Hacker News Reader

Comment by Kon-Peki | Hacker News Reader

Fuel price per gallon is only half the issue - or maybe even less.

Last week I filled up with 93 octane for $6.10 per gallon. Took a ~300 mile road trip with the cruise control set at 72mph. I got passed by people driving 80+ mph every minute or two. In really large vehicles. There's really only two possible conclusions:

1. People don't realize that their driving behavior has a huge impact on their fuel economy.

2. Fuel isn't that expensive.

My car (Audi Q5) got 37 mpg. The window sticker says it should only get 28 mpg on the highway. This isn't even hypermiling - I'm speeding the entire time.

View on HN

You got 37MPG in a (I guess they're technically "compact") SUV with a gasoline (petrol) engine? I'm just having a really hard time with that. That's in the same ballpark as what diesel wagons get (the TDI Sportwagen sees a real world ~43 MPG) and the Q5 isn't exactly a svelte profile, aerodynamically speaking.

Did this roadtrip consist of large amounts of elevation loss? I just have a really hard time understanding how you bested the EPA estimates by almost 28% while cruising at 70+ MPH. I roadtrip in an Outback wagon that can manage somewhere near 30MPG if I keep my foot out of it and is a similar footprint to a Q5, if not a bit more aerodynamic.

If you tailgate someone you can get that magical reduction in air resistance which can increase fuel economy. I hate driving on busy roads for this reason, some jack hole will always be right on my ass and I can't even see their headlights.

If OP has their cruise control set at 72MPH and is tailgating close enough that they benefit from the slipstream, I'm 1.)impressed at their precision driving 2.) terrified we may share the road some day.

I don't doubt that we may someday achieve more autonomous highway cruising where vehicles take 'lead' and 'follow' positions within feet of each other to benefit from drafting, but humans doing it scares me senseless.

In both cases, it's a 10MPH collision at worst...unless car in front is a talentless driver.

He filled up with 93, that's where the gains are from. Try it sometime, you'll be surprised!

That's not true. Gasoline has approximately the same energy content per gallon regardless of octane rating. In Car and Driver's tests, high octane fuel made a difference of less than 1 mpg.

Some high performance engines are designed to use high octane fuel, but there is no reason to use it on normal engines in normal conditions.

https://www.caranddriver.com/features/a28565486/honda-cr-v-v...

> Some high performance engines

Which are...almost every lease I see running around, now.

Lots of cars adjust based on what's flowing in, too.

Even those don't see an MPG benefit from high octane - just a torque benefit.

Seriously? Honest question - how does that work? I've always heard that paying for a higher octane fuel than required by your engine is just "throwing your money away." But if you're saying, at least on the highway, if I can get a 28% gain in fuel efficiency while only paying 20% more for the fuel then I'm interested!

No, thats not what that number is for at all. You heard right, octane is a rating for its stability to premature detonation

https://www.eia.gov/energyexplained/gasoline/octane-in-depth....

Every car I've owned, I've done a compare and contrast on 91 or 93 vs 87 octane gas.

Each time I do the comparison I find that the cost per mile (not cost per gallon) is lower with the higher octane gas. The only time I buy 87 is if I need to fill up a rental car just before returning it.

Have all of your cars been performance-oriented with either high compression ratios or turbocharged engines? If so, it makes sense. For quite some time, such cars have used a knock sensor to detect preignition and adjust ignition timing to prevent it. The adjustment has costs in both power and efficiency.

This would be a surprising result otherwise.

I've replicated this test over the years on inefficient gas guzzling American sedans, fuel efficient econobox hatchbacks, and sporty performance SKUs of mainstream product lines. These are hardly Shelby Cobra and Bugatti vehicles.

That's interesting. It doesn't agree with the results most published tests I've read found, for example https://www.caranddriver.com/features/a28565486/honda-cr-v-v...

If your car calls for regular gas and you give it premium, you're probably throwing money away.

If your car calls for premium and you give it regular, your diminished performance & fuel economy is also most likely wasting money. Penny-wise/Pound-foolish and all.

I've got a Germanic turbomotor, it's going to break if I look at it wrong. It calls for a minimum of 91 octane, but Costco in Illinois only has 87 and 93. So 93 it is.

I did the unscientific math on my Volvo (recommends premium but takes anything) and while the efficiency on premium is better, it's not worth the large price increase over regular.

Higher octane fuel has less energy by volume. That's one of the reasons it doesn't pre-detonate as easily. Try it sometime, indeed, and prepare to be disappointed.

Long trips with no traffic in warm weather (but not so warm as to require a lot of AC) are more efficient for an ICE vehicle per mile than the average trip. OP probably lucked into the right conditions

Why is warm weather more efficient than cold out of curiosity? I would have guessed that cold air being denser means better efficiency.

Excellent questions. It was a there-and-back trip, so every foot of elevation lost was also re-climbed (about 100 feet overall - the midwest is pretty flat).

