What could cause lower than expected MPG [mechanically]?

[uRabbit]

Hey folks! After being car-free for three years, we picked up an '09 Honda Fit Sport AT. I used to have an '07 Fit Sport MT back in 2010, and got 42 MPG in that puppy without trying. Before I even knew about hypermiling.

Now, however, we've got a new-to-us Fit with 121,000 miles. It's the automatic with the select shift. I always use the select shift and I'm the main driver. Most P&G is done engine-on, mostly because I'm worried about using the ignition that much (since you can't bump-start), and because bump-starting is much safer in terms of reaction timing in case you need to act quickly.

That said, I still think I should be getting much better mileage than currently. I've only averaging 30, and the most recent fill-up was 350 miles of all 50-60 MPH roads, barely any traffic, barely any hill-climbing, etc. The fuel efficiency gauge on the dash projected 41 MPG, and from what I know of my previous Fit, it was very accurate. Unfortunately, I got almost 6 MPG less than that, and that seems to be the trend in this vehicle.

Therefore, I'm looking at mechanical reasons for this discrepancy. I've changed out the coolant, AT fluid and air filter, and replaced spark plugs. The transmission shifts like *butter* now. In a couple weeks, I'm changing the oil and filter, but they're not in bad shape now anyways. Tires on all tanks except this last one were at 29.5 PSI all-around. The sidewall says 51 PSI max, so I filled up to 45. I was really hoping that'd get me into the 40's. Bummer.

I do think that the valve clearance needs adjusting, but the car isn't idling strangely or anything, so it's not a top priority. I do need to do it soon, though.

I know that the 2nd gen Fit isn't as efficient as the 1st gen, but still.

Any thoughts?

[oil pan 4]

The usual stuff.
MAF may be dirty or worn, oxygen sensor wear and need to be replaced, remove roof rack if you have one, add low rolling resistance tires.

[mpg_numbers_guy]

It seems like the automatic Fit gets much lower fuel economy than the manual. Ecky can speak into it more with his S/O's Fit, but IIRC manual vs automatic is something like 10 mpg difference?

[nemo]

I'll add brake drag, wheel bearing and alignment.

[roosterk0031]

My XFE had a evap solenoid go bad that was causing the Fuel Trims to max out taking it from 40 mpg down to low 30's, (essential a big vacuum leak) but it would run bad too, rich miss fire at low loads. Replaced $300 worth of other parts till unplugging the electrical connection to it and it returned to nomal. Replaced it yesterday for $32 CEL for it being disconnect hasn't cleared yet waiting to see if it will go away on it's own.

Edit: My CEL cleared it's self today.

[oil pan 4]

Originally Posted by nemo

I'll add brake drag, wheel bearing and alignment.

Good point.
How are the tires wearing?
Brake drag is easy to diagnose at least and usually pretty easy to fix.

[me and my metro]

Toyota Rav4 do the same thing. The sticks get 30 something and the automatics are in the teens.

[ksa8907]

Originally Posted by nemo

I'll add brake drag, wheel bearing and alignment.

These have always been the issues I discover on used cars. Wheel bearings won't directly affect mpg but as they continue to fail they can cause an alignment issue.

[uRabbit]

Originally Posted by oil pan 4

Good point.
How are the tires wearing?
Brake drag is easy to diagnose at least and usually pretty easy to fix.

Good call.

There is some front brake rubbing noise at low speeds. There's also a sway bar end link (I think) that needs replaced.

I've never worked on brakes before, so should be fun!

Tire wear seems fine, but they're relatively newer tires. Previous owners installed them just a few thousand miles before I purchased it.

[nemo]

Originally Posted by uRabbit

Tire wear seems fine, but they're relatively newer tires. Previous owners installed them just a few thousand miles before I purchased it.

The question is were they replace to hide something.