Gas Mileage on the Interstate

[FeuerFrei]

This year Illinois passed a law increasing the speed limit to 70 MPH on most rural interstates. I am pretty pumped about this considering I drive about 40k miles per year. I know that EPA highway MPG ratings are based on real world driving conditions and aren't optimized for setting the cruse control and taking a nap. Having said that, I am very disappointed with the highway mileage on my 2013 Elantra. In order to get close to the EPA rating of 38 MPG, I have to be driving around 55 MPH. I tend to get around 30 MPG if I push it north of 70 MPH.

My question is this, are there any cars out there that are geared to run efficiently at faster speeds? I understand that air resistance is going to play a major role, but I almost wonder if I would be getting similar gas mileage on the highway in a slightly larger car with a bigger engine. If nothing else, the ride would be much more comfortable and I might not lose that much on my overall gas mileage.

Maybe its your car? Or gasoline?

Last weekend I rented an Elantra (2013?) and drove it 360 miles, averaging just under 36 mpg by the car's calculation. Most of the way I was driving at the legal 75 mph limit, dropping to 65 on approaching the metro area. I did use cruise control now and then. Temps were in the 40s. Car had 10112 total miles when I was done.

36 mpg is respectable - that's Corolla territory. Though Corolla can reach 38 easy.

 

[ElMonoDelMar]

Difference in tires?

Probably that and a much lighter vehicle weight.

 

[Hail The Brain Slug]

My brother has a 2014 Chevy Cruze diesel and he set his MPG record of 70.6 with cruise control on at 70 MPH in the desert (super flat). I'm envious since my Cruze is the turbo gas model and I'm lucky to get 38-40 on a flat at 70.

After asking him, his trip average for an all-freeway and highway trip is typically 45-55. I've had some wild variation in mine, getting as bad as 33/34 on one freeway trip but on average I get 35-38 on most freeway.

 

[xanis]

I have a '12 Sonic and I get somewhere around 34-35mpg if I keep the cruise between 65 and 70.

 

[Murloc]

This year Illinois passed a law increasing the speed limit to 70 MPH on most rural interstates. I am pretty pumped about this considering I drive about 40k miles per year. I know that EPA highway MPG ratings are based on real world driving conditions and aren't optimized for setting the cruse control and taking a nap. Having said that, I am very disappointed with the highway mileage on my 2013 Elantra. In order to get close to the EPA rating of 38 MPG, I have to be driving around 55 MPH. I tend to get around 30 MPG if I push it north of 70 MPH.

My question is this, are there any cars out there that are geared to run efficiently at faster speeds? I understand that air resistance is going to play a major role, but I almost wonder if I would be getting similar gas mileage on the highway in a slightly larger car with a bigger engine. If nothing else, the ride would be much more comfortable and I might not lose that much on my overall gas mileage.

In europe it's 75-80 mph everywhere on motorways and people don't buy bigger cars because of that.

It's really all about the attrition I think, bigger cars generally have lower RPM when going 80 mph with the highest gear though, while cars like the VW Polo are constantly at 3500 rpm, I don't now if this is compensated by the lower weight though (regarding fuel consumption).
The elantra looks big enough for comfort but I have no idea if it's cheap or not.

 

[SparkyJJO]

Far too often people think smaller engine = better mpg, but that is not always the case, at all. You need enough HP and TQ to move the car efficiently, and typically if you have more torque lower in the power band you'll be able to hold higher gears better and burn less fuel.

Then of course there is the transmission gearing, vehicle weight, aerodynamics, etc...

EPA numbers are a rough guide but not all that useful sometimes. They said I should have only gotten 28mpg out of my old car, then revised down to 26, while I was getting 32 without even trying. I now get around the revised ratings on my current Camaro, but only after it has been modded and cammed, burning more fuel than stock

 

[Zorba]

FYI: My 2006 and 1999 Subaru Legacy but got right at 30 mpg, from about 65-80mph. My 2014 Forester, gets mid to low 30s until about 73 then starts dropping like a rock.

One thing about air drag, is all other things being equal, drag goes up with the square of speed. So the difference between 65mph and 75mph is a 33% increase in drag, going up to 80 from 65 is a 51% increase in drag. The next question is what percentage of losses are coming from air drag, versus other areas like drive train inefficiencies, etc. I think on my forester, air drag must be a pretty high percentage of the loss, where it is much lower in my legacies.

 

[phucheneh]

Far too often people think smaller engine = better mpg, but that is not always the case, at all. You need enough HP and TQ to move the car efficiently, and typically if you have more torque lower in the power band you'll be able to hold higher gears better and burn less fuel.

