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

I have studied this extensively. Seriously. All the way down to how MPG is affected by coolant specific heat capacity. I'm on it.

But right now its past my bedtime. I will report more later.

In reply to scottdownsouth:

Probably, but that's an unfair comparison for numerous reasons. Hybrid systems don't do anything on the highway, diesel is a more energy dense fluid, a Prius is probably cheaper too.

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

Maybe. Can a TDI?

VW's own Jetta Hybrid is rated at 6mpg more on the highway than its TDI, and 12mpg more in the city. And the Prius has a more efficient drivetrain, so it wouldn't surprise me to see it out perform it.

Kenny_McCormic wrote: diesel is a more energy dense fluid

Many places its significantly more expensive as well.

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

Yup. Aero and lean burn.

In reply to ProDarwin:

Around here it's a whole dollar more than regular right now.

80¢ to 90¢ more here

my new Sonic hatchback can't get quite as good a gas milage as a friends Cruze (same drive train) … I put it down to the aero

Kenny_McCormic wrote: In reply to ProDarwin: Around here it's a whole dollar more than regular right now.

$2.49 for 87, $3.89 for Diesel here. Still $2.99 for 93 instead of the normal 20-30 cent breakpoint over 87. Oh well, that is still really really cheap.

On the other hand, low fuel prices worry me, because they usually signify a faltering economy. And low fuel prices mean low scrap values so the scrap thieves have to work extra overtime stealing peoples' cars and valuables. I don't quite understand the correlation, but that's the explanation I was given when we were quoted $500 for a Corolla we were going to scrap, then two weeks later it was only $300. That was when fuel prices dropped massively when flounder flounder 2008.

Knurled wrote: On the other hand, low fuel prices worry me, because they usually signify a faltering economy.

What I've been reading suggests this may be a battle between OPEC and other energy sources like fracking and solar. OPEC is overproducing to squeeze the profitability out of the competition, by driving prices so low, other sources can't compete. They're hoping to drive development away from new sources that are competitive at the price oil was at six, or nine months ago. This kind of business practice only works short term, so prices will rise again fairly soon, hopefully (for us) without drowning too much competition. I wouldn't use current prices to decide it's time to finally buy that used V10 Excursion you wanted on an 84 month loan.

Mike wrote: What I've been reading suggests this may be a battle between OPEC and other energy sources like fracking and solar. OPEC is overproducing to squeeze the profitability out of the competition, by driving prices so low, other sources can't compete.

So, the exact opposite of the early 70s then. Back then it was, screw the West, we're cutting supply. Now it's screw the West, we're flooding the market.

I wouldn't use current prices to decide it's time to finally buy that used V10 Excursion you wanted on an 84 month loan.

Oil is far too valuable a resource, modern living-wise, to squander on burning it. We should be doing everything we can to minimize how much we burn.

It's a pretty complicated calculation, I've struggled with answering these same questions.

A wrinkle that I don't think anyone has mentioned yet is how flat your routine travel roads are. If you're in the super-flat great plains then a heavier, but more aerodynamic car might edge out a lighter, but less aero car. That calculation might flip if you're cruising up and down W.Va or eastern Kentucky and Tennessee.

I will say that diesel fans frequently ignore/forget the additional cost of that fuel. If you, like me, are really interested in saving money then you don't really care about MPG's, you care about dollars per mile. A quick example, a 2015 Golf TDI gets 45 HWY, my old 2007 Fit only gets 34, but the cost to drive 25 HWY miles at current prices is almost identical ($2.03 for the VW, $2.12 for the Fit). So, the TDI owner can walk around bragging about getting 45 mpg, and get a vanity plate that reads "FUL SIPR" or "HI MPGS," but he isn't actually saving any money. This is useful to think about when comparing cars that run regular vs. premium as well.

What pisses me off is how the government has conspired with the auto companies to keep the price of diesel higher than gasoline. There is absolutely no production reason that diesel should cost more than gas.

I driven small diesel cars in Europe and there is absolutely no excuse that they aren't available here. But government influenced pricing keeps diesel outrageously expensive, so we don't get them here.

maj75 wrote: What pisses me off is how the government has conspired with the auto companies to keep the price of diesel higher than gasoline. There is absolutely no production reason that diesel should cost more than gas.

