Help me increase my milage.

February 27, 2008 5:17 PM

Will I save gas by turning my car off at long red lights?

I drive an old Pathfinder, and the cost of driving is just eating me alive. Please don't suggest buying a new car, as I am already weighing that option, and it'll be next fall at least before this is even a remote possibility. I have a standard, and already coast whenever possible, and just by being careful not to drive like a maniac have improved my milage from ~15 mpg to over 17, and occasionally 18.

Wear and tear on the starter aside, I'm wondering if I will actually save gas in this scenario -- I'm 300 yards from what I know will be a long sit (let's say 1.5 - 2 minutes at least) at a traffic light when I see it turn yellow. If I kill my car immediately, coast up to the light and don't restart the car until the cross-street light turns yellow, will I be saving gas on the whole, or will I use more gas re-starting the car than I would letting it sit at idle for a minute or two? I wouldn't do this at short lights, or busy intersections -- just when on those lonely side-streets where you get stuck forever waiting to cross a major artery.

I know that coasting with the car off, I'm getting ∞ miles to the gallon, but am I burning it back up re-starting the car?

posted by Devils Rancher to travel & transportation (38 comments total)

I think you probably would save gas. First, continue your usual driving habits for a year, but keep very careful records of your gas consumption. Then stop your engine at long lights, while still keeping careful records, for a year. Then compare the two results and let us know how that works out.
posted by thomas144 at 5:21 PM on February 27, 2008

You will save gas, provided your car is fuel injected (you don't say how old old is). Starting a fuel injected car take minuscule gas, but I wouldn't expect your mileage to jump up more than 1 mpg.
posted by sanka at 5:26 PM on February 27, 2008

yes you will.

Now, I just have to go find the reference for this, which I clearly read somewhere. In the meantime, consider this - the Prius does exactly this.
posted by wilful at 5:26 PM on February 27, 2008

Previous thread that you may find useful.
posted by inconsequentialist at 5:28 PM on February 27, 2008

In the meantime, consider this - the Prius does exactly this.

So does the Honda Civic Hybrid. So there's two data points!
posted by limeonaire at 5:28 PM on February 27, 2008

Agreed, at a l1-2 minute light you will save gas. But be careful with the coasting thing -- you could find yourself with no power when you might actually need it. Also, you've probably done this, but pump your tires up hard. If they're old tires anyway, and you're doing mostly city driving, there's no harm in putting in about 4 pounds over the manufacturer's spec (but not over the tire maker's rating), and you'll definitely notice better mileage from that.
posted by beagle at 5:29 PM on February 27, 2008

As I understand it, depending on your particular vehicle, starting the car can consume as much gasoline as a minute or more of just idling.

If, for whatever reason, the wait will be longer than a few minutes, then yes, you should kill the car.

You shouldn't be looking for ways to conserve gas while you are driving. If you need to pinch pennies that badly you should look into not driving at all. You can cast "wear and tear on the starter aside" all you want, until you need to buy a new starter.
posted by crunch buttsteak at 5:29 PM on February 27, 2008

Does the car have power brakes or power steering? If so, then you're flirting with death by coasting with the power off.
posted by randomstriker at 5:36 PM on February 27, 2008 [1 favorite]

Answering the question topic of increased mileage and not the question of killing the engine at red lights here, but:

Have you had an engine tune up lately?
When did you change your oil last?
Have you checked the pressure on your tires lately?
Do you keep unnecessary heavy items in your car all the time? (Like rocks, bricks, tools, small appliances, etc. sounds silly, but some people do keep these items in their vehicles.)
When did you last change your spark plugs?

There's probably a few items that I'm missing, but these are some things you can look into if you haven't already to improve your mpg. In general if your vehicle is in "poor health" your mpg is going to suffer. Good luck.
posted by Stilus at 5:37 PM on February 27, 2008

Wilful, a Prius is exactly what I'll be buying when the money happens. Not gonna happen for a while.

Sanka, it is fuel-injected, so thanks for pointing out that variable.

crunch buttsteak, I'm thinking at the price of gas, I could save more than the cost of a starter, if it even wears out between now and next fall. We're talking about starting my car maybe 5 times a day instead of 2 or 3.
posted by Devils Rancher at 5:40 PM on February 27, 2008

inconsequentialist, I ran a couple searches but missed that thread, somehow. Thanks.
posted by Devils Rancher at 5:44 PM on February 27, 2008

1. Yes, you will save gas by turning the car off at red lights.

2. If you want to buy a Prius in order to save money on fuel, you should really read up and do the math before making that decision. If it's because you just want to make a statement, no need to read up.
posted by The World Famous at 5:46 PM on February 27, 2008

Have you had an engine tune up lately?
New timing belt and radiator last year was all I could afford. I'm not sure what a tune-up involves, beyond timing and spark plugs (see below).

