Rapidly Dropping MPG

Maybe someone can help me out here since I'm at my wit's end. Even the local Toyota dealer service department didn't know what to say.

I've noticed that my gas mileage is dropping with each tank over the past few month. Currently, I don't have a Scangauge to accurately measure but this is what I've got...

Today, at 3/4 tank, I noticed that I'd clocked 32 miles on my trip odometer which was reset when I filled up the tank. (This is all granny-style city driving.) With a 20 gallon tank divided into 4 quarters (5 gallons per quarter-tank), that means I'm getting 6-7 miles per gallon?!?!

The mods that I've done to the truck (2010 4x4 Access Cab V6):

~285/85/16 tires w/ 3" Icon Stage 2 lift kit
~armor: ARB front bumper, AllPro rear bumper, Demello sliders, BudBuilt skid plates
~AFE air intake w/ Magnaflow catback exhaust
~Optima dual-battery setup

Does anyone have any idea of what's going on? Any ideas of what might be causing my mileage issues? Any suggestions?

 

winter blend gas? fouled plugs? dirty air filter? Crap gas from station? Dirty MAF? Bad cat?

a few possibilities....

 

fill it back up and calculate

 

Because temperature drop.

 

What's the temperature.


What's your daily commute.


How long do you idle.

 

to get a true MPG you need to fill up, reset trip, run down till empty, fill up, then divide miles driven by gallons fueled.

 

Still, if he's at 3/4 tank at 32 miles, and was full, it is gonna suck.

 

With bigger tires your odometer is off so you're actually driving further than it says. Im sure it wouldn't be a huge difference but still something worth calculating.


...

 

Around 5%

 

More like 9.8%

OP, create a fuelly account and start tracking your actual mileage and gallons. When fueling, fill up til the pump stops (don't top off or partial fill up). Using the fuel gauge is pointless to estimating mpg. As for the mods... pretty much anything that adds weight to the vehicle will result in decreased mpg.

 

I didn't really pay attention to the tire size before

285/85/16 ?

He's running 35"'s ?

Tire calc says that's a 12.7% difference

 

Winter blend gas...Most likely not since I'm in Southern California and old man winter forgot to visit us here. Unless, winter blend gas is distributed uniformly nationwide to gas stations.

Fouled plugs...I'll have to check for that, although I would be surprised since I'm at 35K miles now and have been religious about the maintenance.

Dirty MAF...Will have to check that.

Dirty air filter...Will check, but not likely since it's only a couple months old.

Bad cat...Possibly, although the cat is a couple months old as well. They did modify the cat/exhaust pathway to accommodate the larger spare tire underneath. I wonder if that would affect the efficiency of the cat?

 

I'm in Southern California so the temperature's really not an issue, especially when it's been in the low to mid 70s this past week.

Daily commute is roughly 10 miles a day in the city.

Idling time is your usual idling at stop lights and stop signs in the city.

Here's the thing...my driving habits and patterns haven't changed in the past several fill ups but my gas mileage is dropping with each fill-up, that's what's causing concern.

 

That's something I'll have to do ASAP, along with making installing a Scangauge a top priority on my list of mods. Thanks for the suggestion.

 

Holy moly! I didn't think it would be that high. Thanks for the tire calc link.

I also noticed on the tire calc that the best gear ratios for better fuel economy with a 35" tire set up are 3.91, 4.11, and 4.27. The stock Tacoma gear ratio is 3.73, the last time I checked.

 

Read this
http://www.fjcruiserforums.com/forums/engine-performance/119773-fuel-mileage.html

 

One thought I had that nobody has mentioned is get a locking fuel cap. Somebody may be messing with you, we used to pull that sort of trick on one another when we were younger. Another possibility is that there's just a thief who's hitting your truck from time to time to fill his gas can. Either way, a locking cap will eliminate that possibility.

mikeg

 

wow .. an excellent piece of work. I have personally noticed better winter mileage when covering our radiator and keeping the engine temp warmer due to reducing the cold wind on the highway.

~285/85/16 tires w/ 3" Icon Stage 2 lift kit
~armor: ARB front bumper, AllPro rear bumper, Demello sliders, BudBuilt skid plates
~AFE air intake w/ Magnaflow catback exhaust
~Optima dual-battery setup

Every mod OP has done except for intake has reduced mileage; exhaust should be MPG neutral; and winter gas even in southern cal. I would expect a MPG drop.

 

not knowing actual distance driven on 35's vs how much you think you gas gauge reads at 3/4 tank and doing the math isnt accurate. Do the math for actual distance driven then fill you tank up to the first click, drive x amount of miles, fill up to first click, then do the math from there.

 

Update:

I took it in to the Toyota dealer and explained that I was consistently getting less miles per tank with each fillup and gave them a list of possible culprits for them to look into (ie. faulty O2 sensor, dirty MAF, blocked catback, fouled plugs). They dismissed the list and said that it's not procedure for them to actually inspect anything unless their diagnosis tool which reads the ODB II memory for fault codes gives them a reason to. Needless to say, they said there was nothing wrong according to the ODBII memory.

I finallly got the Scangauge II and had it installed. Here's another issue...

Due to the change in tire size, the odometer is a little off and the Scangauge needs to be calibrated to reflect this in the Speed setting. This is done by entering the actual speed you're going at as captured by a GPS or speedometer app vs the odometer reading.

According to this tire calc it shows a MPH difference of nearly 13% (65mph vs 74.49mph) but what I'm seeing on my odometer vs GPS and speedometer app is actually closer to 7% (60mph vs 64mph).

http://www.tacomaworld.com/forum/tirecalc.php?tires=265-70r16-285-85r16

What gives? Which one is correct?