MPG on car lower than it is supposed to be?

MPG is affected by many things. Type of driving, fuel quality, temperature and mostly by the right foot. For instance, most MPG figures are determined on a closed test track using what is known as "reference fuel" basically it is a custom fuel without all the additives and ethanol that you get at the pump. It also is not a winter blend fuel. As such that alone lowers the MPG. Now you added snow tires which are softer and have more rolling resistance. Those all add up. Then it depends a lot on where you are driving, mostly in stop and go like a city or town? Long highway miles? Stop and go will cut mileage in half easily. Then there is how you drive. Heavy loads (like 2 "average Americans") or with a lot of extra stuff in the car and that lowers it some more. Fast driving takes some more. What I would do is write down the current mileage. Then drive your normal way. When you fill the tank only fill it until the pump kicks off. Then write down the gallons like 12.34 or 10.78 and drive around 100-1500 miles. Now add up the total gallons and divide that into the miles. That will give you a true idea of the mpg you are getting. Never trust the "miles to empty" or "current mpg" calculations the car shows you. I have yet to find one that is actually correct. That is again because they calculate it based off the reference fuel, not real pump gas. In theory the vehicle should be able to calculate it, but I have yet to see one that was actually correct when compared to a logbook. Plus, the log is also good for things like tire changes, oil changes and other service items. Speaking of which, if you intend to keep the car for years past the warranty, look up the "severe service" tables and use those. Modern cars are set up to reduce the total cost of ownership. However, that means they don't really care if it blows up 100 miles out of warranty, as long as it makes it past the end. So, you get "lifetime" coolant and transmission fluids and oil changes stretched out to 10,000. The severe service actually defines most real-world driving. Like snow, extreme cold, stop and go traffic, and variable loads. Your car uses a "maintenance minder" that will pop up a light to tell you what to check or change. For instance, they warn that if you are driving under severe duty, you should change the transmission fluid every 25,000 and not wait for the minder. I would suggest every 5,000 for engine oil and filter. But calculate the MPG and then decide what you want to do.