Sometimes it’s easy to see that your tires need more air, but it’s far from obvious in many cases. We recently noticed a drop in mileage in Theresa’s car, and I was ready to chalk it up to different gasoline composition in winter that reduces mileage. But I noticed one of Theresa’s car’s tires was noticeably low, so we filled it up. As it turned out, all the tires were under-inflated, even the ones that looked fine.
Then, one cold day I noticed a tire symbol on my dashboard. I thought it was the below-freezing warning light, but I read the manual, and it turned out my car was telling me my tire pressure was low. I have no idea how it knows that, but it was right (I believe my Prius is smarter than I am), so I pumped up my tires.
So check your tire pressure. Both cars are getting significantly better mileage after this simple effort!
Cold weather is a cause of lower tire pressure (and other other things that reduce mileage). For one, pressure is related to temperature (The ideal gas law states pV=nRT where p is pressure, and T is temperature: if everything else stays the same, as temp goes down, pressure goes down). But then as pressure goes down there tire edge moves more around the rim, letting more air out, compounded by reduced flexibility of the rubber.
It is true that gas is formulated differently in winter than summer in order to combust at the right temperature.
But there are some other weather related factors, affecting fuel economy, nicely summarized in Figure 3: Estimates of Fuel Economy Reductions Caused by Various Operating Conditions. In particular, using the defroster is the same as using the A/C in most cars (the A/C blows dry air), and this can reduce fuel economy by more than 20%. Idling and warm-up both use more gas.
So think about these things and make a few simple changes: avoid the defroster; inflate your tires; let your engine warm up (while driving!) before running the heater.