Home heating oil, for one. Jet fuel, for another.

A cynic could argue that gasoline prices are more important because people pay for it directly and see the difference. Diesel pricing doesn't matter because 90% (SWAG figure, maybe low) of its use in the US is commercial. If you are a business and you need it, and the price goes up, you don't pay for it yourself, you just pass the costs along. At least, you do if you want to stay a business.

I fully expect tightening emissions laws to make Diesel powered private vehicles no longer viable to produce as new cars in 10-15 years. Too expensive to make clean. I would not be surprised if the number of people circumventing the emissions controls resulted in private ownership being made illegal.

maj75 wrote: What pisses me off is how the government has conspired with the auto companies to keep the price of diesel higher than gasoline. There is absolutely no production reason that diesel should cost more than gas. I driven small diesel cars in Europe and there is absolutely no excuse that they aren't available here. But government influenced pricing keeps diesel outrageously expensive, so we don't get them here.

not sure it's the government conspiring with the auto industry … I think it's more that they were able to tax the E36 M3 out of diesel … back when it was mostly over the road truckers using it … and knew that the excessive taxes would just be passed on to the consumer … now with more people wanting to drive diesel powered cars, we're stuck with federal and state taxes driving the price up

Aerodynamics, consistent speed, and low RPMs. That's what seems to be the key for everyone I know.

One time in my ecoboost fusion I was able to cruise at 60mph via cruise control for about 90 miles none stop. The car said was getting 45mpg and at one point 49mpg going down hill. But, I'm sure it was more like 38mpg. I wonder how the car computer calculates its mpgs.

yupididit wrote: I wonder how the car computer calculates its mpgs.

The computer knows exactly how much fuel it is injecting. (The fuel injector flow modeling is... precise) It also knows what distance you are traveling.

In reply to wbjones: Yes aero is a big deal. Especially with a hatchback or station wagon. As I recall our 2008 Fit Sport was rated about 4mpg higher on the highway than the standard Fit. Why? I think the main reason is that small spoiler at the top of the hatch. It causes turbulence behind the car, which reduces what we used to call the station wagon effect.

Have you ever been in a station wagon or hatchback with the back "hatch" open, probably while carrying something too long to close the hatch? Or perhaps opened the sliding back window on your pickup? If so, you probably noticed there was a suction bringing foul air into your ride. That suction is caused by a low pressure area behind your ride which also costs you fuel mileage with your hatchback, pickup, or wagon. Therefore, this low pressure result is often called the station wagon effect.

BTW: The same low pressure area, or station wagon effect, which causes you to lose gas mileage also allows airplanes, including choppers to fly. Their wing, rotor blade, is designed to produce a low pressure area on its' top side which provides "lift" and allows the airplane to fly.

Want better fuel mileage? Find an add on device that causes turbulence and reduces the suction in that low pressure area.

I'll start with the things that can change easily instead of diving right into building an engine for MPG.

• Weight. Although on the highway, weight has little or nothing to do with MPG, it does affect how much fuel you use to get up to speed. If you're just on the highway on a long road trip, you won't really notice a difference in MPG. Any difference you do notice will be from the additional rolling resistance of the tires.

• Throttle position. I don't like discussing throttle position because it eventually turns to "pumping losses." It makes me want to punch people in the boob. For any given combination, there are many factors that come into play. Throttle position is not synonymous with throttle flow. The difference between 0 throttle and 10% throttle is on the order of 500% change in airflow, but the difference between 90-100% throttle is on the order of 10%. Generally, the least throttle position will net the best MPG, but not always.

• RPMs. Any talk about lower RPMs being better is horse poop. Take a look at the torque output of your engine. If you torque peaks at 4500 rpms, using a really low highway RPM is pointless for two reasons; 1) you will be operating where the engine makes pathetic torque and therefore require greater throttle openings to make enough torque to overcome moving resistance. 2) if your torque peaks at 4500 rpms, chances are that your cam timing events (specifically overlap) are favoring higher RPM cylinder pressures. Operating at RPMS that are too low will cause far greater intake reversion, intake dilution, and MPGS will suffer. The proper RPM is where you will get the best MPG, not necessarily lower. My current build will be spinning 2400 rpms on the highway, and I can almost guarantee that I could gain some MPGs by spinning it at 2600 because of the large amount of overlap. Spinning it faster (in my case) would increase VE by trapping more intake charge which increases pressure which increases the amount of BTUs released by the fuel as combustion pressure.

