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In cold weather, yoiu need to adjust your tire pressure, else softer tires means less mpgs. When you add air, you could see an improvement.
I think that the tranies take a while to warm up, so best highway mpgs will not be evident until perhaps 1/2 hour into a trip. Manual GB's will take longer to warm up. A seasonal thing too.
Did you switch brands of fuel? Get some higher elthanol blend fuels and you could loose 5-6% mpgs, switch from that to lower ethanol fuel and you see an increase. I find that Amoco provides the better consistant mpgs for me.
Cold air offers greater wind resistance and cold tires are stiff and have higher rolling resistance till they warm up. So the cold weather leads to warm-up costs for the engine, trany and tires. If the tires are soft, things just get worse. I had s set of tires installed once that would not seat on the rims in cold weather, it was -20F and I had to stop every 10 minutes and pump up all of the tires. This was a highway trip and it was windy, a horrible experience. So when I stopped to air up those tires, the side walls were too hot to hold you fingers on them, even in those temps! There is a big energy transfer there. So when you see a SUV with soft tires on the highway in the summer heat, think about tire failure!
From a thermodynamic consideration, colder air means higher efficiency. But such considerations have an unstated hidden assumption of wide open throttle. A spark engine has a high energy loss pulling air through the partially open throttle. For the same compression ratio, the theoretical efficiency of a spark engine (at WOT) is greater than a diesel engine. But an diesel can operate at 20:1 compression plus boost!
In theory for partial loads, in hoter weather there is less wind resistance, and the throttle will be more open as the air is less dense, so there will be less throttle losses. Some vehicles do show better mpgs in hot humid weather, but my 95SET did not. The trionic system with hoter intake air might be faked out into retarded ignition timing which would reduce fuel efficiency. It will be interesting to see how my 2.3 T7 in the Aero does next summer.
Right now I have a tank of higher ethanol fuel in the Aero and the mpgs are taking a hit, but it was .20$ cheaper per gallon at the casino* and I was running on E so it seemed worth while, but I think that it is costing me about $2.50 in lost mpgs, so maybe not such a bargan. (I was driving by, I have never set foot in a casino.)
posted by 208.24.179...
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