Very much this. I drive a 4cyl 5spd econocar, and I'm very aware that there are a lot of V8 cars that can get better mileage on the interstate. The most common example is Corvettes and F-bodies.

Something like the 4th gen F-body was pretty slippery for its day, but overall not too much different in drag between it and my 3. And it doesn't even need the six speed manual (.5:1 6th gear) to get 30mpg- I've seen it in the 4spd autos (due to the single OD gear still being somewhat deep, plus I think they had 2.73 rear ends). They can comfortably cruise in the 2000-2500rpm range with the throttle barely cracked open.

In comparison, my car likes to cruise at more like 3k. At 80mph in fifth, I think I'm edging 4000rpm. Giving me an extra super-overdrive gear would not do a thing to benefit mileage...I'd just be at a lower RPM with the throttle open further. Mileage could even worsen...the more the car has to fight drag, the worse the engine is going to be lugging (as in, struggling under high load) at low RPM.

 

[KentState]

http://www.latimes.com/business/aut...awsuit-20131223,0,2442041.story#axzz2pTXnLJ2T

There was a lawsuit over Hyundai overstating MPG and the Elantra was one of the vehicles. Not sure if the mileage you are expecting is after the numbers were fixed.

 

[ElFenix]

What really sold me on the Elantra (other than the inflated MPG ratings) was how large the cabin felt. I am 6'2"/210lbs and with as much time as I spend in a car, interior space was important. I have much more leg room than I did in any of the other cars I drove (Corolla, Sentra, Focus, Cruze, Jetta and Mazda 3). The car is still really comfy to sit in, it's just been a disappointment in most other areas.

the elantra is midsize, the focus, corolla, jetta, and mazda are not. of the other 2, the sentra has the same pax volume while the cruze has 2 cu ft less passenger volume.

 

[dbk]

CVTs?

 

[bguile]

the elantra is midsize, the focus, corolla, jetta, and mazda are not. of the other 2, the sentra has the same pax volume while the cruze has 2 cu ft less passenger volume.

Isnt the sonata the midsize and the elantra the compact?

 

[ElMonoDelMar]

http://www.latimes.com/business/aut...awsuit-20131223,0,2442041.story#axzz2pTXnLJ2T

There was a lawsuit over Hyundai overstating MPG and the Elantra was one of the vehicles. Not sure if the mileage you are expecting is after the numbers were fixed.

I bought mine after they dialed the mileage down from 40 to 38.

Isnt the sonata the midsize and the elantra the compact?

That's what I was thinking, but maybe I'm wrong.

 

[bguile]

I bought mine after they dialed the mileage down from 40 to 38.



That's what I was thinking, but maybe I'm wrong.

I would hope you know if your car is a midsize or a compact.

Though I am pretty sure the elantra is the compact. I used to always get them mixed up because elantra makes me think of elegance in my mind, so I always assumed its the bigger car.

 

[MDE]

I'm averaging 42 MPG in my 2012 Jetta TDI in northern IL.

 

[marvdmartian]

Realize first that car manufactures aren't going to give you mileage for 70, because they'd all drop their numbers significantly (and the government doesn't say they have to).

That being said, I'm a bit surprised that you're only getting around 30MPG at 70MPH.

My 2009 Hyundai Sonata GLS (curb weight ~3500 lbs, 17.7 gallon gas tank, rated at at 22/32/25 MPG) gets just about the same highway mileage at 75MPH as you're experiencing. I've done two trips from Texas to Georgia (primarily driving I-20 across), and experienced better than rated mileage on a tank of gas each time.....even in the summertime, with the A/C running!

Meanwhile, your Elantra (assuming also a GLS sedan) is 2700 lbs curb weight, with a 12.8 gallon gas tank, is rated at 28/38/32, and getting only around city rated mileage on the highway? Might be worth a trip to the dealer, while it's under warranty, to ask them if they can run a diagnostic on it. Either that, or consider that your cruise control might be hammering the throttle too hard on hills, or your highway driving is affecting it somehow.

 

[phucheneh]

Meanwhile, your Elantra (assuming also a GLS sedan) is 2700 lbs curb weight, with a 12.8 gallon gas tank, is rated at 28/38/32, and getting only around city rated mileage on the highway? Might be worth a trip to the dealer, while it's under warranty, to ask them if they can run a diagnostic on it. Either that, or consider that your cruise control might be hammering the throttle too hard on hills, or your highway driving is affecting it somehow.

It's really not. A great way to guarantee you'll always recieve terrible warranty service is to take a car in and complain about things that 1) you have no evidence of 2) are vastly different depending on driving style and 3) cannot be duplicated by the mechanic.

