tank. So we can infer that one bar is roughly one gallon, even though zero bars means there's still a little bit of fuel left.

89 octane is better for cold cranking, especially in very cold temperatures. And I feel an increase in performance as well as fuel economy.

Also, as stated in the OP, I am in the process of manually calculating gas mileage by dividing the total number of miles driven since last re-fuelling by the number of gallons dispensed in the current re-fuelling. The last time I did this, I got 34.35 mpg.

Even in a much older car (a 2001 Toyota Echo 4AT, to be specific) that I used to drive in my permit stage a few years back, putting in 89 octane yielded much better performance and far better gas mileage than 87 could ever do, no matter what gas station we used.

So I am inclined to think that despite the recommend fuel being 87 octane gas for the Sonic, 87 octane is actually the waste of money, because in my experience, I just don't get that many miles per gallon as 89 octane offers, so the value for the money is less, especially since every gas station in my area says their gas contains up to 10% ethanol by volume, reducing performance and fuel economy even more.

 

[Papasonic]

We know that the Sonic holds 12.2 gallons of fuel in the tank. So we can infer that one bar is roughly one gallon, even though zero bars means there's still a little bit of fuel left.

89 octane is better for cold cranking especially in very cold temperatures. And I feel an increase in performance as well as fuel economy.

Also, as stated in the OP, I am in the process of manually calculating gas mileage by dividing the total number of miles driven since last re-fuelling by the number of gallons dispensed in the current re-fuelling. The last time I did this, I got 34.35 mpg.

Even in a much older car (a 2001 Toyota Echo 4AT, to be specific) that I used to drive in my permit stage a few years back, putting in 89 octane yielded much better performance and far better gas mileage than 87 could ever do, no matter what gas station we used.

So I am inclined to think that despite the recommend fuel being 87 octane gas for the Sonic, 87 octane is actually the waste of money, because in my experience, I just don't get that many miles per gallon as 89 octane offers, so the value for the money is less.

I have not realized the same results as you have described. In regard to cold starts, the fuel with the least % of ethanol will start more readily in extreme cold; not necessarily if the octane is 87, 89, 91, 93, or other value.

As far as octane is concerned, I haven't observed a noticeable difference in performance or economy using different octanes in my Sonic. I've achieved the same fuel consumption with 87 octane BP and 91 octane BP Ultimate.
Some owners have reported results different than mine; I'm just sharing my personal observations.

 

[Swordsmith]

When you run out of bars, the Sonic has roughly 2 gallons left. This can be verified by filling up, driving until you hit zero bars, then filling up again immediately rather than driving further. You'll get about 10 gallons in there. Further, you'll often be able to drive a substantial distance (I've seen as much as 60 miles) before the first bar goes away. Finally, I've seen the car go from 2 bars to 3 bars after driving about 30 miles... try to do the math on that!

The onboard mpg calculator isn't much better. I have a lifetime average of 37mpg displayed, while hand calculations show I rarely get over 33mpg from a tank, and since I have a flexfuel conversion and have run a lot of tanks of E85, I have many tanks where I averaged under 25mpg. You can't get an average of 37 with figures like that.

I agree however with your conclusion; your gas mileage will vary wildly depending on how you drive. As an experiment, I did an entire tank of E85 driving as I normally do, and got around 22mpg, then an entire second tank driving the same route with the same load (four people weighing over 200lbs each) on the same day, but driving gently; slow gradual starts, never exceeding the speed limit and so forth, and got around 33 mpg. Hypermilers push this sort of thing much harder, and get even larger improvements... and if you really push the engine hard, you can get much worse results than my normal driving.

If you want to really get an education, get an Ultragauge or similar device, which will allow you to monitor your instantaneous mpg; you can watch your mpg plummet to 9 as you pull away from a light with a lead foot...

 

[ahtoxa]

Sonic is extremely sensitive to the way we drive it. Drive it like you baby it, and you'll get great MPGs. But drive it like a race car, and watch your fuel bars plummet.

Cars are most efficient when driven conservatively. More breaking news at 11.


But in all seriousness - that's that case for every car out there. Some vehicles have a slightly larger margin and some have smaller. Generally, the larger the motor and the bigger the vehicle is, the smaller is the delta in MPG under variable conditions.

