Sudden Loss in MPG

I’m struggling trying to diagnose why I’ve experienced a sudden drop in gas mileage, around 3mpg from a consistent 17-18mpg on highway driving to 14-15mpg.

I’ve done the obvious things, like replace the air filter, check tire pressure, let a tank of gas through with a couple of new tanks of gas in, nothing is changing.

No codes, no warnings, nothing sounds off, no apparent leaks, just not sure where to go from here. Any suggestions on diagnostics?

 

Mileage?

Misfiring spark plugs can reduce mpg.

 

Is this according to the display screen or hand calcs?

are you getting the same miles per tank?

 

bad gas?
wind direction?

I tend to run Techron every 5k miles - right before oil change. does it work? dunno.
I heart BG43 (or whatever its called) is better.

my truck's ideal mileage is around 70-75 degree ambient temp...it get worse outside of that range.

the only other thing that affects me is wind.

I avg 17mpg. I drive the speed limit. on cruise control. 90%hwy.

wind is the only real variable that I can put my finger on.

I also notice winter blend / summer blend gas.

 

Disconnected or changed out your battery recently?

 

All good points, right now I'm at around 62K miles. I haven't changed the battery or disconnected it in quite some time. No miss fires are showing up on my scanner.

The MPG is from the screen reading and from reading OBD fusion. On the same miles per tank, it has basically gone from 18mpg per tank to 15 mpg per tank.

I'll start doing hand calcs at next fill up, plus I like the Techron comment and will add a bottle at next fillup.

 

One other thought and I dread asking this, but one of the cats is aftermarket (thieves), do you think a bad/clogged cat could be causing this?

 

Just confirming... when you say no codes that you ran an in-depth or 'scan off systems' as some trouble codes may be 'pending' and not yet firing off a CEL or coming up on a 'basic' code scan.

Bad EGR valve can cause a drop in mileage, but you truck is rather low in age/mileage for that to go bad...

 

A few folks have commented that non-OE cats were giving them headaches w/CELs and codes, but I don't recall anyone mentioning drop in MPG...

I wouldn't start throwing too much concern or parts at it though until you've done a hand calculation; the lie-o-meters can be quirky at times

 

I ran a full diagnostic check with a Launch X431 and didn't find anything pending. I think I'll run it in datastream mode and log it on the next few commutes to see if there is anything abnormal.

 

Your truck isn’t a flex fuel, is it?

 

Dirty MAF?

 

A clogged cat could cause mpg drop but i would think newer truck would give code. Maybe not.

Since you have the scanner and if you have a flex fuel engine, read your fuel alcohol % and long term fuel trims.

 

if you're getting the same miles per tank, then nothing is changed except info it seems.

EX...if you got 250 miles from fill up to 1/4 tank, but it read 17mpg

now you get 250 miles from fill up to 1/4 tank, but it reads 14mpg.....

that tells me something is reading differently. unless you've changed tires in that time to a different diameter that it's reading incorrectly, but you've stated nothing like that has changed.

i'm thinking the cats may be causing to read something different.

 

You can also check for a sticking/frozen caliper, but you'd likely see/feel that.

 

Give this a read if your truck is a Flex Fuel https://www.tundras.com/threads/res...ge-level-with-bidirectional-scan-tool.123799/

 

Nope, I don't have a Flex Fuel truck.

 

Good thought. I think I'll check that.

Yea, possibly. I think I need to take the time to do the hand calcs as the readings I'm getting may be completely wrong. I haven't changed tires or anything.

 

Toyota must use some goofy/weighted calculation for fuel economy (not averaged logically). I use my fuel mileage more of as a diagnostic tool than for fuel savings, but there are some times where it surprises me with low numbers for seemingly no reason.

I don't think I helped you at all though

 

What about your tires. I noticed a decrease since my threads have been getting less and less. New I was getting about 14-16 I'm 30k in on the tires and now getting about 9-10 MPG. O and I use Carango app for tracking my MPG. Never trusted the display on the truck

 

Tires are definitely getting worn, but they are still not in the limit for replacement. I ran the scan tool this morning during and after my drive with no abnormal things popping up. I did notice that my mileage is starting to improve again, which has me thinking this is more a faulty calc. Anyway, I need to fill up tonight and have a bottle of Techron. I'll start my hand calcs then.

 

Clean MAF sensor ($10), throttle body ($10), replace PCV ($8).

 

I find going easy on first quarter tank is the key to get better readings, the on dash mpg goes down way faster than it goes up. From quarter to half it moves slower and last half tank It’s only move 0.1-0.2 mpg no matter how I drive…

 

Air flow sensor?

 

I did order a PCV valve recently, so it'll be something I'll replace soon. Will check that others.

 

I get a sudden and immediate drop in MPG when I hookup the trailer

Mike

 

I experienced the same thing this weekend. Typically get about 14, but I was at 8 this weekend…


Joking aside, curious what the OP figures out.

 

I started the hand calcs with a fresh tank and added Techron, PCV valve is in the mail. Haven’t checked the other suggestions, but will.

 

Clogged cat would also cause a noticeable loss of power. Had one on an F150 with a 4 barrel Holley. Service writer at dealership was skeptical when I took it in. Turned out the factory had set the floats too high. They replaced the cat. He called and said I was correct and it now runs like a raped ape.