I never thought that a loose piece of plastic could cost me so much gas mileage. Under the bumper of my 97 960 is a center air inlet with a shelf on the bottom called an air guide by Volvo. It directs the airflow to the bottom of the radiator and keeps the air from going under the car. I found just before I put the car away for the winter that this shelf or air guide was detached and jammed up against the top of the opening in the front. I figure that it was always loose but I never knew there was a problem because once the car would drop below a certain speed it was dropping back into place.
I ended up having to get new screws and clips to fix it right.
Well yesterday I pulled the car out and decided to give it a good run to get all the fluids up to temperature. I took a trip on the interstate up to Erie and back. Set the cruise control at 65 mph and set the Scan Gauge on the trip gas mileage setting just for giggles. I kept staring at the gauge because I couldn't believe what I was seeing in the display.
As I got back home I pulled off the interstate the display was showing fuel mileage of 29.9 mpg. I stopped at the Exxon right off the interstate and filled up. It took 8.42 gallons for my 252 miles round trip. That worked out to 29.92 mpg, and the Scan Gauge was showing 29.9 mpg.
The car is filled with M1 10W30, Redline D4ATF, and Redline 70W90 in the differential and I run 89 octane. I normally run 38 psi in the tires but upped the pressure to 40 psi (cold) that morning.
The normal highway mileage before I put it away was always 27.4 mpg at 65 mph on the highway. The only changes were fixing the air-guide and upping the air pressure 2 psi. The car had the winter blend gasoline from December.