FAQ: Improving Bad Gas Mileage

Take a brick and install it under the gas pedal. Now you will have the equivalent to a small Honda or Toyota in performance as well as fuel economy!

AA wrote:

Take a brick and install it under the gas pedal. Now you will have the equivalent to a small Honda or Toyota in performance as well as fuel economy!

Abaddon wrote:

Yeah, I've noticed that my tune doesn't like the cold weather. Well, actually it loves the cold weather lol. But I have noticed a significant MPG drop since the climate change. All my driving is city. Not very fun to watch the fuel gauge drop......

The other problem is that federal gas (the stuff with 10% ethanol) has less energy content and so gives you less mileage no matter what you do. See this:

http://www.fuel-testers.com/mtbe_ethanol.html

Also - I did not know that the stuff had such a short shelf life (so you can't stockpile it for yourself when prices are low, naturally) - USA pump gas now has a useful life of less than 100 days (unless you add Sta-Bil or another stabilizer to it, which jacks up your costs).

Another thoughtful discussion of the differences is here:

http://www.fuelly.com/forum/609/Difference-in-blend-higher-fuel-efficiency-or-higher-power-fuel

Depending on their fuel combustion strategies, different cars have different MPG losses when burning summer versus winter gas. But my hunch is that the current pump gas actually increases pollution by the loss of efficiency. That is, if you burned a quart of the old stuff to go 7 miles, you now might have to burn a quart of it to go 5.5 miles if your efficiency is cut by ~20%. So if you are getting from point A to point B and the points are 35 miles apart, you'd burn 5 quarts of the old gas (no MBTE, no ethanol) and more than 6 -- well, almost 7 quarts of the new. So the question is how much effluent is generated burning 5 quarts of the old style gas versus 7 quarts of the new? I don't know, but it'd be great if someone who does would comment. For those of you with chemistry backgrounds: The oxygen-bearing compounds in winter-formula gasoline displace other fuel components, so it takes more fuel to get the job done. Technically speaking, reformulated winter fuel has a stoichiometric ratio of between 14.3:1 and 14.4:1 -- compared to normal gasoline's 14.7:1. The decrease depends on teh car, some lose a smidgen and others lose a lot. Your mileage may vary, ha-ha-ha.

So much for that. Your thoughts?

Albertj

I totally agree Albert. Having worked for a retail petroleum company, I spoke with their dispatch employee frequently and he has the same theory you do in terms of the "winter blend" killing gas mileage on cars. You could pretty much tell right when it kicked in... So, in that regard, stupid me for forgetting that. lol He said his 2001 Grand Prix GT would lose about 4mpg on 95% highway commute and that's pretty ridiculous. You're theory on the lack of efficiency of the winter blend causing people to consume far greater gallons and thus being counter-intuitive for the ethanol additive's purpose is spot on. I've said this many times and people in the petroleum industry concur. Not that they mind entirely... they live off margins and how many gallons are sold so they get more $$ out of it. *rolls eyes* I won't rant down that road.

I calculated the mileage today and it's pretty much dead on 18mpg and that was completely city driving with two 20 mile highway trips. Guess my car isn't doing terrible for what it could be. I'm going to work with a local buddy who has HPT and see if we can't do a few scans and lean this sucker out a little. Maybe add a little low - mid range timing in there. That should bring mpgs up a smidge. I'm sure the heavy 18s aren't helping much in stop and go traffic.

Albert, you're never gonna' get a job with the government if you use that kind of actual, literal LOGIC!!

With my Californicated gasoline, I've had that thought for a long time... But, as long as they can make us jump thru hoops, spend more for our evil cars, and meet their own subjective goals, they don't give a damn about the real world...

