Why Did My Mileage Suddenly Plummet?

Ok, here’s the story.  In November of 2009 I purchased a new 2010 Subaru Forester X Premium.   It’s like the NH State Car, or something like that.  Anyway, the car is fine.  After putting 75,000 miles on it, I needed new tires in the beginning of December.   Since I’d gotten 75k miles from the stock tires, I wanted to replace them with the exact same tires – if it’s not broken, don’t fix it, right?

So I did.  I paid roughly $ 1k to have a set of 4 new Yokohama Geolandar G95 tires put on my car, and I had the oil changed at the same time.   Suddenly:  my miles per gallon plummeted.   Where I used to get 29 (driving 60 mph for extended periods of highway driving), I now get 26.  Where I used to get 25-26mpg driving around down and on the highway at 70-75mph, I now get 21-22mpg.  This is a problem.  So I did what any car-ignoramus of my generation would do: I turned to the internet.

My problem is that I’m not sure which variable is causing the problems: it seems likely that it’s the tires, but it’s also possible that something funky happened with the oil change.  This was my usual mechanic, who I assume is quite competent when it comes to oil changes.  I noticed that the receipt mentioned synthetic oil, and this *may* be the first time they’ve put synthetic oil in the car, but I don’t think that should have this kind of negative impact.  Anyway, there are a number of search results for both “mileage drop after oil change” and “mileage drop after tire replacement

For the “mileage drop after oil change” issue, a common response was that this could happen if the oil tank was overfilled.  That does not seem to be the case with my car: the oil level seems pretty normal (tangent, though:  when I went out to check my oil this morning, I pulled the dipstick and the level was way way high.  After wiping the dipstick off and dipping it again, the oil level was normal.   Why? I have no idea.  My father-in-law, who is not a machine-idiot – in fact he’s a machinery wizard, in contrast to me, was perplexed at why the initial dipstick reading was so high, seeing as how the car had sat in the driveway overnight).

For the “mileage drop after tire replacement” issue, I learned that this is to be expected – the new tires, as a result of having more tread, are both “larger” in terms of circumference (slightly!) and also have more resistance.  Thus, it can be expected to see *slight* mpg drops when putting on new tires.  But I’m seeing a drop of 15%-20%, which is way too much.

So I am turning to my audience.   Ideas?  Thoughts? Anyone? Beuller?

There are some other factors like winter gas, cold temps, etc which could have minor effects, but the drop in performance I’ve seen is ludicrous and instant after the service.   A few more data points:  I’ve probably put 1200 miles on the car in the last month, where I’ve seen the problem (I saw it instantly after the tire replacement/oil change).   I have checked the tire pressures and they are fine – right what my car’s door placard recommends: 32 psi front, 30 psi rear.   The mechanic that put the tires on also checked the pressures and the balance and said it was all good.  I am using the dashboard screen mpg data as a guide, but I’m confirming it with actual numbers when I fill up at the pump.   I do not think that my air filter is dirty – it seems fine.  I have not checked my MAF sensor, which was another common suggestion.  I have purchased gas at a number of different locations, so it’s not just a case of a bad tank of gas.



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