A small motor driven conservatively will yield great results, but when pushed, it has to work harder, if you will, so the efficiency will plummet.

 

[dlee1001]

When you run out of bars, the Sonic has roughly 2 gallons left. This can be verified by filling up, driving until you hit zero bars, then filling up again immediately rather than driving further. You'll get about 10 gallons in there. Further, you'll often be able to drive a substantial distance (I've seen as much as 60 miles) before the first bar goes away. Finally, I've seen the car go from 2 bars to 3 bars after driving about 30 miles... try to do the math on that!

The onboard mpg calculator isn't much better. I have a lifetime average of 37mpg displayed, while hand calculations show I rarely get over 33mpg from a tank, and since I have a flexfuel conversion and have run a lot of tanks of E85, I have many tanks where I averaged under 25mpg. You can't get an average of 37 with figures like that.

I agree however with your conclusion; your gas mileage will vary wildly depending on how you drive. As an experiment, I did an entire tank of E85 driving as I normally do, and got around 22mpg, then an entire second tank driving the same route with the same load (four people weighing over 200lbs each) on the same day, but driving gently; slow gradual starts, never exceeding the speed limit and so forth, and got around 33 mpg. Hypermilers push this sort of thing much harder, and get even larger improvements... and if you really push the engine hard, you can get much worse results than my normal driving.

If you want to really get an education, get an Ultragauge or similar device, which will allow you to monitor your instantaneous mpg; you can watch your mpg plummet to 9 as you pull away from a light with a lead foot...

I suppose then that the bar drops are not as linear as we think they are? Also, I never quite understood why it takes so long for the first bar to drop. On my full tank of gas this time, it took 49.8 miles for the first bar to drop.

 

[ahtoxa]

It takes a while for the first bar to drop because the float in the tank is submerged when the tank is completely full. The level of gas slowly foes down to where the float begins to move, and this can take a while.

Additionally, some vehicles have a slightly irregular tank shape, where the tap can be wider than the bottom, so it seems that the gas gets used faster when the level is lower.

 

[kevjam79]

I've been putting 91 octane in my tank. I don't have a tune. Is that gunna mess up my car in the long run


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No. The engine will actually increase longevity with higher octane. The large timing pull that 87 creates is harsh on all components and runs the cylinders and head hotter. IMO, all 1.8s should be running 91 or higher. The compression alone demands it.

On the latest TSB for the stutter/cold start, GM is requesting owners to run high octane, top tier fuels only, while they find a fix at TAC.




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[kevjam79]

Also, I have done a MPG track for the first 27 tanks. I ran three tanks of 87, followed by three tanks of 89 and 91 and 93. I saw 1 to 1.5 MPG increase with each octane during the summer. A high MPG of 35 and low of 27.

The biggest factors that affect the 1.8 is excessive idling and cold outside temps. On 91 top tier, my butt dyno seems to feel the smoothest power delivery. I know the on-board torque monitoring is limiting the total capability of the map but I am comfortable with that. Maybe a Trifecta tune someday.


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[dlee1001]

Recently I've noticed that my fuel bars are dropping every 20 miles consistently. I suppose that is due to the sub-zero temperatures in my area and remote starting almost every morning.

 

[ChrisJ]

Why is this a surprise to anyone?
If you think "small engines fluctuate a lot in gas mileage" you should try a bigger one.

All cars are sensitive to how you drive them.
Smaller cars with smaller engines are the least sensitive.

 

[ChrisJ]

No. The engine will actually increase longevity with higher octane. The large timing pull that 87 creates is harsh on all components and runs the cylinders and head hotter. IMO, all 1.8s should be running 91 or higher. The compression alone demands it.

On the latest TSB for the stutter/cold start, GM is requesting owners to run high octane, top tier fuels only, while they find a fix at TAC.




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I ran nothing but 87 in my 1.8 and I have run primarily 87 in both of my 1.4s.

I'll take the manufacturers' recommendation on what minimum is allowed over yours.

Does 93 produce more power on a 1.4? Yes. Did I see an increase in MPG to offset the cost difference between 93 and 87? Nope.