Losing MPG

[Defender]

I have a 2003 GMC 2500HD that I bought new. For years I have averaged right at 19 mpg. Recently, out of the blue, my mpg has dropped to about 17. My injectors have all been replaced (30,000 miles ago). The truck has 140,000 miles on it. New air filter...fuel filter replaced about 20,000 ago....do regular service as far as oil changes etc. No changes in driving habits. Any ideas about what could be causing this? Appreciate any input.

[DmaxMaverick]

Depending on where you live/drive, it may be as simple as a fuel formulation change. If you've been changed to winter formula, a mileage drop is expected. Also check with local fuel formulation mandated changes. If not, you may have something to look at.

[CoyleJR]

Did you recently replace your tires? A more aggresive tread will decrease your fuel mileage.

[Defender]

Thank you both for your help. the drop in mpg happened before the fuel changeover. And yes, i recently replaced my tires, but the drop occured before that. Good ideas, though. I am baffled.

[dually2002]

What about the air filter?

[Kennedy]

Thermostats/engine coolant temp?

Wheel bearings/steering/suspension?

[Defender]

Kennedy, thanks. Everything checks normal. Replacing fuel filter tomorrow. Hope that helps. Also going to run some injector cleaner through it. Again.

[Kennedy]

I'm not really a fan of "some injector cleaner" rather a continual use of a quality fuel treatment like FPPF Total Power.

If the thermostats are original they are likely getting weak.

[Defender]

Nobody has ever called me a mechanic so pardon my ignorance. How does a thermostat affect MPG?

[rapidoxidationman]

Cold engines don't run as efficiently as engines at operating temperature. If a thermostat is stuck in the open position, the engine might not reach optimum operating temperature so efficiency (MPG) is lost.

[Defender]

Got it. Thanks. Can I trust my gauges? The needle rarely moves and when it does I know the reason.

[More Power]

I have a 2003 GMC 2500HD that I bought new. For years I have averaged right at 19 mpg. Recently, out of the blue, my mpg has dropped to about 17. My injectors have all been replaced (30,000 miles ago). The truck has 140,000 miles on it. New air filter...fuel filter replaced about 20,000 ago....do regular service as far as oil changes etc. No changes in driving habits. Any ideas about what could be causing this? Appreciate any input.

My 2001 LB7 averages 17 locally - always has. It'll inch up a bit on longer trips, with the best I've ever seen at 21 on one tank of fuel.

It's now October. All across the northern teir of the US and Canada, the fuel suppliers will begin offering "winterized" #2 diesel fuel as the only fuel you can buy for on-road use. Winterized #2 is a blend of #1 (a thinner lower gell point fuel) and #2. The blend percentages can vary depending on how cold it's expected to be. #1 isn't as energy dense as #2, which produces a little lower fuel economy.

For more info about dealing with the upcoming colder weather, have a look at this link: http://www.thedieselpage.com/features/coldweathercc.htm

[Defender]

Just a followup.....I replaced the fuel filter and ran some Stanadyne through her and got back to 18.9 MPG. Appreciate all the tips.