When did you change your oil last?
I'm pretty regular about this, though I slip past 3000 miles sometimes. Never more than 6000.

Have you checked the pressure on your tires lately?
Aired 'em up when the weather cooled down last November to 36 lbs. They look good, right now, but I think I need a more accurate gauge.

Do you keep unnecessary heavy items in your car all the time?
That actually occurred to me just a few weeks ago, andI managed to get about 60 pounds of junk out of the car. I'm a wee bit of a slob.

When did you last change your spark plugs?
Been about a month.

Good questions all, though.
posted by Devils Rancher at 5:51 PM on February 27, 2008

Does the car have power brakes or power steering? If so, then you're flirting with death by coasting with the power off.

I'm very judicious about this. No cars in front of or behind me? 3-400 yards of straightway, hopefully, a downhill slope, I coast. In traffic it would seem stupid. I had to swerve to avoid (locking up my brakes, and screeching to a halt in a parking space, once) 4 accidents on the way home from work last Friday. It's the beginning of the semester, and I drive near UT campus, which means only about 30% of the cars on the road right now are driven by people who have any clue where they're going, and 50% of them are on the phone.
posted by Devils Rancher at 6:09 PM on February 27, 2008

Doesn't over-inflating the tires mean they'll have less traction, which could be dangerous in an emergency stopping situation?
posted by amtho at 6:45 PM on February 27, 2008

Mileage tips

Keep your tires properly inflated, or even run them higher than the vehicle calls for (but check the tire itself for the maximum PSI).

Drive easier. It sounds like you're already doing that, at least on slowing down, but what about when you accelerate? Hard acceleration uses a lot more gas.

Drive slower. This is a huge factor if you're driving on at highway speeds. Huge. I had an 89 Blazer that would get me 16-17 around town and 18-19 on the highway at 75+. On one trip I set my speedo at 68 and got something crazy, for the vehicle, like 23-24. On another trip I was stuck on icy roads behind a semi for hours; conditions were such that I didn't want to pass and ended up going about 30. I broke 25 mpg.

So if you're driving long distances and highway speeds set the cruise and set it slow. If the limit is 65 set it for 55-60, and 65-70 at 75. You'll probably be the slowest one on the road and it may not be that safe in heavy traffic. But light traffic it really pays off to slow down.

Re: Prius, TWF makes a good point: 2. If you want to buy a Prius in order to save money on fuel, you should really read up and do the math before making that decision.

It may be years before you break even, especially if you own the Pathfinder outright. So if your goal is only to save money that may not realistically happen.

Just running through a few numbers. The cost of the Prius is about $20,000, as far as I can tell, and lets assume you buy it outright with no trade-in. Financing obviously makes the cost higher; a trade-in will reduce the cost. Won't figure in maintenance at all. An older vehicle generally has a higher maintenance cost per year but the complexity of the Prius may make it expensive to fix.

So let's say you drive 20,000 miles/year and gas costs $3.50.

The cost to drive the Pathfinder for a year is 20,000 miles / 17 mpg *$3.50/gal = $4100/year in gas. The Prius, using 50 mpg, costs $1400. So the difference is $2700/year. Divide that into $20,000 and you get 7.4 years to break even.

If you drove 30,000 miles/year, keeping the other values the same, you break even in 5 years, and it's nearly ten years at 15,000 miles/year.

I'm not saying the Prius is a bad purchase but the idea of buying a new car, even a much more efficient model, to save money generally doesn't pay off. You should be realistic about your expectations and run through the numbers. Would you buy the Prius outright? If not, what kind of down payment, if any, and what finance rate? How much is the Pathfinder worth as a trade-in, realistically? You could make a little spreadsheet to do this. Try for different gas prices, like $3/gallon and $6/gallon.

If you've got other reasons to get the Prius, that's fine, though I think we're often too quick to assume that new means more savings and is better for the environment, once you figure in the impact of manufacturing and disposal. I remember another question in which someone wanted to convince his dad to replace a thirty year old chest freezer because a new freezer would obviously be so much more efficient it would quickly pay for itself. But what about the hidden costs? How many new replacement freezers would be purchased in the next thirty years? They don't make them like they used to, it's a cliche but there's truth, and you can bet, on average, any new freezer wouldn't last near as long as the old one. Environmental impact is really much more complicated than just naively assuming something new will use less gas or less electricity that the savings will be immediate and obvious and so much better for the environment while the old goes in the landfill or the crusher, while our disposable society becomes less and less sustainable, even when people think they're thinking green.

