I know that it seems every other thread on here is about fuel mileage but I can't figure out why my mileage dropped so severely! I have a stock 07.5 crew cab 4x4 and all of a sudden my mileage has dropped. Since I changed to the 285's about 10k miles ago, I would get 17.5 - 18 mpg on my 70 mile round trip work commute and if my wife drove it back and forth we would get 14-15 with her commute. In the last 2 months I can't get any better than about 12 mpg even driving it on the highway and I have no idea why. The truck has 37k on it now.

Here's what I have done to try and figure out the problem:

new AC Delco air filter
oil change
fuel filter change
transmission fluid and Allison spin on filter change
2 bottles of Stanadyne (4 tanks)
unplugged the battery for an hour in hopes of a reset or something
filling up at different brands of stationsAny ideas on what could cause it? Water in the fuel? Clogged injector? I'm still new with diesels.

*please don't post a "get rid of your dpf and program it" response, I'm leaving this truck stock, it used to work just fine.

Thanks for the help.

you need to adjust your speedo - when your speedo says your going 50 miles an hour you are actually going about 60 with 285 tires. I do not think dealer can go above 265? but who knows i use a PPE to adjust it.

yep - been there, done that. You need to reprogram for tire size change. I used a Hypertech Max Energy programmer and did it and it worked great. You don't have to use the power pramming funtion to do it. Dealer couldn't do it.

It's only off by about 4 mph at 75mph, the mileage was fine up until about 2k miles ago so 8k miles had good fuel mileage on the 285's.

It's only off by about 4 mph at 75mph, the mileage was fine up until about 2k miles ago so 8k miles had good fuel mileage on the 285's.


I have 37K on mine and getting 15 city, 18-20 highway. You need to see your dealer for programing or injector issues if you are truely getting 12MPG on the highway in a 2500 rig.


Hand calc your MPG. Use a tire size calculator to figure the percentage difference. Mine is off 4% and 5% with winter and summer tires. I never go by the DIC - it reads 16.5 MPG all the time.

http://www.miata.net/garage/tirecalc.html

Also my DashHawk displays true corrected speed.

:cool:

All of my mileage is done by hand, the DIC is never accurate but the gallons used is always within .1 to .5 but still calculate every tank on all of my vehicles. I had a problem on my S10 years ago where the mileage dropped but it was a tbi throttle body and you could watch the inector pouring fuel into the throttle body instead of spraying it in - too bad you can't look at the injectors on this thing because that would make this easier to diagnose.

I think I will call the dealer and have them look at it. Do we have a deductible or anything after the 36k mile warranty is up?

Thanks for the replies!

something is wrong...i got same truck, same size tires and it gets 15-16 highway. It should get 12 pulling a decent load.

Yeah i was at 12 - 13 mpg after tires were switched for about 2 weeks until i adjusted for it with the ppe -- now i am averaging around 16 somewhere i just quit resetting the dic and have not worried about it. I can tell you it seems as though it has even been in the 15 mpg range lately - i think the regen is occuring more often and for longer periods - seems like sometimes i have to drop it in 5th and get the rpms up to around 2k for the regen to complete or it just stays in regen 20 min or longer. I can pretty much tell the dofference now when in regen i may just be getting used to it - but feels sluggish during regen - drop it in 5th for 5 min or so and power seems to come back

Here is a link to a tire speed calc
http://www.tacomaworld.com/forum/tirecalc.php

I had 285/75/16 in my sig and they are actually 285/85/16 which brought me off like 7-8 mph at 60

I vote injector / programming issues either from dfp signal or otherwise. There have been other threads reporting similar.
37k, get it in quick. I would have done it before assuming you were still within 36mo.

One of the few that laid things out with symptoms, background and attempt info. Nice job OP!

Most need to read better! you are too used to those one liner posts "my milage dropped, Why?" and start shooting off every possibility. :mad:
Tires not the issue, He has solid background data, run his tires 10k and experienced the prob only last 2 mo. (9 wk x 5 day x 80mi max) = 3600mi

The other thing I do periodically is perform a used oil analysis (UOA) to check on the status of my oil and contaminant levels, as well as to keep an eye on fuel dilution, which can be an indicator of injector issues.

The other thing I do periodically is perform a used oil analysis (UOA) to check on the status of my oil and contaminant levels, as well as to keep an eye on fuel dilution, which can be an indicator of injector issues.

Is there any good way to tell if it's fuel getting by the pistons on regen or if you've got a jacked up injector?

Get your speedo dialed in,sure helps to know where your at.

I did this for about 2 tanks of fuel, I left my truck in 4 wheel drive, and it killed my MPG down to 11-12 around town.:eek:
Im sure its something simple.

UPDATE: Reset the computer one more time before calling the dealer and just drove the pizz out of it. 130 miles of towing my boat, 75 miles of highway and lot's of stomping on the pedal and I averaged 13.6 for the tank. I wonder if it was stuck in the dpf cycle or something. I'll see how the next tank goes and if I am back down, I will call and take it in.

I have a 2008 dually extended cab LBZ completely stock that has never got better than 13.5mpg empty on hwy. stealer says thats normal??

I've got a 07.5 crew cab, and the best I have ever gotten is 14 mpg, averaging around 10 empty. Any thoughts?

Yeah i was at 12 - 13 mpg after tires were switched for about 2 weeks until i adjusted for it with the ppe -- now i am averaging around 16 somewhere i just quit resetting the dic and have not worried about it. I can tell you it seems as though it has even been in the 15 mpg range lately - i think the regen is occuring more often and for longer periods - seems like sometimes i have to drop it in 5th and get the rpms up to around 2k for the regen to complete or it just stays in regen 20 min or longer. I can pretty much tell the dofference now when in regen i may just be getting used to it - but feels sluggish during regen - drop it in 5th for 5 min or so and power seems to come back


I believe 20 minutes to be about normal.

Yeah i was at 12 - 13 mpg after tires were switched for about 2 weeks until i adjusted for it with the ppe -- now i am averaging around 16 somewhere i just quit resetting the dic and have not worried about it. I can tell you it seems as though it has even been in the 15 mpg range lately - i think the regen is occuring more often and for longer periods - seems like sometimes i have to drop it in 5th and get the rpms up to around 2k for the regen to complete or it just stays in regen 20 min or longer. I can pretty much tell the dofference now when in regen i may just be getting used to it - but feels sluggish during regen - drop it in 5th for 5 min or so and power seems to come back

How does one know that the regeneration is actually taking place, I understood this to be cyclic thing, with no driver intervention etc.

Most tuners will indicate that, or if you get en EGT readout during regen (tuner/guage required), you'll see EGT's up dramatically during a regen. MPG's drop dramatically as well during regen. When my EGT's are up...and I hit reset in the MPG...I'll see 30% lower mileage, instantly!