Interesting that your dealership always used synthetic oil. I'm up to 35,000 miles and my engine has not yet had any synthetic oil. The dealership used regular oil for my free oil changes, and I always specify that I want regular oil for my el-cheapo EZ Lube oil changes.

 

Everything you have said makes perfect sense to me. These are all things I suspected, but it's nice to have some confirmation. These are all reasons why I was originally inclined to blame the tires over anything else. I am not sure how the tC's stock tires rank against aftermarket tires, but these new ones are definitely performance oriented. The old stock tires were good for straight-line acceleration but very poor in corners. The new tires are the complete opposite, causing acceleration to be slower but staying glued to the road in the corners.

 

Since you went to el cheapo for the oil change, did you make sure they actually changed the oil and oil filter?

 

Even though it's el cheapo, the nice thing is that they have cameras mounted above and below the car bays, and you can actually watch the whole thing in real-time on the video monitor inside. I think this is just a new thing at my local EZ Lube. The picture is in full color and is really clear and crisp. I can definitely confirm that the old oil was drained out and the filter removed, because I watched it happen. I also checked the oil level after I received the car to make sure it had been filled up again.

 

Well I have been noticing this in my tC too. I really don't know how you manage to get about 380 but I usually get about 320-340 on a tank of gas. But lately for some reason I've been only getting about 280. I'm still driving the same. Did they change the gasoline lately??

 

RodC-
those direzzas are very sticky tires if I am remembering correctly. And yes tires can and do affect your fuel economy! Definitely. Just because a tire says it's 215/45-17 size, doesn't mean all those that are 215/45-17 are the same. Some have wider treadwidths, others the diameter can be 0.5-0.6" different than the stock tires. Also, how much do those dunlops weigh? Another factor people forget is how much a tire weighs. More weight = More power required to get it moving = More energy used = less fuel economy.

In all this I never read what your miles per gallon were. what was it before and what did it drop down to? I'm guessing about 3-4mpg would be about right if the tire was heavier, stickier, etc. But no you're not crazy to think that tires affect mpg's, because THEY DO!

Sorry ya had to find out the hard way, but next time you look for tires, you'll remember.

 

Well I'm not sure it's really a big problem afterall. I mean, the mileage has gone down quite a bit, but I love how sticky these tires feel in the corners. It's like night and day over the stock tires. I think the trade-off might be worth it.

I lost about 5 to 6mpg, but these tires are indeed stickier and heavier. Considering how much I had worn down my stock tires, the stocks I was using were actually even lighter and less sticky than their specs would suggest. So switching up to a new set of performance tires with deep treads and a softer compound was a sudden big change.

 

Not trying to be nit-picky but you really can't be sure of your exact mileage the way you're estimating it. I'm sure you've noticed the fuel gauge is very non-linear ... after slowly dropping from past Full to 1/2 the gauge drops like a rock to Empty ... makes it difficult to determine just how much fuel you're actually using.

To accurately figure mpg you would fill the tank to the pump shut-off, set a trip odometer to "0", then after next fill (again to pump shut-off) divide trip odo reading by fuel pumped = mpg. (To be totally **** you would use the same pump at the same station every time also.) Keeping a running mpg sheet will help you track overall mileage over a period of time.

This may be beyond your needs, but you may be curious as to what you're actual mileage is, you may actually be getting better mileage than you think ...

Also, it's best not to wait until near empty to fill up to minimize chances of sucking tank dirt/debris into your engine.

 

I think you may have misunderstood. That's exactly what I'm doing, so I can make a reasonably accurate estimate of my mpg after each tank.

 

I'm almost certain it's your tires. Rolling resistance does affect MPG. Most tires to be "OE" (like the Bridgestones you probably had) are created with lighter compounds, slightly shallower treads and sidewalls, and less rolling resistance. Dunlop tires are known for causing less rolling resistance basically because of the compound they use. And from what I recall, this has been proven to affect fuel efficiency by 15 - 20% (can't recall exactly the source, but I know the numbers are correct).

There is a reason crappy OE tires are more expensive.

 

Here's the link I was looking for This might help.

http://www.tirerack.com/tires/tirete....jsp?techid=29

 

Have you gotten an alignment lately?

It's most likely your tires I'm just trying to throw in another theory.

 

Yeah, like I said in the OP, the alignment was done right after the new tires were installed and it was found that one wheel had been way out of alignment for some time prior.

 

RodC- Have you also taken a look at your MAF sensor? They tend to get very dirty/oily on the tC. CRC industries sells a "MAF sensor cleaner" spray. I found that there was about a 10mpg drop in fuel economy due to that sensor being dirty. That might also be part of the problem too. 6 mpg is a lot for tires, the most I've ever witnessed was about 3mpg loss. so check out that MAF sensor, I bet it's dirty if you've never cleaned it.

 

Never cleaned it. How often should it typically be cleaned?

 

about every 15,000 miles, do you already know where the MAF sensor is located?

 

Yeah I do. I have an intake. Moving the MAF sensor was part of the intake install process.

 

oh okay, cool, just making sure if you knew where it was. here's a link to the CRC MAF cleaner spray. It's like $5, just spray wait 5-10 minutes for it to dry and reinstall. There are two sensors, an amber looking bulb on the outside (Idle Air Temp sensor) and the MAF sensor is down the tube inside. This cleans both. Dries pretty quick too.

http://www.crcindustries.com/auto/co...x?PN=05110&S=N

 

Cool. I actually ordered it yesterday after you first mentioned it. I also ordered Injen's air filter cleaner and filter oil.

 

cool :D let us know what ya find.