This is getting pretty tangential so I'll stop.
posted by 6550 at 6:46 PM on February 27, 2008

Does the car have an automatic transmission?
If so, your savings will be wiped out the first time you accidentally shift into reverse instead of drive.
posted by hexatron at 6:53 PM on February 27, 2008

when doing the prius calculation, it is worth considering the strong likelihood that the price of gas is going to go up significantly over the next 5 years, or more.
posted by Good Brain at 7:13 PM on February 27, 2008

6550, I've thought about the ROI, and that makes it a tough decision. I want to keep the Pathfinder for caving/camping trips, anyway. My tendency with cars is to buy used, so I'd probably look at 1 3-to-4 year-old Prius. I've noticed in general, cars lose about half their value in the first 1/4th of their life span, but I'm not sure about hybrids. Gotta start watching the classifieds before I think too seriously about that.
posted by Devils Rancher at 7:44 PM on February 27, 2008

In the meantime, consider this - the Prius does exactly this

And that's nothing new. My old Dodge did this back in the day, but I paid good money to have it fixed.
posted by I EAT TAPAS at 8:14 PM on February 27, 2008

Check out this story.
posted by JohnnyGunn at 8:34 PM on February 27, 2008

I would suggest reading up on Hypermilers - there a lot techniques for squeezing extra miles per gallon.

If you are buying a Prius to save money (because of rising gas costs), I would steel your passion by also factoring in other costs to the calculations offered by 6550 above. Although Toyota offers an eight year warranty on the Prius, there is debate on exactly how long the battery systems of the cars will last (the replacement cost is $3000.00) - and if you are stranded miles from the nearest Toyota dealership, you certainly don't want a mechanic like Goober Pyle working on your hybrid car (it just might void the warranty).

Greenest Cars of 2008

I personally feel that the vehicle with the best gas mileage may not equal the cheapest car to own and operate for the life of the vehicle...
posted by cinemafiend at 9:48 PM on February 27, 2008

Of course if you have diesel available in your neighbourhood, you'll likely get something as efficient and more affordable than a prius.

But these have all been good points raised.
posted by wilful at 9:56 PM on February 27, 2008

For safety reasons I would definitely not do this.

If someone behind you loses their brakes or otherwise does not stop while approaching the light, you would not be able to take any preventative measures to avoid being hit.
posted by loiseau at 11:35 PM on February 27, 2008

I'm very judicious about this. No cars in front of or behind me? 3-400 yards of straightway, hopefully, a downhill slope, I coast.

And if a cat or dog runs out into the street? A child?

For safety reasons I would definitely not do this.

If someone behind you loses their brakes or otherwise does not stop while approaching the light, you would not be able to take any preventative measures to avoid being hit.

Precisely.

If saving the cost of gas is so important, how about buying a bicycle for spring/summer/autumn?
posted by dirtynumbangelboy at 3:58 AM on February 28, 2008

Cold restarts are hard on the engine. So best if first off/on is after the engine has warmed up a bit.
posted by Kevin S at 4:15 AM on February 28, 2008

In many states, coasting is illegal. Click-n-Clack say to turn off the engine if it's going to be more than a minute or 2. Get any unneeded weight out of the car. No, not the spare and jack.
posted by theora55 at 7:29 AM on February 28, 2008

If saving the cost of gas is so important, how about buying a bicycle for spring/summer/autumn?

Bicycles are the spawn of Satan, and you wanna talk dangerous? but still, I think I need one for those quickie grocery store runs when I forgot one item I need for dinner, or zipping up to Hillbert's for a to-go order. No way am I going to commute on Airport blvd. on a bike. Ya'll would be posting dots in my honor within the year.
posted by Devils Rancher at 8:24 AM on February 28, 2008

Even though you said mean things about bicycles*, here is the quote you probably are looking for:

"Did you also know that more than 10 seconds of idling uses more fuel than restarting the engine?"

Now you do.

from "You, Your Vehicle, and Fuel Efficiency" from NRCan's often excellent Office of Energy Efficiency.

You might also want to check out their "Auto$mart Guide" which covers these matters in more detail.

Also, please heed the advice of others in the thread. Don't coast up to the stoplight with the engine off.

