Declining fuel mileage - trying to determine/eliminate the causes.

[Marv]

Noticed over the past few days that my truck's fuel mileage is starting to decrease. Hoping it's from larger tires and the trailer I've been towing, but figured I'd get you guys/gals thoughts and opinions.

Tires were changed from stock 275/65/18s to 325/65/18s.
Most of the last 1 - 1 1/2 tanks was burned while pulling a 30ft gooseneck equipment trailer (both empty and loaded with about 13,000 lbs of hay - 8-10 MPG while pulling the trailer).

Obviously the above two things will play a major role. However, the thing that gets me is the "Miles to E" estimator on the dash. Usually when filling up, I'd have at least 450 miles to E. Shortly after the tires and first couple trailer pulls (empty), it would only show ~360 miles to E.

At the moment, the guage shows 1/2 tank, but the estimator indicates 113 miles to E. What's up with that??

Truck still gets ~18-20 cruising on the highway empty and averages 15-16mpg around town.

Unfortunately I haven't had a chance to fill back up and reset the MPG calculator to see what it'll do. I have a hard time believing the tires would make that big of difference.

So....I think I'll change the fuel filters as well and see what happens.

Anyone have any thoughts/suggestions? Anyone experienced this before?

Thanks in advance!!!





And sorry about the novel!!

[tech10002]

The miles to empty thing doesn't use the current mileage. It uses an average of the last so many miles. I can't recall the number, but it's at least a few hundred, so your indications are normal considering your last few hundred miles have been towing a huge trailer. If it's still indicating normal with day to day driving, I wouldn't worry about it.

If you really want it to read based on your current average, disconnect the batts for a couple minutes and reconnect. That resets everything on mine.

[Raul@Stealth Automotive]

you will see that kind of decrease with the larger tires... i went from 18-20mpg when i was stock to 16mpg if i was lucky when i went up to 315 70 R17

[DoubleDpsd]

definitely the tires. I wouldn't recommend disconnecting the batteries necessarily cuz that will erase your kam and it will take longer to restore it and while in the process your mileage will not get better anytime sooner. that's why a lot of people complain of loss of mpg's after getting a "new" flash cuz their kam was erased in the process. I went from getting 500 miles to a tank to thanking the gods above if I get 400 now with these 35's.

[Marv]

Originally Posted by tech10002

The miles to empty thing doesn't use the current mileage. It uses an average of the last so many miles. I can't recall the number, but it's at least a few hundred, so your indications are normal considering your last few hundred miles have been towing a huge trailer. If it's still indicating normal with day to day driving, I wouldn't worry about it.

If you really want it to read based on your current average, disconnect the batts for a couple minutes and reconnect. That resets everything on mine.

Well, after driving it to work this morning (~50 miles, mostly highway) empty and without 15K lbs behind me, I think it's getting back to normal. Pulled away from the house this morning with it saying 12.5 mpg and by the time I got to work it had climbed back up to about 15 mpg. By the time I get home tonight, it'll probably be back up to normal for highway driving (17-19 mpg).

So, tech, I think you're on to something....it probably just hadn't had time to reset itself since pulling the heavy load.

I'll keep an eye on it and see what it does over the next week or so.

Thanks!

[Marv]

Raul, Darsey, I think you both are correct on the tires as well. I figured I'd forfeit some mileage when I put them on there, but that's the way it goes I guess.

Thanks!

[Betaracer]

Going to larger tires throws off the odometer/speedometer too. You are traveling further than indicated, so you need to figure out your correction factor by using a GPS.

[Marv]

Beta, that's a good idea too.

I've read that the dealer can re-flash the computer to compensate for larger tires and correct the speedo and odometer.

Is this true? Can they take care of this?

Thanks!