Based on the Real MPG figures achieved by other Hyundai Tucson owners that you can view here - www.honestjohn.co.uk/realmpg/hyundai/tucson-2015 - we would suggest that 39mpg is actually quite good economy for a car of this size. We would also not expect a car with 39,000 miles to be suffering any significant issues with carbonisation, so we would suggest this is not necessary at this point.