This car has Audi's "energy recuperation system", which is actually a "mild hybrid" system but you'll never see those words on the website or Monroney or any advertising - "mild hybrid" has a terrible public reputation despite the fact that it works (but doesn't have much of an effect on the EPA test cycle). Almost every single Audi (and Volvo) sold in the US has this system. It is also the base model, so the tires and wheels are the base 18-inchers which weigh a LOT less than the 20+ inch wheels that everyone loves (and kill fuel economy). The transmission gear ratios are perfectly matched for the speeds I choose. I use high-quality gasoline from Costco, etc.

Go read other people on Audiworld or other forums. I'm not an outlier. These modern turbo engines are great for highway cruising, but drink fuel like it's free if you want "spirited driving".

As far as I can tell, you won't see "hybrid" on Audi's site because the "energy recuperation system" isn't a hybrid system at all -- it simply banks energy from the alternator and then reduces the load on the alternator (and the load on the engine, as a byproduct) when requested, allowing the car's energy needs to be drawn off the battery during that time instead.

From Audi's literature:

>While the vehicle is coasting or braking, the alternator voltage is raised above the level of the basic electrical system. During these phases the alternator can convert the vehicle’s kinetic energy into electrical energy, which is stored temporarily in the vehicle battery. When the vehicle subsequently accelerates or is driven at constant speeds, this energy is used to relieve the load on the alternator, allowing a fuel saving of up to 3%, depending on the driving cycle.

I am fully aware of how efficient a modern turbo engine can be, and perhaps you did, in fact, best the EPA estimates. Did you calculate these numbers yourself, or rely on the onboard computer?

I keep track of everything myself.

That's fine, we don't have to call it a mild hybrid. But whatever you want to call it, it's working really well on my car. It only has about 7k miles on it, but the efficiency keeps getting better as it breaks-in. I have to imagine that this is probably as good as it will get.

Let me ask you something, though: if I went back and edited the original post to say 30 mpg, would you agree that how someone drives makes a huge difference in their fuel costs. Because that was my main point :)

Yes - I'm certainly being pedantic. You're correct in that driving style can have a huge impact on fuel consumption. You don't have to hypermile it; just not playing speed racer between red lights, cruising at 70 instead of 85 helps, etc.

I was just kind of blown away by the claim. Not that long ago, high 30's MPG on the highway was the domain of economy cars (think Corolla's and the like) and euro diesel cars. To hear that a gasoline SUV, albeit a smaller one, could be close to 40MPG on the highway made me skeptical and curious of how it was achieved.

Many new Audis also have a newer version of Quattro (marketed as Ultra) that mechanically disengages the rear drivetrain.

My A6 wagon gets about 10mpg on the highway better than my in-laws Outback, and I assume this was a large portion of the reason.

Sounds about right to me. I have the wagon version of this vehicle (A6 Allroad) which has pretty much the same engine and similar weight, just different aerodynamic profile.

If you put it in cruise and don't go fast I can hit just under 40 - but I've only had it for a couple months. This is for a 21/28 rated vehicle. Forum posts seem to validate this as well.

City driving it's exactly what you'd expect - couple miles under EPA estimates.

Edit: this is manually calculated, but with a sample size of one. Strangely the onboard computer seems to underestimate in this vehicle, which is the exact opposite of every other car I've driven. Probably the most unexpected bit of owning this car so far.

Yeah, the number of drivers sitting in their cars with the engine running while parked shows me fuel prices really aren't that high.

The amount of gas spent idling is quite low. A compact sedan uses around 0.16 gallons per hour. Intuitively, this makes sense. The work required to accelerate a car is much higher than the work required to move the engine through its cycles.

For reference: I drive SF <> LA monthly in a 2008 Avalon with advertised highway 28mpg. Cruise control set to 75. My average is just above 30mpg. Windows shut, AC on. Costco in Gilroy had 87 for $6.10. Stations along the 5 ranged ~6.70-7.50. My car says premium only but since gas in CA is so fresh I've done 87 for 10 years, no issues.

Since you're someone who tracks numbers, you might try doing a run with 91 or 93. I've done the comparison on every car I've owned and in every case the dollars per mile (not dollars per gallon) is better with the higher octane.

Spurred me to dig out an anecdote: 2005 Toyota Solara, 3.3l V6, vacation trip from CA to UT and back, Sept 2016, moderate A/C use: 6 days, 2550 miles, 28.9 mpg (100% lowest-octane "regular" gas used). As others have noted, driving smoothly with minimal/gentle acceleration has a major mpg benefit.

Is that 37 miles per American gallon or 37 miles per Imperial gallon?

"Don't worry about the price of gas, just change what and how you drive!"

Unlike "what" you drive, changing "how" you drive can have extraordinary and immediate benefits. Driving at 80 MPH uses around 30% more fuel than driving at 55.

Obligatory Top Gear

https://www.youtube.com/watch?v=F04MXepYiBs

Just changing the drive mode on my Honda from normal to "Eco" gets me +10MPG in the city, and all that does is change the throttle pedal response map.

Reply on news.ycombinator.com