So, what *really* makes for good interstate fuel economy?

As my search for a new, more efficient/tolerable to sit in for 800 miles a week continues, I've found myself wondering what REALLY makes a car efficient on the interstate for long periods of time. This seems to be a tough question to answer, and the misleading EPA figures don't help. For example, I found a very reasonable New Beetle Turbo S for sale locally; basically a sportier 1.8T Beetle with a few more psi and a 6-speed. My thought was that the taller 6th gear would improve economy over the 5-speed on the freeway, yet the rated highway mileage is much worse than the regular 1.8T 5-speed! Confusing. And then you have sites like Fuelly, where it would SEEM you'd be able to realistically track MPG, but some people's entries are all over the goddamn board and a lot of people don't seem to understand how to drive for economy.

So, what makes for good freeway MPGs? Is it tall gearing? Light weight? Aerodynamics? Strangled engines? Are people REALLY getting 30+ MPG out of BMW 540i's on the freeway as is often claimed on this board? What gives? Considering I get paid for mileage and my driving will be 95% freeway, it makes sense to get something as efficient as possible, yet spending that much time behind the wheel also makes it a priority to get something that will be comfortable and pleasant...on one end of the spectrum would be something like a Toyota Matrix, which has the apparent economy but isn't terribly comfortable. On the other end would be something like an E39 528i, which is a very comfy ride but probably lacks the economy. So, what's the secret here?

Aerodynamics, smooth driving, use cruise control. Try and avoid accelerating or braking. I did get 30mpg on some occasions driving 80mph in our Mustang convertible, but the only way to do that was on cruise control. As soon as you are changing speed you're screwed.

Consistency in speed for as long as you can take is what it boils down to, pedal modulation is something important to pay attention to when you do have to take cruse off.

Over the years I've always found that our mid-size cars do better on freeway economy than my various hatch backs. I'm assuming it's mostly aero at work, because the hatchbacks normally do better in city driving. My gut says a 4 cylinder, manual trans ,4 door sedan would do well, like a Fusion, Mazda6,etc.

That, and duct tape all the door openings that aren't routinely needed.

I drive more miles than you, but always have to haul tools, tow, or haul people, so my options are limited.

The most comfortable long distance car I've ever driven is a VW Jetta. If I was just driving (and getting paid for mileage), it would be hard to beat a TDI in whatever configuration you want. If the wagon is a little less economical, who cares? What, 52 mpg instead if 55??

Plus, there are sport suspensions available, etc.

At highway speeds, aero is #1. After that is gearing. But it doesn't really matter if you are driving a TDI.

BTW, don't assume a 6th gear means taller final drive ratio. It may mean the manufacturer was trying to raise fleet performance by short shifting the lower gears to maximize in-town ratings.

2nd gen Priuses are now pretty cheap, and they are roomy. But the seats are not as comfortable long distance.

Both are available with Bluetooth, hands free, Sirius, etc.

There is a lot of very good info on this subject over on Ecomodder.

I got 29 miles/gallon @ 80 mph in my C6 6 speed on the highway. Torque, aero and gearing! That's the way to eat highway miles.

anything can get better mileage. I often find that my usual daily commute sees me getting better than EPA rated. When I had my 318ti.. I was routinely over 30mpg.. even my Disco see's better than EPA.. I usually get 16 to 18mpg in my mixed driving commute.

Now having said that.. like was mentioned above, you need to practice restraint. I rarely exceed the speed limit by more than 5mph and the average speed for my commute is around 50mph.. so your mileage will vary.

But.. I want to emphasize this. Comfort is key. Just because a car can do 50mpg.. will you want to sit in it? I found the Prius has some of the worst seats I have ever had the misfortune to park my posterior in (the MB 190e has the worst I ever sat in).. this would make me speed -more- just so I can get out of the car sooner

I would agree with the tdi suggestion I have no first hand experience though. On the highway I could get 40-42 with my 2013 focus that wasn't even really broken in yet. But, that was really trying.

