medeka

I own 1999 Maxima and 1999 Sentra. I have been folloving a fuel economy of both cars regularly since they were new. I noticed extremely strong dependence on outside temperature. At the same type of relatively long distance, uninterrupted highway driving the Maxima gets 28mpg in Summer (80+F) and about 22mpg in Winter (~15F). The mileage follows the temperature almost lineary between these limits. The similar features for Sentra are 35 and 29 mpg. I tried to ask in several Nissan garages but nobody gave me reasonable explanation for this up to 25% variation. Any idea of what it is? Could it be improved in Winter? I used to have several other cars but the fuel economy of none of them was so strongly temperature dependent. Simple air density variation with temperature does not seem to explain it.
Ales

Eric L.

The answer is simple. When it is cold out, the engine takes longer to warm up to operating temperature (around 210 degrees, or where the temp guage pegs itself during normal driving). Hence, the ECU sets a "fast idle" to warm the engine up quicker on cold days. This is because when the engine is warmed up to operating temp, exhaust emissions are reduced. This uses additional gasoline.

On warm days, the engine gets up to operating temp sooner and uses less gas.

vmok

Hmm... could be Eric... but then he also noted that he is heavily highway dependent. I know for my daily 55 mile each way commute from Sunnyvale to San Francisco, the car hits operating temperature rather quickly and stays there throughout the trip... I think that might affect .5 mpg or so in the grand scheme of things.

However, I don't have any better theory either.

Only other thing I can think of is in the winter, if it rains a lot, there is a lot of rolling resistance created between the tires and water. Also, when you're adjusting the cold inflation pressure of the tire in a cold winter, it might be operating at a lower overall operating temperature, thereby gaining less psi due to heat generation/friction with the roadway, with the lower actual tire operating pressure lower than in the summer.

Another theory is that the gas company switches fuel formualations depending on the season to help cold start/vaporlock/etc...

-V