The best of places will entertain you...the advisor will write a line for the mileage complaint, and they won't charge you for diag if it turns out that you're wasting their time. If they can't find a problem, they can't file a warranty claim. With the exception of some makes allowing a limited number of 'no fault found' warranty claims. But generally...they want money for their time.

And the mechanic gets screwed no matter what, so he will not give half a shit. Hopefully you don't have other complaints, 'cause the mileage one is going to sour him on doing a good job with the others. Not saying it's right, but it's the truth.

So basically, if you truly suspect something is wrong with your car that has no symptom other than low mileage (no CEL, no problem with the way it performs, ect), you need to start logging what's going on and present a convincing complaint. Measure precisely how many miles you're getting per a precise amount of gas (like a full tank to the fuel level light, with confirmation of the same number of gallons pumped each fillup) and then...well, you're still kind of boned if you can't prove that the mileage used to be significantly better while doing the same type of driving. Then I'd ask...trouble with the wife? Trouble at work? Just generally feeling angry?

Something as simple and silly as a change in mood (which then affects driving style) is more than enough to account for how much fueling can change within the bounds of an OBD2 system (how much the PCM can up the fueling without triggering a CEL).

 

[Blades]

I'm surprised no one has mentioned gear ratios. Although I've never heard of a gear swap for Hyundais. In my turbo trailblazer SS (20mpg highway) I run open loop and command what the Australians call "lean cruise"; or an air fuel ratio of 15.5 for moderate loads. It helps.

Clean the MAF/IAT sensor.. change the air filter too. If the aforementioned "change in mood" is believed to impact your fueling (highly doubt it), then reset the fuel trims.. Laziest way to do this would be to pull the fuse for the ECM or disconnect battery for a little bit.

Depending on how much ethanol is in the gas and what grade you use... the results can vary between from person to person, area to area. More ethanol in there, quicker it will burn.. Same for premium gas, it'll burn quicker. Worse with the ethanol if your car has a flex fuel sensor and adjusts stoich to match for the blend of ethanol..

Put that pig on a dyno and tune it!

 

[Pandora's Box]

I have a 2013 Elantra too. Winter kills the MPG. I do mostly highway driving, get about 33-34 MPG in the winter. In the summer it shoots to 38-42. Winter blend gasoline, and the temperatures is brutal on MPG. Speed limits around me are 65 to 70mph. The stock tires on the car suck for winter driving, get yourself a set of winter tires/wheels.

 

[Aaviel]

That seems pretty low.... I can get 37-40 on my f30 328i if I do 65-75.

The winter air + winter tires + winter gas destroys that notion though. I'm currently averaging around 33-34.

 

[ElFenix]

Isnt the sonata the midsize and the elantra the compact?

nope, sonata is a large car.

click on specs

 

[phucheneh]

nope, sonata is a large car.

click on specs

Meh. They're mistaken. Elantra is a compact. Competes with Civic, Corolla, 3, Focus, ect.

Sonata is a midsize and, mirroring the examples above, competes with: Accord, Camry, 6, Fusion.

The Accent is their subcompact and the Azera is the fullsize.

On a totally random note, why have I never noticed that Honda does not make a fullsize to compete with the Maxima and Avalon? That's weird. Car companies are usually all about filling in every little niche, and Honda is the only company I can think of that's missing one of the big four mentioned.

 

[ElFenix]

Meh. They're mistaken. Elantra is a compact. Competes with Civic, Corolla, 3, Focus, ect.

Sonata is a midsize and, mirroring the examples above, competes with: Accord, Camry, 6, Fusion.

The Accent is their subcompact and the Azera is the fullsize.

On a totally random note, why have I never noticed that Honda does not make a fullsize to compete with the Maxima and Avalon? That's weird. Car companies are usually all about filling in every little niche, and Honda is the only company I can think of that's missing one of the big four mentioned.

they're not wrong. size classes are based on cubic footage of passenger + cargo volume. anything >=110 and <120 cubic feet is midsize. the elantra is a fairly big car inside. it's bigger than the 1996 accord was.

the current accord is larger than the maxima, and only 1 cubic foot smaller than the avalon. both the avalon and accord have shrunk with the latest model change, previously they were the same size.

 

[LTC8K6]

nope, sonata is a large car.

click on specs

Those specs would not really be related to comparisons of the size of the car for aerodynamic/mileage considerations. They are based on interior volumes.

When talking about large and small for mileage discussions, it's better to simply compare using the actual vehicle dimensions and compare like sizes.

 

[skyking]

If you drive manual transmissions, the diesel Passat does well.
http://www.fuelly.com/car/volkswagen/passat/2014/diesel%20l4/sedan