Overnight gas mileage dropped to half of 17 mpg. The car runs excellent

Half of 17 mpg is 8.5 mpg. But anyway, your mileage computer calculates mpg based on miles driven vs gallons gone from the tank. Someone must have siphoned half your gas out. Does your fuel gauge look like it suddenly dropped? Your car's computer thinks the engine consumed it....I think someone stole it. Check your mpg the old fashioned way. Top up your tank.....drive 100 miles.....divide by gallons needed to refill tank. Is it back to 17 mpg? Should be.

If you use your low octane gas all the time try using the middle or high octane gas.