This is the first time owning a Volvo. I used a 15 gallons of gas going 163mile

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I check air filter, I use premium gas and did an oil change. air in tires are good. Im on my second tank of gas. So far I have gone 75 miles and used 1/4 tank of gas, the compuer says I have 375 miles to empty and I'm getting 22.5 gal per hr. What seems to make the problem better or worse? nothing How long have you had this problem? 1 month