Some hypermiling questions

June 30, 2008 10:39 AM

A couple of questions about hypermiling: 1. Have there been any studies that prove that driving on the highway with the windows open significantly decreases gas mileage? If so, where can I find them? 2. Have there been any studies that prove that using the air conditioner in the car significantly decreases gas mileage? If so, where can I find them? 3. Does shutting off my car (2006 Corolla) at stop lights/signs decrease the life of my car's starter (or any other parts)? Thanks!
posted by GernBlandston to travel & transportation (20 comments total)

The Mythbusters tackled the windows down vs. A/C issue. Here are some details.
posted by nitsuj at 10:41 AM on June 30, 2008

Mythbusters retested the car myth after a number of complaints-- they did so in episode 38: "MythBusters Revisted 2" in which they found that it depends on the speed of your car.

"The fundamental flaw in the MythBusters’ test was that the point where the drag becomes powerful enough to inhibit a car’s performance with windows down was inside their 45 - 55mph margin at 50mph. Going less than 50mph it is more efficient to leave your windows down, but going greater than 50mph it is more efficient to use your A/C." - Mythbustersresults.com
posted by Static Vagabond at 10:58 AM on June 30, 2008 [2 favorites has favorites]

My starter coils went out after 8 years at 80,000 miles. $320 in parts + labor.

I wouldn't bother turning off the engine to save gas at idling. We're talking a gallon or two per year, at the most.
posted by yort at 11:05 AM on June 30, 2008

Shutting off the car at stop lights/signs is illegal in most jurisdictions, the stated reasoning usually being that it's unsafe. If, indeed, this is true, then it might decrease the life of your car's driver.
posted by box at 11:16 AM on June 30, 2008

Here you go: Fact or Fable: Fresh Air vs Air Conditioning
posted by knave at 11:28 AM on June 30, 2008 [1 favorite has favorites]

Afterthought: Interesting that those blue & green lines never intersect. A/C always uses more fuel, at every speed. There is no magical crossover point.
posted by knave at 11:29 AM on June 30, 2008

On a recent long highway trip in a Volkswagen jetta, we ran our own experiment, and found that at 70-75 mph, with the A/C on, fuel efficiency dropped by more than 8 miles per gallon, compared to windows down and no A/C at the same speed.

The car was a diesel, TDI, specifically, if that makes a difference. YMMV.

Also, stopping and re-starting the engine will shorten the life of your starter, but starters aren't very expensive, especially if you learn how to install it yourself.
posted by M.C. Lo-Carb! at 11:30 AM on June 30, 2008

I found the SAE study paper referenced by my earlier link. It talks about crossover points, so perhaps I interpreted the graphs incorrectly.
posted by knave at 11:41 AM on June 30, 2008

Shutting off the car at stop lights/signs is illegal in most jurisdictions

That's good to know... Had a friend who was seriously contemplating doing this. I can now mention this in addition to the compelling argument of: "Seriously? But... well... the environment and all... but...y'know... oh hell, that's just fucking retarded!"
posted by Debaser626 at 11:46 AM on June 30, 2008

Actually - I've heard that it takes alot more fuel to start an engine than to leave it idle for a minute or two.

However - this is in Alberta, where we have many, many jokes about people idling their cars...
posted by jkaczor at 12:03 PM on June 30, 2008

2. Have there been any studies that prove that using the air conditioner in the car significantly decreases gas mileage?

Do you mean proper, academic studies? How about this one - in case you don't have American Chemical Society access, I'll summarise the results: The paper proposes a model where air conditioning presents a base load when on, and an increasing load as temperature increases - in other words, if T>5 then Fuel Consumption=max(c,a*T+b) else 0.

The values for a, b and c are given in grams of fuel per kilometer (and grams per kilometer per degree centigrade) and are tested for urban, rural, and highway driving, (as defined by the Common Artemis Driving Cycle) and in shade and direct sun. The CADC is a multi-speed test representative of real driving, but for simplity's sake I'll tell you the highway stage takes place at around 125 kilometers per hour.

