06 Tacoma Gas Mileage Issues

[budro]

Ladies and gentlemen, I have searched high and low (and a little to the sides) for answers, but I seem to be the only one who has this problem. So the issue I am having on my stock (tires are now 265/75 vs OE 265/70) 2006 Tacoma (access cab, 4x4, tow package) is that after moving from 6000ft to 1000ft MSL, my gas mileage has dropped 3-6 mpg. Previously, I got 16-19 (in line with EPA estimates), now I get 10-15.9. Before you jump to the air density conclusion, let me lay out the history of the truck.

Dec 2006 - In GA, purchased new with 45 miles on odometer - gas mileage 16-19
2006-2011 - multiple cross country trips (GA to NC to OR to NC) - no noticeable effect on mileage. Serviced on time and according to schedule.
Aug 2011 - Moved to CO from NC, elevation now 6000ft from ~1000ft - no noticeable effect on mileage
Dec 2011 - Road trip from CO to OR - no effect on mileage
Feb 2012 - Only issue with the truck - right front bearing started to go out around 90K (replaced immediately)
Oct 2012 - New tires - went from 265/70 Bridgestone Dueler A/T to 265/75 Goodyear Wrangler Duratrac - no effect on mileage - speedometer reads 3% under against GPS, I account for this in the mileage.
May 2013 - Week long trip to KC to look for houses - no noticeable effect on mileage
June 2013 - 100K mile service done on truck in CO
Aug 2013 - Moved to KC, elevation is ~1000ft. This road trip was the first time I noticed a decrease in mileage - avg just under 16 on the trip. Commuting now, I average 10-12.
Dec 2013 - Back to CO for holidays, same gas mileage (10-16) until I got back to 6000ft and then it shot back up to normal parameters (16-19 for the trip). As soon as I got back under 6000ft, I saw the decrease again. Back down to 10.5 on the current tank (city driving).

So that is my conundrum. Has anybody had any experience like this? I talked to the two shops that worked on the truck in CO and they swear they didn't do anything that could have affected my mileage. I constantly get the runaround about better economy at altitude, but it doesn't make sense that I would lose so much coming down from the mountain and it only started in Aug of last year - not three months prior.

If you have any ideas or thoughts or experience, I am willing to hear them all. And answer any questions not covered above if it might shed some light. Thanks for your help.

 

[ken_79-fj40]

That is odd. I'm at about the same elevation as you but work right near the ocean. I've got a 4.0/auto/4x4 with 265/70/16's on aftermarket wheels and I get 20 consistently. Before I changed the wheels/tires from steelies with 245's I got 21.5. Tire/wheel weight does affect mileage but not as much as you're seeing.

 

[liljlandon]

Pull efi fuse or battery terminal for ~30 minutes and see if the computer needs to reset itself?

 

[SIZZLE]

Very odd usually altitude gives the opposite affect. Throwing any codes? Taking a stab I'd guess a faulty oxygen sensor.

 

[budro]

One "fix" at a time: Pulled the EFI fuse and now have a fresh tank of gas to check. I'll report back when I fill again.

I had considered the oxygen sensor, but I'm not getting any idiot lights. Anyway, that will be fix number two.

 

[Tigerstripe40]

What is the percentage of ethanol in your fuel?
I found that when the gas stations started selling 10% ethanol my MPG's went from 22-23 down to 16-17 (summeR) and 12-14 during the winter...

I have an 08 quad cab, 4x4, auto, running 265-75-16's fwiw.

there is a gas station in Price, Utah that sells ethanol free fuel and it goes back up to 22-23mpgs...

 

[budro]

That's interesting. I've suffered the E10 fuel since at least 2010 in NC, same in CO, and KS. Only reason I know this is that I bought a boat in 2010 and was very careful about filling it with "pure" gas. I had enough trouble with it to risk E10 corrosion.

Unless I am seeing the cumulative and catastrophic effect of the fuel, I can't imagine that is the cause. Appreciate the idea though. If I do figure this out, maybe someone else will benefit from all the knowledge.

 

[ken_79-fj40]

We have e10 here too and I'm still getting 20. Back when they just started getting e10 in my area it was all that was available near work but we still had pure gas near home. I noticed a 1-2 MPG difference depending on where I filled up. E10 really made a mess out of a brand new edelbrock carb I put on my Capri over last winter. It would barely even run. But no fuel stabilizer and I let it sit for 4 months. Oddly enough I never had issues with it when I had the FJ40.

 

[SC T100]

How many miles on the current set of plugs? I swapped out the original plugs (97K!) on my T100 and my mileage went up 25% instantly.


other thoughts o2 sensors as stated above...could be slow to react which might explain very low mileage in town.


Check the temps on your wheels after a long-ish drive. if one is hot you might have a dragging brake.

 

[budro]

Time for an update.

I pulled the EFI fuse at the end of February. As you may be able to tell, I really don't have much of a commute. However, let me start with my feelings before I jump into the data. I feel like the truck was performing better. Snappier acceleration and such. I attributed most of that to my head. However, my first tank of 30mph commuting got me 15mpg. That is a significant jump from 10-11mpg on my commute. My second tank was about 15.5mpg. Then I went fishing. An hour each way of open highway. I would guess I averaged about 60 mph. I got closer to 16.5mpg (including some commuting on that tank). My last tank was a couple days of commuting and then emptied the tank at highway speed - I got 16.8mpg. That may not seem significant, but half was at 30mph (~15mpg) and half at 60mph. So that seems to put my top end around 18.5mpg - right where I would expect an 8 year old truck to be.

Anyway, it would seem that for my gas mileage issue, pulling the EFI fuse worked. I have been pleased, although I would love to see a true 19mpg again. I appreciate everyone's help and ideas on this.

SC - to answer your question - I got new plugs with the 100k service last June. These are the second set and I have been changing them every 50K or so.

 

[iwashmycar]

Just a shot in the dark, but a faulty thermostat can cause bad mpg. If it goes bad and / or is simply not opening and closing when it needs to it can make the truck think it is not warmed up all the way and the ECU will compensate by running rich.

Do you have a Scangauge or Ultragauge to monitor things closer?


So did YOU change the plugs??? You mention the mileage dropped off sometime after that 100K service. If YOU didnt set the gap, I would double check them. Could have even jacked up a plug wire or something silly like that, reducing spark. At least take them out and look at the color / burn pattern if you havent.