• Just go slower. Again, not really effective in all cases. Going very slow (like 10 mph) is wasting a ton of fuel. Conversely, going 150 mph wastes a ton of fuel overcoming drag. Since aerodynamic drag exists exponentially as you get faster, every vehicle has a crossover point. In a 76 Chevy pickup, that might be 35 mph. In a new C7 vette, that might be 95 mph.

I often wonder about this also.

I'd love a full size truck, but there is no way in hell, apparently due to aerodynamics, that a full size pickup will make better than 23-25 mpg. Even the dodge half ton diesel appears to top out in that range.

http://www.edmunds.com/ram/1500/2014/long-term-road-test/2014-ram-1500-ecodiesel-fuel-econom-update-for-may.html

So i'm seriously considering a car with a trailer hitch + uHaul for the occasional hauling needs.

While true that the cost of diesel is higher then gasoline especially in the winter. For me that isn't really an issue as I replaced a car that needed premium fuel for best MPG and performance. Since I drive less in the winter then in the summer and since summer diesel runs about the same as premium fuel, for me the TDI saves me money.

Not everyone will see a savings over their present car. What works for me may not for someone else. All I know is that I fill up half as often as I used to and the cost of a fill up is only a bit more then what I paid before.

Really, who could predict that the price of oil would drop, or that it would drop so much that $60 a barrel is not out of the question? I do think that that price if made will not hold and I think $80 is more likely a price that oil will stabilize at going forward 1-2 years. Of course any kind of disruption in production in some middle eastern country will cause oil to go back up as we have seen in the past. I not pulling this out of thin air, I spent 24+ years in the oil industry and keep up on the international situation because of this.

In reply to curtis73: I've read more than one or two treatises that say the best throttle position with conventional carbs. is wide open while pulling the highest gear possible. You're operating as close to engine stall speed as possible. This results in a higher air to fuel ratio since the suction which draws the fuel is hugely reduced. But watch out for compression knock!! Pin holes in pistons can get expensive.

Rupert wrote: In reply to wbjones: Yes aero is a big deal. Especially with a hatchback or station wagon. As I recall our 2008 Fit Sport was rated about 4mpg higher on the highway than the standard Fit. Why? I think the main reason is that small spoiler at the top of the hatch. It causes turbulence behind the car, which reduces what we used to call the station wagon effect. Have you ever been in a station wagon or hatchback with the back "hatch" open, probably while carrying something too long to close the hatch? Or perhaps opened the sliding back window on your pickup? If so, you probably noticed there was a suction bringing foul air into your ride. That suction is caused by a low pressure area behind your ride which also costs you fuel mileage with your hatchback, pickup, or wagon. Therefore, this low pressure result is often called the station wagon effect. BTW: The same low pressure area, or station wagon effect, which causes you to lose gas mileage also allows airplanes, including choppers to fly. Their wing, rotor blade, is designed to produce a low pressure area on its' top side which provides "lift" and allows the airplane to fly. Want better fuel mileage? Find an add on device that causes turbulence and reduces the suction in that low pressure area.

well aware of all those points .. .that's why I said that the Sonic coupe and the Cruze (both with the same drive train… and the Cruze being slightly bigger/heaver) get better milage than my hatchback …

and even though it has a small roof extension (flat spoiler ?) the rear window gets covered in dirt, of if in the rain, covered with spray worse than any station wagon or hatchback I've ever driven … all the Civic's, the Integra (much more sloped) all the Suby wagons … none attracted as much dirt/spray … i.e. much more suction behind the car … so my hopes of Cruze like 40+mpg probably won't happen

In reply to wbjones:

Some of the extra cost of diesel fuel may be the cost of producing the new environmentally devoted fuel.

Gas vs. diesel. Cost per mile is the question or maybe the answer. Ruling out the hit of buying the diesel.

The hit of buying the diesel is often recuperated at time of sale. Price out a gas 97 Ram vs a diesel for some real lulz.

I gotta believe aero comes down to a huge part of it, why else could a Corvette make close to 500 horse and still knock down 30 mpg on the highway.