Crappy Fuel Economy

dirtyfoot · August 27, 2017

2008 Silverado 1500 5.3L, 4 Speed trans, 2WD, 3.73 rear diff, 275/55R20 Tires, 100K Miles

Burning about 12.5 MPG City/15 MPG Highway with regular, about .5 MPG better with premium, and have been getting this mileage for quite some time.

Plugs/wires/trans fluid/ air filter/diff fluid all changed within 25K. Oil changed regularly.

I already have a black bear tune, but thinking maybe a canned programmer might help me get better economy. I never tow with the truck, it's a daily driver. Not looking for any increase in performance, just better economy.

Maybe change O2 Sensors? Exhaust does seem a bit sooty.

Any thoughts?

Bad Bowtie2 · August 28, 2017

AC Delco upper O2 sensors are a great ideal & with the mileage are about due IMHO.

What brand are these other replaced parts??

Has the front end been aligned?

Is the truck all stock otherwise? You did say you already have a tune...

Is this a 2-wheel drive or a lifted 4x4, for example??

dkjbama · August 28, 2017

Canned tune will not improve on the blackbear. My truck is almost identical, except it's a 4x4 with 17 in wheels and stock size Michelins. I get 12 in town (very short commute). 17.8-18.2 @ 70-72 mph on the interstate.

How fast are you driving? 75 mph drops me down into the low 17's high 16's. 80 mph would likely get me down into the high 15mpg range. These trucks have a lot of drag from wind resistance. Costs money to go fast.

As stated above, it may be time for O2 sensors, but you can't expect miracles.

Nanotech Environmental · August 30, 2017

Also, if you live in or near the salt belt, check your fuel lines, particularly where they are clamped to the frame. They may have a pinhole in them from rust. Your truck is at the prime age for this to happen.

I had a similar issue a couple years ago on my 06. Fuel mileage seemed to be quite crappy & there were no other obvious symptoms. It was leaky fuel lines.

Z45 · August 30, 2017

That's sound normal for a 2008. The 20 inch tires may be robbing some mileage due to increased unsprung weight and more rolling resistance. Going back to the stock size may help. Bigger tires always impact fuel mileage to the worse.

In simple terms, a vehicle's sprung weight is supported by springs of one kind of or another (coiled springs, air springs, Belleville springs). ... So unsprung weightincludes the wheels, tires, brake assemblies, differential, solid drive axles, hub motors, and anything directly connected to the wheels is unsprungmass

intheburbs · August 30, 2017

If you do the math, depending on how much you drive, it'll probably take many years to recoup anything you spend in gas savings. It's a truck, not a Prius.

If you do drive a lot of miles, I'd suggest a beater commuter car like an older Civic or Corolla.