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Old 10-19-2017, 06:29 PM   #85
rtcat600man
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Originally Posted by Lizey93 View Post
For the first 3 months I had my car I only ran 87 in it and averaged 25.1 across that entire timeframe with about 75/25 highway. For the last 7 or 8 tanks I have been using 89 and I am averaging 22.5 with the same drive. I am switching back to 87 when I fill up tonight to see if it goes back up.

Just an FYI. Part of the reason for decrease in mileage during the past 7 or 8 tanks is because the weather changed on you. The warmer it is the better the fuel economy, that is due to the fact the air is more dense. The denser the air, the less fuel required. Thus better MPG. I have 7 years worth of data to show my mileage for over 100K miles.

Below is just one of my charts.
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Old 10-19-2017, 08:35 PM   #86
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I’ve been running premium 93 octane now for about 2 months and basically no change in my gas mileage. The car does feel peppier but that could be due to my lighter wallet.
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Old 10-20-2017, 01:04 AM   #87
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Originally Posted by rtcat600man View Post
Just an FYI. Part of the reason for decrease in mileage during the past 7 or 8 tanks is because the weather changed on you. The warmer it is the better the fuel economy, that is due to the fact the air is more dense. The denser the air, the less fuel required. Thus better MPG. I have 7 years worth of data to show my mileage for over 100K miles.

Below is just one of my charts.
Hot air rises because it is less dense than cold air.

x amount of air requires x amount of fuel to produce x amount of power. Denser air requires more fuel as measured in volume. As measured in molecules the same number of fuel molecules has to be proportional to the # of air molecules, no matter the density, to achieve the optimum air/fuel ratio. Just as cold air is more dense, fuel is also more dense at lower temperatures.

In the old days with carburetors fuel jets and idle settings had to be changed to achieve the optimum air/fuel ratio. With fuel injected engines the computer adjusts the ratio using the MAF sensor and the O2 sensors. So no more pulling out the tools when driving up and down the mountains or pre-heating the air before introducing it to the engine when the temperature drops.

Driving at a steady speed requires x amount of power no matter what the temperature is, except that air resistance may be slightly reduced in a warmer atmosphere (or higher altitude). If the air is less dense it requires a greater throttle opening to achieve the amount of air/fuel to obtain the same power as produced with denser air. Anyone who has driven through the mountains knows this. It is the reason superchargers are common on aircraft.

What does result in lower fuel mileage in the colder states is the change in fuel mixtures for summer and winter driving. Also cold days usually force you to warm the vehicle before driving. Not only for comfort, but to keep the windows from fogging. It also increases the viscosity of the lubricants which result in more drag on the rotating masses. Also, running the defroster, wipers, and lights (due to shorted daylight hours and an increase in precipitation) put more load on the alternator resulting in increased drag.

Another cause of lower mileage in colder air is the increased power the engine can produce with the same throttle opening as warmer air. It gets more than a few of us excited enough to hammer the throttle when accelerating.

Spinning the tires can also result in lower MPGs, but since the odometer doesn't know you aren't going anywhere fast, it is not a very accurate indication of true MPGs.
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Old 10-23-2017, 09:09 AM   #88
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Originally Posted by CamaroFred View Post
Hot air rises because it is less dense than cold air.
This is true. I can't confirm what CamaroFred said in regards to why you're seeing lower gas mileage in colder weather (though what he said makes sense). Cold air is more dense. Cold air also means more air in your fuel/air mixture into the cylinder. That's why we like to install COOL Air intakes on these cars. Cool air = denser mixture = more power.
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