I just bought a 2007 Volvo S40. The previous owner put 87 octane gas in it and I read that it's best to put 91 octane in, which I did yesterday when I filled up for the first time. My question is, why am I getting bad gas mileage?? Is this normal or is there something that I can check, change or replace? I filled up yesterday at noon and have traveled about 140-150 miles and it is showing I have used a little more than a 1/3 tank of gas. I hope this isn't normal because at this rate I won't be able to afford the 91 octane gas almost twice a week when the gas prices go back up... The previous owner had it checked out and oil changed about a month before I bought it and according to them everything checked out good with the car.

 

which model? Modern engines have knock sensors which adjust timing and mixture to accommodate a range of octanes without damage to the engine. running 87 simply cuts back on the top horsepower available and you'd probably never notice the difference. Dunno what you mean by bad mileage - you should expect 20 city / upper 20s highway, with winter blend gas taking away 1-2 MPG. Also if you do pure stop and go, even 20 is optimistic. Considering the gas tank is 16 gallons or so, you should expect 300 miles range for city driving, 400-425 highway. So if you went 150 miles on 1/3rd of a tank, that suggests you're getting upper 20s MPG - exactly what's to be expected.

 

I was completely mistaken when I put I used 1/3. It was more like 1/2. Today I am below half a tank and a little over 200 miles. It's about at a 1/4 or a line over a 1/4. I've noticed a drag in it when I accelerate and when I let off the gas it feels like the brake is applied when I don't push anything. I'm trying to figure out if that problem is causing the gas issue.

 

It's an S40 2.4

 

Best thing to do is measure mileage from a fill up - lines on the gauge are too inaccurate. Fill the tank normally (ie don't try to top off). note the mileage, and again at the next fill up to calculate miles/gallons. Again the result will vary if you are doing only city vs all highway.

If you think something is dragging you'd have to do a few tests - does the car coast normally if you pop into neutral? If you jack up the car, do the rear wheels turn easily by hand with no grinding noises (testing for stuck e-brake shoes etc). To check the fronts you'd need a floor jack/jack stands to lift both wheels together. Do you have a check engine light on? If the engine were running rich - ie say due to a bad O2 sensor, you'd probably get a warning light and an error code. Other possible causes include intake air restriction (air filter, preheat flap) , fuel pressure sensor or temp sensors are bad, clogged catalytic converter, restricted exhaust. If you had a wheel bearing going bad, they normally make noise (ie a wub wub wub at 30-40 MPH)... Are the tires properly inflated to 36 PSI? any signs of wear on the edges (bad alignment)? When was the oil last changed? What does the transmission fluid look like when you wipe the transmission dip stick on a white paper towel (red is good, reddish brown is ok, dark/brown or black is bad)...

 

You have a fuel mileage computer built into the car? (mine does) Instant and cumulative.