Why the decrease in my MPG?

How many miles on truck? What maintance have you done? Have you cleaned the throttlebody and MAF sensor since you've owned it. We need this type of info to help you. Also welcome to the club.

X2 what Silveradotrailblazer said. Also look at your air cleaner and tire pressure. A few of seemingly small things can add up to a problem. And welcome to the club.

Thanks for the welcomes.

All I've really done is changed the oil and check fluids. Aside from that I'm pretty useless when it comes to vehicles.

The truck has 76,000 miles. I've done about 20,000 of those.

Is it easy to check those thinks on your own or should i take it to a mechanic?

Well the spark plugs are good for 100,000 miles so your good there. As goldie says, the air filter and tire pressure. I would clean the throttlebody with throttlebody cleaner, not carb cleaner and use a soft toothbrush. Also clean the MAF with some CRC MAF cleaner. Its in a silver can. Clean per cans instructions. Use ONLY MAF cleaner to clean the MAF, otherwise you WILL ruin it.

Dont know if your already running synthetics in the engine, trans, and rear axle, if you arent next lube change might be a good time to make the switch.
Take a good look at how much stuff your carrying in the truck also, extra tool box's and spare parts add weight that cut into your mileage, if you dont need them leave tham at home in the garage.
How many add on parts have you put on the truck to make her look pretty? Big front bumpers (your from Texas thats almost a requirement) bug shields, window vents, all theses items "dirty" up the trucks profile and make her burn more fuel to get around.
As was posted first check the aircleaner and tire pressure then look at averything else.

I changed the air filter already. I haven't added a bunch of stuff to it (like a bumper lol) the tires are just 31in.

I use mobil 1 oil

I run Mobil 1 in the entire truck, engine, trans and f&r axles. As I mentioned before clean the throttlebody and the MAF.

What are you running for tires? Are they new recently?

I've been getting the same poor milage out of my 2000 Silverado with the 5.3 and my wifes 2003 Lincoln with a 4.6 in the past 6 months or so. Both started about the same time. My 2011 GMC 5.3 4x4 is getting better milage than both my others. I have been told that all the gas stations have started using the 10% ethanol blend. I'm thinking this may be why the milage is dropping on the vehicles that weren't calibrated for that type of fuel and it's not effecting the newer E85 vehicles as bad.
Just a thought. My older vehicles stay tuned up and maintenance is never neglected so that's about all I can think of.

X2 on the ethanol LSUMan. Some places they're up to 15% so that could easily be where his MPG went.

thanks for the input ill let y'all know how it goes.

---------- Post added at 03:10 PM ---------- Previous post was at 03:02 PM ----------

What octaine do y'all prefer?

onejon said:

What octaine do y'all prefer?

Click to expand...

Hey Onejon,
run what GM recommends for your truck. It'll be in your owners manual & on the gas door. Running higher octane than what they recommend is just costing you money & doesn't do anything to increase your mpg. Higher octane fuels are for higher compression engines & won't benefit you.

X2 What Stephan said! For my truck, 87 is recommended. And at various times I've tried 91 and 92 as an experiment with no resulting increase in performance or mileage.