Sudden drop in MPG

I have a stock 2011 V6 2WD TRDSport 67Kmi. Up until a month ago I was getting about +19.5mpg. Then one tank of gas came up about 60mi short (15.8mpg). Since then, every tank of fuel is looking to be heading down the same path. I am not driving any differently than I ever have and I always get gas from the same gas station. Only thing different is new tires (but they are stock Mich. MS2)...

I usually fill my tank until it is almost coming out the side and usuall get around ~340 mi before the light comes on. I then usually add around ~17.5 gal

To try and rule things out one step at a time, I went to the same gas station for a fill-up and found that I got 16mpg. I then changed spark plugs ( found a nice link on this site-thanks), diff fluid, and checked filters and belts. Old spark plugs were consistently gapped at .053-.054" and new plugs were gapped at .040". Filled up tank at new station (only took about half a tank, but it calculated out to about 16.5mpg). Ran it around town for about half a tank and found my numbers to be around 18.5mpg. With the increase in mpg, I guess I can rule out the tires... Maybe I got a really bad tank of gas, or maybe I just needed to tune it up...? After my last couple fill ups, I am getting around 17.8mpg. So where's my original MPG?

Any Ideas? Bad gas? O2 sensors? Also, air filter is clean and tire pressure is good.

 

Where are you located?

In the winter (cold weather)....the chemistry of the gas will change so it will function effectively in cold temps. Plus a variety of other reasons why the winter gas is different than summer gas.

My truck drops in MPG every winter about 2-3mpg when its in 30's without warming it up in the mornings. Gas mileage gets even worse when I let it idle to warm up in the mornings.

For some reason...our trucks don't like the winter gas chemistry. My husbands Acura isn't affected at all.

Some gas related links:
http://www.bellperformance.com/blog/bid/49483/Fuel-Mileage-Dropping-Blame-The-Winter-Blend-Gasoline
http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/summer-fuel1.htm
http://www.popularmechanics.com/car...-blend-gasoline-whats-the-difference-13747431
http://itisscience.wordpress.com/2011/11/28/winter-gasoline-yes-there-is-a-difference/

 

If it is cold and snowy..... 1. Cold air is dense and when your truck sucks air in, more gas is needed to correct the gas to air ratio. Pack in more air and pour in more gas. 2. Spinning tires in snow or ice will sip some extra gas. 3. Warm up the truck in the morning? More gas. 4. Winter mix of gasoline will have a tendency to affect gas mileage.

 

I've had this truck since it was new and have always paid attention to my mileage. This is the first time I have seen it drop. I'm in Orlando FL. It doesn't really get cold here. Maybe 3-7 days a year it is actually cold. I let my truck idle for about 10sec before I put it in drive. My driving habits haven't changed since I bought the truck, but my MPG has...... ???

 

This might sound like a stupid question, but are you positive the new tires are the exact same size as your last. A change in size will change the mileage calculation. Just a thought.

My mpgs drop significantly in the winter. There has been some cold snaps reaching all the way down to Florida. Has it changed temps down there than what you're use to?

 

I'm having the same issue, oddly enough for the first time in 3 years of having my truck.

Most of my tanks were 18+ MPG, or 19.5+ when I used non-e gas. Sometime around October, my mileage started to decline... significantly. I stopped using the non-e gas, because that wasn't even helping, plus it cost more at a time when I needed to save up. Most of my tanks these days are 15-15.5 MPG.

I'm in Portland, and we've had a few cold streaks here, but again, my issues started back when the weather here was still relatively warm, and while my route to work has changed, the travel to and from is actually better, and I put new tires on mid-Dec (yes, they were the same size - stock 245/75R16 - aside from normal minor variances, at least).

I'm at a loss, as is my Toyota mechanic buddy. I added a bottle of injector cleaner at his suggestion, not much help. I know I need an oil change, likely next Friday (gonna treat my only lady to a good evening ), and he suggested a radiator flush as well. Plugs were changed this weekend last year, about 20k miles, and filter was changed then as well, I may have changed it again recently but can't remember, but this would be the first time the filter made much if any difference.

 

Well if you changed the tires and you suddenly started getting bad MPG my guess is tires had something to with it. That being said you might read this. as another possibility.
http://www.fjcruiserforums.com/forums/engine-performance/119773-fuel-mileage.html

 

Personally, I'd change the rear diff fluid to synthetic asap and think carefully about the condition of the original shock absorbers on the trucks suspension.

 

I'm gonna say the new tires caused it, tires have a big impact on mpg and if you've replaced your original tires with stock ones that probably means your original ones were pretty much bald right?

Since your new tires have probably half an inch more tread on them they now have a larger diameter, your odometer now isn't reading as many miles for the same distance travelled. This is going to show up as a reduction in your calculated mpg.

All season type tires tend to get less and less rolling resistance as they wear up to a point somewhere around 50% treadwear. Also since the treads are deep on your new tires, they catch air and cause turbulence when they spin which causes resistance like a fan on your tires. These little things add up to reductions in mpg. .

How often do you check your tire pressures? A few psi lower than before will affect your mpg.

I personally have not seen much if any change with tune up items like spark plugs or air filters with Toyota V6s, but I have definitely seen a change in mpg if I went a little too far between oil changes.

Because of the difference in your new tires diameter affecting the odometer reading, your probably really only getting slightly lower than what you were before. It's just that your calculations are thrown off from the odometer. This is why a lot of people use a gps as their odometer to calculate their fuel mileage. The gps doesn't change with your tire size.

 

Run through a few more tanks before getting too concerned - the ECUs take forever to relearn.

Were those the original plugs at 67K miles?

 

Edit: Coast2coast explained it better.

 

Yes. I double checked a couple days after the change.

 

I read that there is a possibility that the t-stat could be stuck open. This could cause the engine to run cool, inturn the coolant temp sensor would tell the ECU to keep the vehicle in warm up mode.

So I changed the t-stat (next cheapest part). Didn't help my case...16.5mpg still

I have a friend that has one of those bluetooth modules that links to a smartphone. He has an app that he purchased , so we hooked everything up and checked it out. Everything passed except for...

MID:$10 TID:$0c
Misfire counts for last/current driving cycles(calculated) - O2 Bank 4 Sensor4
Max: 53,285 Min: 22,808
Test result value: 56,224
FAIL

--and--

MID:$50 TID:$0c
Misfire counts for last/current driving cycles(calculated) - O2 Heater Monitor Bank 4 - Sensor 4
Max: 53,285 Min: 6,424
Test result value: 56,224
FAIL

Which sensor is this????