The whole formula changes for winter. Because of the colder ambient temps, gasoline can be mixed more cost effectively and not boil as it would in summer months. That's why you can't store the winter mix, because it would boil once it gets warmer out. More info:

A Primer on Gasoline Blending

Gasoline is composed of many different hydrocarbons. Crude oil enters a refinery, and is processed through various units before being blended into gasoline. A refinery may have a fluid catalytic cracker (FCC), an alkylate unit, and a reformer, each of which produces gasoline blending components. Alkylate gasoline, for example, is valuable because it has a very high octane, and can be used to produce high-octane (and higher value) blends. Light straight run gasoline is the least processed stream. It is cheap to produce, but it has a low octane. The person specifying the gasoline blends has to mix all of the components together to meet the product specifications.

There are two very important (although not the only) specifications that need to be met for each gasoline blend. The gasoline needs to have the proper octane, and it needs to have the proper Reid vapor pressure, or RVP. While the octane of a particular grade is constant throughout the year, the RVP spec changes as cooler weather sets in.

The RVP is the vapor pressure of the gasoline blend when the temperature is 100 degrees F. Normal atmospheric pressure varies, but is usually around 14.7 lbs per square inch (psi). Atmospheric pressure is caused by the weight of the air over our heads. If a liquid has a vapor pressure of greater than local atmospheric pressure, that liquid boils. For example, when you heat a pot of water, the vapor pressure increases until it reaches atmospheric pressure. At that point, the water begins to boil.

In the summer, when temperatures can exceed 100 degrees F in many locations, it is important that the RVP of gasoline is well below 14.7. Otherwise, it can pressure up your gas tanks and gas cans, and it can boil in open containers. Gas that is boiled off ends up in the atmosphere, and contributes to air pollution. Therefore, the EPA has declared that summer gasoline blends may not exceed 7.8 psi in some locations, and 9.0 psi in others.

A typical summer gasoline blend might consist of 40% FCC gas, 25% straight run gas, 15% alkylate, 18% reformate, and 2% butane. The RVP of the gasoline blend depends on how much of each component is in the blend, and what the RVP is of each component. Butane is a relatively inexpensive ingredient in gasoline, but it has the highest vapor pressure at around 52 psi.

In a gasoline blend, each component contributes a fraction to the overall RVP. In the case of butane, if there is 10% butane in the blend, it will contribute around 5.2 psi (10% of 52 psi) to the overall blend. (In reality, it is slightly more complicated than this, because some components interact with each other which can affect the expected RVP). This means that in the summer, the butane fraction must be very low in the gasoline, or the overall RVP of the blend will be too high. That is the primary difference between winter and summer gasoline blends.

Why Prices Fall in the Fall

Winter gasoline blends are phased in as the weather gets cooler. September 15th is the date of the first increase in RVP, and in some areas the allowed RVP eventually increases to 15 psi. This has two implications for gasoline prices every fall. First, as noted, butane is a cheaper blending component than most of the other ingredients. That makes fall and winter gasoline cheaper to produce. But butane is also abundant, so that means that gasoline supplies effectively increase as the RVP requirement increases. Not only that, but this all takes place after summer driving season, when demand typically falls off.

These factors normally combine each year to reduce gasoline prices in the fall (even in non-election years). The RVP is stepped back down to summer levels starting in the spring, and this usually causes prices to increase. But lest you think of buying cheap winter gasoline and storing it until spring or summer, remember that it will pressure up as the weather heats up, and the contained butane will start to vaporize out of the mix.

And that's why gasoline prices generally fall back in the fall, and spring forward in the spring.

http://www.theoildrum.com/node/2964

Prices fall in the fall???

Maybe before oil & gas started to be traded like an investment commodity...

So winter has come to full swing here in michigan and im having a horrible problem with gas mileage. I went from getting around 25 MPG in the warmer months to getting around 10. I plugged in our Scanner and no codes are being thrown. Im kinda stumped on this. Anybody have any ideas?

winter gas, city/suburban driving, and you probably need maintenance like o2 sensors or plugs/air filter... there was a big thread on winter gas, you might should find it and have a look.

Also be sure to check your tire pressure.