Aero & gearing/efficiency.

Remember, HWFET doesn't have a lot of true highway crusing, so its possible that much of the test does not see the benefits of the 6th gear. Also, the Turbo S may have different aero than the Turbo. Both have a terrible drag coefficient.

The secret here is get over it and drive a Prius.

DeadSkunk wrote: Over the years I've always found that our mid-size cars do better on freeway economy than my various hatch backs. I'm assuming it's mostly aero at work, because the hatchbacks normally do better in city driving.

That's aero. Its easier for a long car to have a very low drag coefficient (Tesla, Lexus LS, etc.)

Each car seems to have a sweet spot. My DOHC Neon I just sold got 40+ mpg highway...as long as I kept my speed under about 105 kph. Above that it really plummeted. I think it's a combo of aero and rpm (gearing) for that car.

I think that part of it is a good ignition and fuel delivery system. My 328i gets much better fuel economy than my 91 Camaro. All of the difference isn't just from a larger engine with two more cylinders.

Of course low weight, low drag, correctly inflated tires all make a huge difference. No extra outside wind grabbers like a roof racks or even window draft blockers. And windows closed when at all possible. Other than that, I use cruise as much as possible though never to accelerate back up to cruise speed after a traffic slow down.

My most successful single tip is to coast more. I try to look far ahead & kick cruise off and coast when approaching slower traffic so I never touch the brakes. A hundred miles at a time with no brake applications and slow throttled up acceleration is one of my main goals.

If you own a slushbox. Try very hard to keep it from kicking down even one gear on the freeway.

Pretty much anything gets good mileage if a t56 is behind it and the final drive is right.

Light weight, decent aero, torque off idle to maintain a cruise, and a really tall top gear.

That's how i like to do it, it's less boring.

If you are asking about the beetle specifically, check gearing of the trans, just because it's more gears doesn't mean better cruising rpm, the 5 could be set up for economy and the 6 performance, and tire size/compound.

Accelerate firmly to speed then stay there.

Volvo got 36mpg on last trip. Not bad for a huge car with an old-tech turbo engine and automatic. Actually, that's freakin' awesome.

Rupert wrote: If you own a slushbox. Try very hard to keep it from kicking down even one gear on the freeway.

Or unlocking the torque converter, which is typically the first kickdown.

Here's another TDI convert! I bought a new 2012 Jetta a little over a year ago. I have yet to get less then 40 mpg and if the miles are mostly highway then your talking 50 mpg or better. My best to date was 52 mpg, all interstate with speeds from 60-70 mph. There are ways to boost even that to near 60 mpg but it requires a turbo back exhaust which eliminates the cat and soot filter assembly. It also requires a re-flashed ECU. Then you can even "roll coal". The above is not EPA legal and I have not done this.

Aero, gears, lean burn.

Speed. Everything else is nice but driving slower is the biggest single thing. Wind resistance goes with the cube of speed.

70 mph has 60% more resistance than 60

icaneat50eggs wrote: Speed. Everything else is nice but driving slower is the biggest single thing. Wind resistance goes with the cube of speed. 70 mph has 60% more resistance than 60

Wind resistance varies with the square of speed. Power varies with the cube.

my 86 Caprice with the carbureted 305/700r4/2.56 gear drivetrain combo would average 28mpg with the cruise set at 80 and the AC blowing ice cold, and it was like driving the most comfortable couch you've ever sat in down the road.

The true answer to high fuel efficiency is a hybrid or a diesel powered car.

In reply to jimbbski: Does adding an intercooler increase fuel mileage with a TDI?

I think a tall final gear helps quite a bit. Not as sure about direct injection. My skyactiv Mazda3 turns about 2600 rpm in 6th gear at about 79 mph, and gets 39-40 mpg doing it.

TDI all the way ! Can a Prius do 80 mph and get 50 + mpg ?