Tanking gas mileage

I’ve got a 09 Ford Fusion, four cylinder, five speed manual transmission with about 6k on it.

During the break in (thousand miles)the car was driven very gently, the cruise control was never used, speeds rarely topped 65, the a/c was off because it wasn’t needed, and I was averaging 32 mpg with 80% highway use.

After the break in my driving habits didn’t change. The only difference was increased use of the A/C. The mileage dropped to about 31 mpg.

After getting the oil changed at 5k miles, the car has been averaging exactly 25.4 miles per gallon. Is it possible the oil was changed with a heavier grade that was in there previously? Would that account for such a drastic drop in fuel mileage?

The tire pressure is where it should be.


Heavier oil could make a difference, but a 5 mpg difference?

That is a big difference. The mpg read out on the dash confirmed my math. The mileage took a dive after the oil change.

Your car is rated for 20 MPG city and 29 MPG city. 25.4 MPG is dead on for what you should be getting overall. How did you come up with your figures? You can’t rely on on-board MPG computer, nor can you just take an accurate measure from one or two tanks of fuel.

Who changed the oil, and did they use the correct oil?

Was this a Quicky-lube joint? If so, I suggest you check the dipstick. They may have overfilled the engine, which could account for the decrease in mileage.

Don’t rely on that dashboard mpg readout. They are not all that accurate yet. They are good for general things, but there are ways to fool them. All kinds of changes can change mileage and those computers don’t take all of those factors into account so they may show false numbers.

Just for fun try this. Fill your tank and note the odometer. Nest time you fill up see what your on board meter tells you you got and see what the old fashion math (divide the miles since the last fill up by the number of gallons. Repeat two more times and see how consistent each is. The overall manual calculation will be most accurate.

To calculate the mileage I take the odometer reading and divide by the gallons purchased. I usually try to use the same pump and let it shut off automatically. I calculate mileage almost every tank. Why? I’m not sure.

The oil was changed at a Ford dealer (using a free coupon!) I suppose the extra oil they left on my engine and inside door panel were worth it for the free change. Sans the freebie, I’m going to take it to my local independent going forward.

I did check the oil after the change and it was where it should be.

I realize my mileage was higher than the ratings, but my driving was/is mostly highway. At one point my car WAS getting 32 mpg average.

I’m leaning toward the issue being a oil thicker than the weight that was used before, but would that account for such a big drop off? Anything else to consider?

Take it to the Dealer tell them you are only getting the advertised mileage and you are a unsatisfied customer and want something done about it.

Thanks, very helpful.

First, you don’t know that you got exactly 25.4 MPGs. Your least precise measuring tool (the odometer) only gives you tenths of a mile. Therefore, you only really know you got about 25 MPGs.

Second, there are so many variables to consider that I seriously doubt it is the oil. You say the only difference is that you “increased use of the A/C.” That could very well explain the difference, although, like I said, there are too many variables that you haven’t eliminated to be sure.

So change your oil to the recommended weight . . . do it yourself and it’ll cost you $15. But I’ll bet that a 20% drop is not (completely) due to a heavier weight, if that’s what happened. How’s the tire pressure? Driving habits or route changed? A/C will make a difference, maybe a mile or two per gallon. Are you still driving 80% highway? That’s where my guess would be. More aggressive driving, A/C on, low tire pressure and heavier oil MAY drop you to 25 mpg . . . but FoDaddy called it on this one . . . dead center between highway and city. Rocketman

If you look at your gas mileage from one take to another then you’re calculating your gas mileage WRONG.

You need to take an AVERAGE over several tank fulls to determine your TRUE gas mileage. I have a little book that I write down my mileage and how much I filled up every time I fill up. My gas mileage may fluctuate even 5mpg from one tank to the next…but if I take the AVERAGE over 4-5 of those fill-ups my mileage is very very close.