Over the summer my riv was getting great gas mileage. With all the mods I have and everything I was still pulling between 25-27 MPG on the freeway. Now im only getting 11. I hooked up a scanner to my car thinking it would probably tell me my O2 sensors were bad and I was suprised to find that the car was throwing absolutely no codes. Is there anything specific I need to be checking on the riv? Because I know the O2 sensors will cause a significant drop in mileage but they seem to be working fine. Any help or input would be greatly appreciated because I cannot keep affording to drive this thing with this kind of gas mileage. And especially since im trying to sell it I would like to have this fixed before someone buys it.

Fastbuick95 wrote:

Over the summer my riv was getting great gas mileage. With all the mods I have and everything I was still pulling between 25-27 MPG on the freeway. Now im only getting 11. I hooked up a scanner to my car thinking it would probably tell me my O2 sensors were bad and I was suprised to find that the car was throwing absolutely no codes. Is there anything specific I need to be checking on the riv? Because I know the O2 sensors will cause a significant drop in mileage but they seem to be working fine. Any help or input would be greatly appreciated because I cannot keep affording to drive this thing with this kind of gas mileage. And especially since im trying to sell it I would like to have this fixed before someone buys it.

I am thinking winter gas and weak sensors, coupled with gas gauge error.

Need to measure your mileage using pump slips (receipts). What you do is fill the tank at a certain pump to the 'click' only, note the temperature on the pump slip and reset your trip odometer. Then in a day or 2 at the same temperature use the same pump to refill the tank just to the cloick, again. Get the pump slip. Calculate the mileage based on the trip odo reading.

As for your sensors if a GM tech has ever flashed your PCM he probably (unbeknownst to him) installed an update that lets the PCM "compensate" for weak sensors, so it won't set a code when they are weak. In practice, o2 sensors degrade over time. If you never misfuel they last very long. If you never misfuel, 100,000 miles is not an unreasonable service interval. The Bosch people would have you thinking the interval is much shorter.

SO I hear what you're saying your sensors di'n't set a code but they may be weak anyway. A scanner will give you an oscilloscope-type reasout so you can plot the rise/fall time of the blips on the o2 reading as the exhaust pulses pass it. Did your scanner do that and if so what did you see?

If you are comparing highway mileage on a freeway trip with mileage around a city you will be disappointed. Rivs do not get good mileage in city driving. 11 is on the low side, admittedly.

Why don't you list what mods you have so others can comment on their gas milage impact?

Albertj

Another problem though is my gas gauge doesnt work. it just sits there on E all the time lol. All of my mods are in my signature. But im talking im getting 11 on the freeway right now when I was getting 25-27. I always had to go by the odometer reading because of my gauge. Last weekend I filled up with $50 on premium gas and drove to ypsi and back, all of it was strictly highway driving maybe other than 5 miles. I did the calculations while I was sitting on the side of the road waiting for my friend to come get me due to running out of gas and I was getting 11.95 MPG on the freeway and thats terrible.

Do you think I should just replace the sensors and see if that helps?

My guess is a combination of selective maintenance and spirited driving preferences got you to this point.

Problems include:

- your gas mileage calculation is in error. at $3.60 a gallon a Riv will hold almost $70 in gas (appx. 19 gallons) and your gas gauge is broken so although you can reckon and estimate you don't know how much is ever really in there--and MPG measurement by odometer is imprecise because you will always be off by as much as 2 miles. I already outlined how to calculate MPG with a broken gauge. But I won't make you do it.
- you have a number of mods listed but I don't see plugs (autolite 104) and thermostat (180) and I don't know if your DHP tune expects the 180. And is your DHP tune optimizing mileage or power? The more power you are generating the more gas you will burn for a given system efficiency. How does that tune react to winter gas?
- you have not scoped your o2 sensors so you don't know if they are weak or not.
- you do not say what your engine RPMs are for a given speed. For all I know you were whizzing around in 3rd gear; you can check. At an indicated 70 MPH (some places that is the hiway limit) you should have around 1800 RPM indicated on the tach. And if your torque converter is not locking up that's a couple or 3 mpg lost right there.