* Not really that dangerous if you ride carefully (and do you know what else is really dangerous? driving a car). Also, biking is awesome.
posted by ssg at 11:23 AM on February 28, 2008

Bicycles are the spawn of Satan

Except for the whole non-polluting, good for your health bits, I suppose. I wasn't talking about danger to you, though, I was talking about the danger you pose to other people by not actually being in control of your vehicle while coasting.
posted by dirtynumbangelboy at 1:31 PM on February 28, 2008

DNAB & ssg -- Okay, bikes are Great™, but they're peripheral to the issue. It's not uncommon for Texans to not survive on bicycles -- those white ghost-bikes are becoming distressingly common around here, and the route I'd have to take to work is a shoulderless, major artery for cement and gravel trucks, as there's a big-ass cement and gravel plant across the road from where I work. I want to live.

Many days, I have children to take to far-flung schools, as well as getting myself to work. "Don't drive" simply isn't an option. "Drive less" is somewhat of an option, why is why I might entertain the idea of a bike for around the neighborhood. Don't think I'll be bringing home a week's-worth of groceries for 6 on one, though.

The point about staying in control of the vehicle is well-taken. It's a little hard to steer w/o the power steering, granted, but then, so was my '63 Biscayne. As to the brakes, they seem to maintain their power-ness (whatever you'd call it) for a good while after killing the engine, like at least 30-45 seconds. I don't want to run anyone over, so I'll limit this practice, I promise.
posted by Devils Rancher at 4:25 PM on February 28, 2008

But, uhhh.. 'limiting' isn't so much the issue. You're not in control of your vehicle--or, I'll grant, not reliably in control--when you turn off the engine and coast.
posted by dirtynumbangelboy at 7:45 PM on February 28, 2008

I'm obtuse, you're obtuse, let's leave it at that. I haven't hit so much as a curb in 5 years, and I am capable of assessing control. I'm probably one of the safer drivers in this part of the world. If it's 3 AM, and I'm on a lonely road all by myself, and the light a half a block up turns red, I doubt anybody's gonna die if I kill the engine. Like I say, this isn't something to be done in traffic. I'm really not reckless, I assure you.
posted by Devils Rancher at 8:55 PM on February 28, 2008

Doesn't your steering lock when you turn off the ignition? If not, you're going to want to seriously alter your habits when you get a newer car, because no steering will be a huge problem, as will the lack of power to your brakes when you cut the engine.
posted by The World Famous at 9:00 PM on February 28, 2008

Doesn't your steering lock when you turn off the ignition?

No. There's three positions on my ignition switch, like on most modern cars. (It's a 1991, with a standard transmission) Only turning it completely to the "all the way off and take the key out" position locks the steering. Perhaps it would be better if I swore on a stack of bibles right now that I will never, ever, ever turn my car off again as long as I live, unless I'm safely ensconced in a parking space, safely between the yellow lines, with equal distance on either side and my front wheels full up against the parking abutment, or when I have come to a full and complete stop at the gas pump, but before I leave my vehicle, for fear of releasing a static charge that could blow up the neighborhood. I will set my parking brake in a maniacal frenzy on all these occasions.

I promise not to de-claw my cat, either.
posted by Devils Rancher at 4:39 AM on February 29, 2008

Since you have a stick-shift, you can have your cake and eat it too -- there is no need for you to turn off the key while coasting and possible lose control and hit the organic bicyclist.

The answer? Engine brake your way to the red light, and then shut off the engine while you are waiting there. Fuel injected engines use either zero or almost zero fuel when you are engine braking, and you have the substantial benefit of keeping your power steering and power brakes active and ready for use.

So you are cruising along in 4th gear at 40mph, light ahead turns red -- if you take your foot off the gas (leaving the car in gear), you are then using no/almost no gas. Downshift to 3rd, then 2nd, then 1st, using the engine braking to come almost to a stop before needing to use the brakes; shut off the engine only when you are fully stopped and you know you will be waiting a while. Bonus: since you are not coasting, you remain fully legal.

Obviously this causes some incremental amount more wear on your clutch and a bit less wear on your brakes, and clutches are more expensive than brakes, so in theory you might have to spend all of your (very small amount of) gas savings on increased maintenance. But I use engine braking every time I drive, and my clutches last well over 100k miles usually, so this is no guarantee of disaster.
posted by Forktine at 6:03 AM on February 29, 2008

I had always assumed that engine braking would use more gas, since it ups the rpms. You see what happens when I assume?
posted by Devils Rancher at 8:06 AM on February 29, 2008

Update: No matter what I do, Ican't seem to get the bugger above 17.6 mpg -- the last two tanks, anyway -- though I think I was hampered on a couple of occasions by severe headwinds and driving rain. No coasting -- just turning the car off at long lights. Results -- inconclusive/nil.

Think i'll save my starter.
posted by Devils Rancher at 7:31 PM on March 23, 2008