I know I've seen threads like this on here but I cant seem to find them.

Over the past year or so my MPG went from 25 to 20, same kind of driving, no real engine mods and same driving characteristics. The car has an IceBox intake setup, some lightening, mid muffler delete and thats about it.

When I bought the car it had winter tires on them so I had them changed and I saw a slight decrease in mileage, which I found very odd. Did the 105k service, check tire pressures, changed air filter and still decreased in mileage.

Car has 112,000 miles. Any ideas?

 

without telling us your average mpg and average mph, this thread is useless.

 

Yes, there is a direct correlation between average mpg and average mph.

You've checked all the normal things. Next on the list would be bank 1 sensor 1 and bank 2 sensor 2 O2 sensors. I replaced my bank 1 sensor 1 and saw an increase in mileage. They degrade after awhile and the stock ECU even has tables for O2 sensor degradation as this is a wear and tear item. If the O2 sensor starts going bad, it may not be relaying accurate information to the computer and the computer may be giving too much fuel resulting in lower mpg.

Check your spark plugs for richness.

 

Has the mileage been decreasing consistently? Have your driving habits changed? The reason I ask is because I've experienced a similar decrease in mileage but noticed (once I started paying attention) that I drive the car harder now than when I first bought it. I drove like a grandma when I first got a hold of my TL, as if I was afraid I was going to break it or something. Now that I've paid it off, torn it apart via the TL diet and modded it a bunch I'm not as easy on it as I once was. As such, the lead foot has dropped my mileage by about 3-4mpg over the last few years. I decided to drive it easy for the last month just too see what the mileage would come out at. Sure enough, back up 3-4mpg to the mileage it used to be.

Otherwise, I'd look into maintenance items - replacing/cleaning spark plugs and O2 sensors, tire pressure (this can make a bigger difference than you might think, even a few PSI) and possibly a good synthetic oil. Careful on the tire pressure though. Depending on the tire, too high a pressure can cause premature wear on the center of the tread.

 

Just follow the manufacturer recommendations for tire pressure found on the drivers side door sill. Check it in the morning when the tires are cold.

 

Too many factors go into FE, it's really hard to narrow things down if you're talking about a minor drop over several months. Once you start modifying the car these minor changes can certainly make things awfully complicated. You will get worse FE with dramatically reduced IAT's.

The VAST majority of the time, nothing is wrong with the car and its all the driver and conditions.

 

I went from 25 miles/gallon to 20. I think some of it is contributed to the new tires I put on there a few months ago although the car had winter tires on there and now it has sporty all seasons.


justnspace I'll get you the average mph when i get home

 

Good call with the average mph. First, you need to compare your average mph back when you got 25mpg to now to see if there actually is a problem or if your driving habits changed. If you don't know what it was back then, posting the average mph will still help us to see if you're in the normal range.

The others were right on with the 02 sensors and checking the spark plugs to see what's actually going on.

 

Avg MPG: 20

Avg MPH: 27 mph

 

I swear I'm not lying, mine is EXACTLY the same. It's higher than normal for me because I drove out of town twice last week. It's been fluctuating between 20/27 and 21/27.

It looks like your car is lightened a bit and you probably don't have the wide soft tires as I do so your might be a little on the low side but you're definitely within the normal range.

 

I checked mine this morning:

33 avg mph.
21 avg mpg.

 

17 mph
15 mpg

low and slow

 

Over 9,000 miles i am at:
22 mpg
29 mph

 

Over 9,000 miles i am at:
22 mpg
29 mph

 

For giggles, reset the ECU?

 

Cool thank you fellas.