I've been noticing a mileage drop over the past month. My driving habits have stayed the same, but my mileage has gone from a monthly average of 15.56 MPG to 14.60 MPG. (I calculated this via an iPhone app I have been using for two years, which is pretty accurate).

Only change in that period of time was an oil change which seems to correspond with the start of the drop in mileage. Mobil 1, 5W-30, Wix filter. Oil level is good. Air filter is good. MAF sensor cleaned about two months ago. Fuel filter about 8,000 miles ago.

Plugs? Car just cracked 30K.

Yes, it's a blown Roush, so I'm not exactly too concerned about fuel economy (it is what it is), but I'm more interested in what is causing the drop.

 

Keep an eye on it since it usually takes quite a few data points to confirm a real change. Tire pressure will change with ambient air temps. The ethanol content of your fuel plays a big roll since the % ethanol is usually different from station-to-staion and even at the same pump from delivery-to-delivery (I've seen as much as a 4 MPG change when using fuel from the same pump on bi-weekly 400 mile one-way trips). Stopping for more traffic lights, idling, yada yada. After keeping strict fuel mileage records for over 30 years, a 1-2 MPG difference or more during what you perceive as "identical" driving conditions is quite typical.

 

Might be time to swap out the fuel filter too. Mine didn't last very long, maybe 35k at most before the car started to bog down and see a drop in MPG.

 

Maybe the summer blends switched to winter blends. I notice a 1 to 2 mpg increase on summer blends of fuel myself.

 

Do they even do that in LA?

 

I think they do, but I'm not sure when that changeover takes place.

 

+1

I never got great mileage, but once I started going through 1/4 tank in 20 miles, I knew something was wrong. The filter was filthy and now it drives a lot better. Still doesn't get great mileage, but it is improved. Worth giving it a look, but there are a lot of variables

 

Changed the fuel filter about 8,000 miles ago. Unless I got a horrible tank of gas that helped clog it up, I doubt that is the problem. Car runs great, plenty of power, just the loss in mileage.

Also, I tend to go to different stations (Chevron, Shell, Mobil), not always the same one.

 

Does anyone know if the Fuel Injector Cleaners do anything worthwhile? Like the stuff you add to your tank every once in a while. I'm wondering if that would help. I've been getting mixed reviews on it

 

I try to add Techron Fuel System cleaner routinely every oil change or 10,000 miles, whichever comes first, in both of my cars. If I suspect a problem, I'll run SeaFoam thru the system. I consider it cheap insurance but don't get my shorts in a ruffle if I miss it once in awhile.