MPG Decrease Over Time

Hi Everyone,

First would like to clarify that this isn't a complaining "my mpg is bad thread" - rather, I've found that my MPG has been on the decline the past few months/tanks.

I've tracked every tank that has gone into the truck since I have owned it, over about 6,500 miles now. My lifetime average is 18.52 mpg over 19 tanks of fuel. The vast majority of these miles are my route to work, so there is a lot of consistency in the speeds, stop lights, hills, etc. There are ups and downs too, on tanks where I've driven longer highway distances or towed my trailer.

What is odd to me, is that the past few tanks have been close to 3 mpg lower than the previous tanks. Further odd, when I look at last years tanks of gas for the same timeframe (Jan/February) I had much better mileage a year ago - when temps were much colder than this mild winter we are having. I'd expect a warmer winter would mean slightly better mpg. My driving style hasnt changed and is pretty lax in the Tacoma, for reference my 18.5 overall average isnt bad when you consider about 95% of those miles are not highway miles and on a 4 mile commute in which the engine never reaches optimal temp. I even fill with the same gas station almost every time.

The truck is a '13, 2.7, 4x4, manual trans with now just under 40k miles. It is completely stock. It runs just fine, had an oil and filter change about 500 miles ago with the same brand/weight oil and filter that I always use. Both diffs and xfer case have fresh gear oil in the past year and the trans even more recently (took some experimenting to find a GL4 fluid that shifted well). So while I haven't chocked this up as something wrong with the truck yet it is jumping out to me and I'll continue to monitor the mpg over time on my spreadsheet.

I'm posting to ask if there is anything to look out for? 40k miles seems young for an o2 sensor to get lazy but I don't know the replacement interval for the 2.7. I do believe the truck has its original spark plugs but I believe these are 100k mile plugs, correct me if wrong. Air filter looked good at last oil change. I've noticed no changes in how it runs, feels, or sounds. Welcoming any and all advice.

 

My 2008 2.7 would lose about 2 mpg during the Winter months, but would come back to normal in Spring.

 

I hand calculate my mileage and track it at every fill up. My truck usually get around 17.2-17.8ish. Last few months it been 15.4-16.0. All of the maintenance was up to date. This weekend I cleaned my MAF sensor- with special MAF spray. I then filled my tank at my regular gas station. Refilled today and calculated my mpg: 17.8. Hopefully my next few tanks will show the same increase in mpg.
Good luck, my lost mpg's were driving me nuts.

 

I've tracked every tank for 5+ years, always peaks in July and troughs in February. Part of it is due to winter blend, part of it is I take shorter drives in the winter. If you want to see this expressed visually: http://www.fuelly.com/car/toyota/tacoma/2011/ziggynagy/98090/fuelchart

 

Has your throttle body been cleaned, air pressure in tires? My mileage typically drops off in winter also. That's with every vehicle I have owned. I have found that + 2 psi can make a helluva difference in the coldest months.

 

I never knew MPG would change according to air temperature

 

Not air temp. Oil companies change gas formulas during winter. The gas burns cleaner, but is less efficient.

 

This is true, but engines also run richer in colder weather. Then there are the people who need to warm up the truck on cold mornings getting 0 mpg. I know we've had some of those mornings where I live. My 2014 V6 gets about 20 mpg in the warm months, but I actually dropped to 16.5 on one fill up during the coldest part of winter.

 

There's a good chance those short commutes are catching up to you. The engine is not getting warm enough to correctly burn the fuel. That unburned fuel forms a goo in your cats. this confuses the computer and the truck throws more fuel at the problem. Go for a long drive at highway speeds once a week (drive for half an hour at highway speeds). You might be able to burn out the goo, since the truck is so young still.

 

Like most I see a dip in winter mpg and agree it's due to a mix of winter blend fuel and cold air requiring more fuel to burn. However, my point is, a year ago the winter was much colder than this winter and I'm returning worse mpg.

The truck is definitely a victim of short commutes. But I've never heard of this goo in the cat converter before. The only negative that I know of is the oil probably doesn't reach optimal temp to burn off moisture. That said, this truck has a brand new cat converter installed by the dealer when I purchased, believe it was due to a recall.

I've never cleaned the maf. Seems like one of those things where it's possible to do more harm than good.

I'm going to continue to keep an eye on it. As the weather warms I'm hoping to see the mpg align with my records.

 

The winter blend and short commute combo is quite the 1-2 punch. I didn't know cleaning the MAF sensor could yield increased MPG's. What's this special spray that you use @Front sight

 

I would run some fuel injector cleaner through my tank and see if that rectifies the poor gas mileage. If you run pure gas you will get better gas mileage than 10% ethanol blend.
Air Cleaner dirty? There are alot of facors that can affect gas mileage. Take a systematic approach and try and eliminate possible anomalies.

 

my mileage shot up the last couple weeks its been warmer. Fuel didn't change

 

When you start up the truck, before the cats reach operating temps, the engine runs in open-loop mode, which relies on the MAF and not on the O2 sensor. Clean the MAF sensor with dedicated MAF cleaner. Visually inspect it for dirt and grime buildup on the sensor.

Replace PCV valve since you are more likely to have vapors in the crankcase.

Make sure you are using 0Wxx engine oil and 75Wxx diff oil.

Make sure the driveshaft U-joints are greased every year.

Try a different gas station in case the pump flow meter has drifted out of calibration.

 

Thanks tomwil, this is the exact spray I used. It's been 2-3 years since the last time cleaned my MAF. I didn't notice any more power to the wheels lol, but the truck does feel slightly more responsive to throttle input. All my other bases were covered with maintenance so I gave this a shot. So far so good. I'll see how the next fill up goes.

 

Spark plug time!

 

It isn't really that cold air requires more fuel to burn. Cold air actually means you're getting a greater mass of air per the same volume, so you will operate at a lower throttle setting to even it out.

I've been monitoring my engine temperature lately. On a very cold day, the engine hovers around 86-88 degrees C and gets fairly bad mileage. On a warm day, the engine temperature is up higher, more like 92-93 degrees, and fuel consumption goes down. Might be time for a new thermostat, or blocking off the radiator.

 

Only When My Right Foot Is In It... 99.9% Of The Time

 

Well finally changed my original plugs st almost 290,000 miles! 08 5 lug! I can definitely feel a difference, we'll see if mpg improved, I'll bet it does significantly! The gap was almost double
Very easy diy. Owners manual says.043 on the gap FYI