Checked all the filters (air, cabin, fuel, all clean) when I bought the car Feb. 1017 in N.J. Drove 2,500 miles to Utah. Put 4,000 more miles to date. Warm weather finally so decided to order and replace air and cabin as preventative. Checked fuel mileage on initial drive (2,500) home and using regular vs premium. The actual vs the car "computer" was ALWAYS better by 2-5 m.p.g. and premium was 1-3 m.p.g. better than regular. This was at constant highway freeway speeds with cruise control on 90% of the time.

Lately mileage has dropped from around 30 hwy to 23.

Pulled the filters to replace with great ones I found from AutohausAZ. Both were filthy (cabin doesn't impact the next info). The air filter was so dirty it looked like you spayed it with asphalt sealer tar. Ran a tank of gas through my kitty and car computer said 28 m.p.g., the actual (miles vs gallons used) is 33 m.p.h.!!!
Unbelievable that an air filter would/could impact mileage that much. 10 m.p.g. difference? Poor kitty was suffocating!!

 

And I thought our air in the UK was dirty. I feel for your poor lungs

 

Yeah surprisingly with Utah being a high desert one would think air quality would be decent. Unfortunately, during late fall through winter and early spring, all the "junk" in the air moving East across the desert, Great Salt Lake piles up against the Rocky Mts. and traps the smog in the valley. They love to use the phrase "inversion" when it looks like L.A. Smog, lol.

When it rains or been a while between snow storms it looks like you went four wheeling with the dirt and grime on your car.

 

Here's a couple of pics of our lovely air =

 

Dang; here I thought the pollen was bad in the SE USA!

Is great real life info on the importance of proper maintenance including filters. Thanks!

 

Just a small update, pure freeway or main road driving, no stops, I get computer saying 28-29 with actual (gallons/milage) @ 35 m.p.g.

A tank with mainly in town stop/go, not above 45 m.p.h. = 30 m.p.g. Still amazing a filter affected the Jag that significantly.

 

Today = pure, short trip, in town stop/go driving on the whole tank. Computer showed 24.8 actual, gallons/mileage was 22.3.
Gotta love that computer, too low on highway driving and too high on short trips, smh.

 

Looking at the fuel maps, I can see how it happens.


Clogged filter tells the MAF you are going slower than you are, and the heated sensors say the car is running lean. Computer dumps more fuel to "fix" the lean, but now the back side sensors see more fuel coming through the exhaust unburned, which screams "rich". Conflicting signals put the computer into open loop, and the system will revert to fail-safe....15-18 mpg.