Getting better gas mileage out of a fuel hog?

February 7, 2007 12:14 PM

Fuel economy filter: I drive a 1998 Dodge Durango, so I know my gas mileage is going to be low... But I was wondering if I use Mid-Grade or Premium gas if I'll be able to squeeze some more mileage out?

I drive a 1998 Dodge Durango, 5.2L V8, and I want to stretch my buck a bit further as far as gas mileage goes. I added a cold air intake to the engine to get a little better performance / gas mileage. In the owner's manual it says that the engine is designed for 89 Octane gasoline, but would running a higher octane, especially with a better air intake system, improve gas mileage? What else could I do to increase gas mileage?

posted by ganzhimself to travel & transportation (19 comments total)

Higher octane won't improve your mileage and there's no reason to spend the money on it unless your vehicle calls for it.

Keep your tires properly inflated. If you do a lot of interstate driving go slower. In my vehicle I see a marked change from 75 to 65. 55 would be even better though I wouldn't drive that slow on a 75 limit interstate. Your driving habits affect economy a lot. If you drive fast, accelerate hard, lots of speed changes, that kind of thing, your mileage will suffer.

What kind of mileage are you getting? I'd guess typical driving would be less that 18 for that vehicle and I'd be surprised if you ever got more than 20 outside of exceptional circumstances.
posted by 6550 at 12:25 PM on February 7, 2007

The Low-Down on High Octane Gasoline: "It won't make your car perform better, go faster, get better mileage or run cleaner. Your best bet: listen to your owner's manual."
posted by peeedro at 12:27 PM on February 7, 2007

Higher octane won't improve your mileage and there's no reason to spend the money on it unless your vehicle calls for it.

I got consistently better mileage out of premium fuel compared with super in my '89 RX7. That might have been a rotary engine thing.

The easy way to find out is to keep track of the mileage you get out of each tank, and the remaining gallons. You can then compute the exact amount of gas consumed. Run three tanks of each kind of gas you want to test, and compute your MPG values.
posted by b1tr0t at 12:29 PM on February 7, 2007

No, higher octane gas won't help you at all.
posted by knave at 12:31 PM on February 7, 2007

What else could I do to increase gas mileage?

Keep your fuel system clean, and your tyres at their proper pressure.
posted by Kwantsar at 12:34 PM on February 7, 2007

Draft things that are bigger than your truck.

Note: this is dangerous, and probably illegal, but it works.

The quick answer is to drive more gently. Slow on the take off, and look as far ahead as you can to the next light to brake early and coast to red lights.

From physics, Force = Mass * acceleration

Your engine provides the force, your truck (and you) are the mass. More acceration = more force. More force = more fuel.

Therefore, the less you change your speed, the better mileage you'll get. Your brakes will theoretically last longer, too.
posted by Wild_Eep at 12:48 PM on February 7, 2007 [1 favorite]

I keep my tires inflated properly, I check them once every two weeks. Over the past 6 months I average 14-15 MPG, getting about 16 on the highway. I generally drive it gently, but every once in a while it's fun to "drop the hammer."
posted by ganzhimself at 1:03 PM on February 7, 2007

Everyone's saying that higher octane gas won't improve your mileage. That's usually true.

But some engines have knock sensors and automatic spark retard when knocking is detected. The spark retard decreases engine performance and impairs your mileage a little.

A little blurb on this page suggests the 5.2L V8 doesn't have a knock sensor, so that's out for your car.

There are not many ways to increase mileage. As others have said, overinflate the tires. You'll reduce rolling drag at the cost of a much bumpier ride.

Driving style can also be altered productively. On the highway, don't exceed 55 mph. This is probably the #1 way to improve gas mileage; you can boost your highway mpg 3 or 4 points this way. You have to pretty much park in the right lane and let others blow by you, though, which is annoying.

In the city, never brake; or drive in such a way that you have to brake as little as possible. Every time you brake your car you are creating a necessity for accelerating afterwards; every time you accelerate, your gas mileage goes to hell. Depending on your current driving style you may or may not be able to squeeze out a few mpg this way.