For highway driving in the sun, a=0.2790, b=-4.3917, c=0.6512.

Of course, by this measure driving faster actually improves your AC's fuel consumption per kilometer because for the same AC fuel consumption per minute, you get more kilometers.

If you want more details, MeFi mail me.
posted by Mike1024 at 12:04 PM on June 30, 2008

Previously (on idling at stop signs). Actually that's asking about fuel consumption (the cut off point starts arouns 30 seconds for modern cars) but it also covers the mechanical stuff.
posted by tallus at 1:01 PM on June 30, 2008

I wouldn't bother turning off the engine to save gas at idling. We're talking a gallon or two per year, at the most.

Nonsense. All hybrids do this automatically, and with marked results.

Apples and oranges, obviously, but still.
posted by ChasFile at 1:07 PM on June 30, 2008

piggybacking someone else's question : does anyone know how much damage you'd do to your engine 'bump starting' your engine at speed?

I'm thinking of the suggestion that you kill the engine in a European style floor shift while travelling at speed and then restart the engine by bringing the clutch up. I can't decide if it is torture on the engine or not.

Maybe I should go write my own question...
posted by twine42 at 1:42 PM on June 30, 2008

When I bought my 2005 Skoda Octavia (if you don't have Skodas in the states, think of a saloon version of the VW Golf) I was told by the dealer that VW aircon units were more efficient than windows down at motorway speeds. Mind you, in the UK motorway cruising speed is often 80mph+.

Unfortunately I can't test it because I decided I'd rather have a sunroof than aircon. I'll never understand that sales logic...
posted by twine42 at 1:47 PM on June 30, 2008

twine42, if you just give the clutch a momentary bump in a high gear (i.e. at 20mph, I use 4th gear), it won't hurt the engine. It'll do some wear to the clutch, but I'm not sure how much. Don't just let the clutch out and drag the engine up to 3000RPMs or something, just give it a quick bump, and let the engine get into an idle, and then use the gas and clutch to properly shift into the right gear.
posted by knave at 2:06 PM on June 30, 2008

twine42 - I think that is also how the Prius starts its engine at speed... and I'm pretty sure that the electronics in the car actually shutdown individual cylinders in the engine (i.e. supply gas only to 1 or 2 cylinders depending on how much power is needed) to save on gas.
posted by Arthur Dent at 4:54 PM on June 30, 2008

What's a starter coil?

-- Starters are made to be used. A good starter motor will last a long, long time. Using it more often might cause it to fail at year 8 instead of year 10, but it was still going to break at some point. The worst thing for a starter is improper starting technique, ie, keeping the key turned to start while the engine has already started. And running it for more than a few seconds at a time. If it takes that long to start the engine, fix the engine.

-- Shutting down and restarting takes more gas, but significantly less than the old days of carburetors and old fuel injection. I think the margin of gas savings is into the seconds now. 30, maybe?

-- Bump starting is fine, if you do it right. Just like shifting gears, you have to use the proper gear and the proper clutch technique or you'll anger the engine. And starters are way cheaper than clutches...
posted by gjc at 6:08 PM on June 30, 2008

All hybrids [shut off the engine] automatically, and with marked results

You sure about that?

There's a *lot* of technology in a Prius that gets it its good city MPG rating. I can get 40mpg in my Miata going 40mph, or an *hour* of driving. Any guess on the comparative fuel usage at idle? How about a SWAG of 5%, giving us *20* hrs -- 1200 minutes -- of idling on 1 gallon of gas.

Assuming 20 stops per day at one minute each, that's TWO MONTHS of idling per gallon, 6 gallons per year with these guesstimates.
posted by yort at 9:12 PM on June 30, 2008

twine42 writes "Unfortunately I can't test it because I decided I'd rather have a sunroof than aircon. I'll never understand that sales logic..."

The A/c Unit probably doesn't have the capacity to cool to the car to the degree required with the added load from the sun shining in the sunroof.
posted by Mitheral at 1:19 AM on July 1, 2008