[Elbyron]

Since gas mileage was recently brought up in the "Where do you normally fill up?" thread by rookie888, I thought it would be a good idea to have a discussion about gas saving strategies. His post contains a link to an article in the Toronto Star, which lists 10 gas-saving ideas from Jim Davidson's book, 75 Ways to Save Gas. The comments on this article seem to be quite negative, but I hope that the disucssions here can be a little more open-minded. In particular, I'd like to know what you think about nitrogen tires, synthetic oil, and various gas-saving driving techniques. I'm not interested in hearing about how I can use my vehicle less often - I'd just like to share and compare strategies for using less gas per kilometer.

There's no doubt that maintaining a good tire pressure improves fuel economy, and nitrogen makes this easier to do (especially when temperatures fluctuate). I personally use pure nitrogen in my tires, but it's more to avoid the hassle of checking tire pressure and to prolong the life of my tires. I just get the nitrogen levels checked & refilled when I take my car in for maintenance. I use synthetic oil for the same reasons: less engine wear means less repairs and longer lifespan. But it's great to know that these measures might also be slightly improving my gas mileage!

Probably the best driving technique is to avoid going too fast on highways. Most vehicles have an optimum gas mileage when travelling at a speed of 90 - 100 km/h, and gas usage at higher speeds increases quickly (at about 150km/h you're using twice as much gas per km!). I found a chart with mileage vs speed for 8 different vehicles. The rate that the mileage drops off (the slope) is pretty similar for the 8 different cars, so it stands to reason that most cars will suffer a similar rate of mileage loss.

On city streets, going too fast isn't usually a concern, though you should always think about going a little out of your way to use a highway instead of stop-and-go roads. Besides being stuck in a traffic jam, the biggest waste of gas on city streets is having to constantly stop and accelerate. Here are some of the techniques I use to minimize this penalty:

1. Don't waste gas to get to that red light sooner! Preventing the guy in the next lane from cutting into your "shorter lineup" just isn't worth it. Release the gas pedal and coast up to the light. Another advantage to coasting is that the light might turn green before you get there, meaning you won't have to come to a stop at all (while that guy who raced ahead now has to accelerate from zero!)
2. Time the lights. Look ahead to the next set of lights and try to determine if it will be green or red by the time you get there. Make use of the pedestrian walk lights (espcially the countdown type), as well as your gut feeling. On your regular commute you'll probably get to know how long each light is. If you think a green light will turn red, let go of the gas (resist the urge to try and beat it). If it's going to stay green, maintain speed or accelerate a little to ensure that you make it. If it is currently red but you think it will change soon, let go of the gas and change to the lane with the shortest line up, to make it less likely that you'll have to brake.
3. Leave lots of space between you and the vehicle in front of you. Hopefully you do this anyway, for safety reasons. Drivers on city streets tend to vary their speed, and if you're tailgating them then you're likely applying the brakes every time they slow down a bit, and then re-accelerating to stay on their tail.

Next, there's aerodynamics. Having a roof rack attached can significantly reduce mileage, especially at higher speeds. Most of the air flows over the top of your vehicle, so interfering with it on the roof is going to cost you. Mythbusters showed that using A/C actually uses less fuel than it does to have your windows open, AT HIGHWAY SPEEDS ONLY (I think 80km/h was the break-even point). The Toronto Star article mentioned keeping your vehicle clean for smoother airflow - I'm not convinced that this is going to make a significant difference but I like to keep my car from getting too dirty anyway.

My last gas saving strategy is to avoid uneccessary weight in your vehicle. Don't go removing your spare tire or emergency blanket, but if you've got heavy cargo that you don't need, then unload it before you leave home. That includes a detachable hitch - if you're not going to use it for a while, take it off. A full tank of gas may be heavy, but a low tank is not only more likely to explode in an accident, it also increases condensation (thus diluting the gas and making is less efficient) and can damage your fuel pump which relies on the gas to cool it. I always fill up full, but I never try to go beyond the first click.


Million Dollar Journey also has an older (Jan '07) article about Saving Gas by Maximizing Mileage.

 

[Elbyron]

how does the nitrogen inthe tires tihng work? Never heard of it, but I'm fascinated!!!

When you fill your tires with dry air, you're putting in about 78% nitrogen, 21% oxygen, and 1% other gases. But water vapor can account for up to 5% of the air under the worst conditions. The density of water vapor fluctuates more than dry air, so removing humidity can help keep your tire pressure more consistent. Supposedly the water vapor can increase wheel rust, but I haven't heard of this being a common problem. Using pure nitrogen avoids any problems caused by water vapor.

Another benefit for pure nitrogen is that you eliminate the smaller oxygen molecules, which more easily diffuse through the porous tire walls. So nitrogen tires will maintain their optimal pressure longer, leading to better mileage and more evenly distributed tire wear. That's the theory anyway; I haven't seen any scientific studies that show that pure nitrogen makes a noticeable difference.

Nitrogen at normal temperatures is inert, so it won't attack the rubber in your tires like oxygen does. Oxygen attack has been researched and proven, though its effects are mainly only seen in older tires.

There's also a safety benefit, since nitrogen doesn't support combustion like oxygen. Aircraft tires are normally filled with pure nitrogen, but on March 31, 1986, Mexicana Airlines flight 940 had a tire improperly filled with air instead of nitrogen. An overheated landing-gear brake caused the tire to get hot and explode, rupturing fuel and hydraulic lines. 167 people died in the crash. Of course, the chances of your tires getting hot enough to catch fire is very slim, but at least you can be assured they won't explode!

One final side benefit - you get cool green valve caps and the bragging rights that go with them!

By the way, if you get your new tires at Costco (be prepared for a long wait), they will fill them with nitrogen.

 

[Elbyron]

Mythbusters looked at A/C versus windows, and as well slipstreaming behind a big rig. For the second, you could be a good ways back and still be taking advantage of the reduced drag.

I've heard of slipstreaming aka "drafting" as a common strategy in car racing and cycling, but I never considered it to be significant enough to try tailing a large vehicle on the highways (safety concerns aside). But I found the results of the Mythbusters episode, and it turns out you can improve your mileage by up to 39% if you're insane enough to tail 10ft behind a big rig. True, you can stop faster than the big rig, but at highway speeds that 10ft gap could close faster than you can hit the brakes. Other dangers include rocks being flung at you by the big rig's tires, inability to see what's ahead (debris on the road might pass under the big rig but not under you), driver of the rig might not be able to see you, and there's even a chance of being killed by an exploding tire. In the same episode of Mythbusters, they confirmed that an underinflated big rig tire could disintegrate violently and that the flying debris has enough velocity to decapitate a dummy right through the windshield. Of course the chances of this are probably far less than say, an animal jumping out in front of your vehicle and decapitating you. Dangers aside, another reason NOT to tailgate is that it can really irritate the big rig driver.

Another thing to consider is that many big rigs I've encountered often travel in excess of 100km/h and by following at the same speed, you aren't getting your optimal gas mileage. Depending on the speed and how close you tail the rig, it may actually cost you more gas to keep up.

Personally, I think 100ft is reasonably safe for me to follow a big rig. The driver can likely see me at that distance, and probably wouldn't be very annoyed since you aren't exactly tailgating. If that gets me 11% better mileage, then it's probably worth trying. RV's, moving vans and other large vehicles would probably also work pretty good, maybe even better since they are lower to the ground and would thus provide a better wind-shield for a car.