All in, I really don't know about replacing your o2 sensors. There's all this other stuff to fix or eliminate as cause. Others who know more about this may chime in to help, I suppose.

Albertj

Ok, The gas prices here arent $3.60 a gallon for premium lol. I feel bad for those people who have to pay that much. Granted it is going up, but I filled at $3.20 and I still had maybe 3 gallons in the tank. Im pretty sure alot of it has to do with winter gas and my tune. My brother had a WRX STi and granted they are complete different cars, but his got terrible gas mileage in the winter as well because of his tune. My thermostat is actually a 160 and its been that way since it was tuned so I would assume that the tune is accounting for it. Id like to learn more about this scoping my O2 sensors. I have no idea what your talking about or how to do it. Mind telling me more?

if you use car-code (http://www.obd-2.com) or another software you can plot the output of the O2 sensors. The plot shows what the sensor voltage is and the rise time. From those you can get a sense of whether you rsensors are "slow" or whatever. That's all there is to it, really.

By the way, check the inflation in your tires.

Albertj

Running a 180+ thermostat will help your winter milage improve as well. I prefer a 195 stat for best economy results.

Running 160º t-stat in temps below 60ºF can be bad news. If your PCM isn't tuned for it, you will get audible knock on a Series II SC. And in my case, the engine would only "warm up" to 150º on colder days. That's not healthy for a daily driven car. Obviously, fuel economy will suffer.

Fastbuick95 wrote:

Ok, The gas prices here arent $3.60 a gallon for premium lol.

Well, screw you and the gas you rode in on... I'm at $3.65 and counting!!

However, I'll think about you tomorrow when I'm driving to the beach at 70 degrees on salt-free roads...

damn, I would gladly pay a couple extra bucks to have 70 degrees again lol

Fastbuick95 wrote:

damn, I would gladly pay a couple extra bucks to have 70 degrees again lol

And I would gladly pay less to escape the anti-American, anti-Everything Pelosi, Boxer, Feinsten, etc...
Just keep burning up that gas! You too, will have global warming...

Correction: It's 75 and climbing...
Looks like Punxsutawney Phil was right!

Looking at older posts, i see that people sometimes mention worse gas mileage with colder spark plugs and a colder thermostat. Would this really happen? I don't really get it, it seems these mods would only make the engine run more efficiently.

It does happen to a small degree with a cooler t-stat, but it is a negligible difference. Spark plug heat ranges do not make a difference with regard to economy.

The theory is, the motor is engineered to optimally operate at a specific temperature. For us, the temperature is ~200ºF. At that temperature, all components in the engine and transmission should operate at peak efficiency. When you lower (or raise) engine temps, you leave the range of peak operating efficiency, resulting in slightly decreased economy.

A cooler t-stat decreases coolant temps, which forces the engine to operate at a lower temperature. A 180º t-stat is only down a slight bit, and I haven't noticed a difference in MPG. Colder than that will cause the engine to run at a temperature close to warm-up cycle, causing poor efficiency and much more wear to the engine's internal parts.

Cooler range plugs affect combustion temps, but have no real effect on the overall operating temperature of the engine, so should not impact MPG.

The reasoning behind using cooler plugs and t-stat is to lower KR (knock) levels, which makes the engine better for safely producing power and torque under boost, but this does not make the engine more efficient for fuel economy, which really applies only when you are not running much boost (lower RPM, less load).

Cliff's notes: Conditions for peak power efficiency ≠ conditions for peak fuel efficiency.

Hope this helps.

Thanks a lot i'm hoping to start doing some smaller mods this summer, nothing major like a cam or anything, but i think i'll make a thread with some of my plans to see what everyone thinks and ask for suggestions