I'd be surprised if you could increase your total mpg more than 2 or 3 points overall. Most of the mpg number is built into your engine, transmission, body shape and gross vehicle weight by the laws of physics.
posted by ikkyu2 at 1:04 PM on February 7, 2007

As others have said, one of the easiest (safe) ways is to try to drive so that you brake and accelerate as little as possible. Remember that most hybrid cars charge the battery with regenerative breaking. You can apply that same principle to your non-hybrid by using the brake as little as possible. (I'm constantly amazed at the people who practically floor it on their way to the light that just turned red. If they went with a slower acceleration they'd save gas, their brakes, and possibly get where they're going faster (don't have to accelerate again))
posted by chndrcks at 1:05 PM on February 7, 2007

ikkyu2 is right. Some engines designed for regular can get a slight boost in performance when using higher octane fuel by adjusting the timing. Even if you could get this with your engine though any gains would probably be outweighed by the extra cost of the fuel.
posted by caddis at 1:16 PM on February 7, 2007

Thanks for the answers. I wanted to make sure there was no gains to be had if I filled up with premium.
posted by ganzhimself at 1:21 PM on February 7, 2007

To be even more concrete about your driving habits:

- keep your engine below 3000 rpm. Over revving your engine has drastic effects on your consumption. Three thousand rpm is the sweet spot on most cars and trucks (assuming you're not using a diesel).

- Don't idle for more than 30 seconds. Starting you car uses about the same amount of gas as idling for 30 seconds (or less for modern vehicles). Most stop lights are in the 2-3 minute range. this is the reason hybrids win so big.

- Get all the gubbins off of your vehicle: no roof racks, light kits, roll bars, etc.

- get you car tuned properly. Detuned and unmaintained engines can really increase consumption.
posted by bonehead at 1:30 PM on February 7, 2007

Your best bet: listen to your owner's manual

Actually, your best bet is to listen to your engine under load. If it doesn't ping with regular - that's all you need.
posted by Neiltupper at 1:33 PM on February 7, 2007

There is one other valid reason to occasionally tank up with one of the premium brands that includes an extra shot of detergent additive: you keep your injectors and intake valves cleaner, forestalling economy robbing deposits. If your vehicle already has lots of miles on it, this might be a good, zero risk thing to do, although if your intakes and injectors are already heavily gummed up, you can't expect premium fuel alone to solve the problem. I run 2 or 3 tanks of V-Power premium, which has at least 5x the minimum amount of detergent additive specified by EPA, through my vehicles every 5,000 miles or so.

In case your vehicle already has very gummy injectors, a gummed up throttle body assembly, or valves with major deposits, you might need a solvent based fuel system cleaning, preferably one where the injectors are removed from the fuel rail and checked. There's only so much you can do about gummed intake valves on most vehicles, without disassembling the engine for a valve job, but there were machines made to blast walnut shell fragments down intake manifolds (and then collect the debris by vacuum) to clean them, that were somewhat effective. Other companies sell equipment and supplies to allow service shops to do a solvent based fuel system and intake valve cleaning on the vehicle, but how effective these are, for the money involved, I can't tell you.

Fuel filters on high mileage vehicles also need to be changed, and fuel pressure and pressure regulation on injected vehicles needs to be checked.
posted by paulsc at 1:46 PM on February 7, 2007

Accelerate slowy and slow down slowly:

http://money.cnn.com/2006/05/01/Autos/driving_for_mpg/index.htm
posted by zaphod at 6:16 PM on February 7, 2007

Actually, your best bet is to listen to your engine under load. If it doesn't ping with regular - that's all you need.

No, your knock detector might be changing the mixture to avoid pinging, at the expense of power, instead, just like was explained earlier in this thread.
posted by mendel at 7:32 PM on February 7, 2007

Don't sweat it. Burn that fuel like we've got it.

But, seriously: (your driving)^n. I find that keeping the engine under 4k and using cruise control really helps fuel economy.
posted by Netzapper at 9:11 PM on February 7, 2007

Starting you car uses about the same amount of gas as idling for 30 seconds (or less for modern vehicles). Most stop lights are in the 2-3 minute range.

I'd be interested in backup for the first statement; I'd think that the number would be quite a bit less than 30 seconds.

The second statement is of course ludicrous. Time your stop lights. Unless you live in some slow-witted flyover state _[ducks], there's no way that "most" stop lights are that long. They may only feel that way.
posted by intermod at 4:40 AM on February 8, 2007

The 30 seconds of idling = start-up thing was commonly discussed during the 1970's gas crises. I don't know about 30 seconds, but starting the engine appears to use less fuel than a two minute idle:

Test #6 Avoid Excessive Idling

Result: More important than we assumed

Cold Hard Facts: Avoiding excessive idling can save up to 19 percent

...

Method: We took two cars and drove a 10-mile route stopping 10 times for two minutes. We shut down the car each time. Then we drove the same route at the same speed and let the car idle for two minutes.

posted by caddis at 7:16 AM on February 8, 2007