I have a 2002 VW passat V6. With my primary local short commute, the gas mileage is around 15mpg. I notice that in hot summer days the gas mileage can get to about 16 to 17 mpg. On cold days it only gets to 12mpg. Is this perfectly normal or is there anything needs to be replaced? Thanks.
It is normal for an engine to burn more fuel in cold temperatures.
As long as you have been doing regular oil and filter changes, maintaining correct tire pressures, etc. as per owners manual, you’ll be fine.
Yes, the warmup cycle takes a lot more fuel in cold weather. The only good thing about cooler weather is that you don’t need the air conditioner. The best remedy is a block heater if you live in an area with cold winters. It will warm up the engine very quickly, and your engine will last longer. Using a block heater, I found there just a little difference between summer and winter mileage. Your running gear still has to warm up, so cold trips will always take more gas.
Are you certain that your thermostat is working correctly? One that opens at too low a temperature or is stuck open will cause the engine to run cold. That increases fuel consumption.