Ah dude I'm in the same boat some what anyways. When I first got my truck I was gettin 90 miles to a quater tank. After I put a tonneau cover that dropped it abit. Then my magnaflow exhaust and well that dropped it too since I like to hear it. But recenty I got the superchips and puttin it on 91 performance it increased it to about 95 a quater tank.

 

It will be interesting to see if the 89 gives a little better mileage over the 87 octane. You may have to calculate mileage over a couple of fillups to make sure that the computer adjusts (if it does that) to the gas and to make sure that you have diluted the 87 sufficiently.

I'm getting around the same mileage around town, but I have a habit of getting into the throttle a bit.

 

if it makes you feel any better i get a wapping 9mpg in my 02 5.9....5 pulling my boat

 

Reset your PCM, changes to the truck effect how the engine will deal with your throttle input/engine demands. Disconnect the battery for 20 minutes and hook it back up, that'll dump the memory and give your truck a fresh start.

Your driving style sounds like a bad measure of MPG, too much fluctuation in habits, speed, engine load, etc. Remember things like heat, humidity, temperature, wind all can affect mileage. Same thing with different brands of gas.

It sounds to me like you're experiencing normal mileage fluctuation and maybe just getting on it a bit. And you may want to get your tire sizes checked. 4% may seem insignificant, but there is only a 4% difference between 12.48 mpg and 13.0 mpg.

 

2 inches is like when your speedo says you are going 60, you are going 62.5

Like Pyro said, you are actually getting a little better gas mileage, but not anything significant like 12-16 we are talking about like 12-12.3 or so

 

On that first tank of 89 I got 12.7 MPG. I just filled up with 89 again to see if it gets better.

I hear a squeak in the rear and am wondering if I could have a bad brake caliper or something that could be causing some drag. That would be a cause for lower MPGs... at least in my logic it would be. I'll have to take a look at that.

 

My truck came with 17 in rims, Since I changed my tires and went with the Goodyear kevlar belted tires (very heavy) my gas mileage dropped as well. I went from 14.5 in the city to 12.3, hwy went from 18-19 to 16. If I pull a pair of jet ski's on the hwy, I get 12.5. Life in a truck. I'll probably flush my injectors next and clean the throttle body next to see if it helps.

 

Second tank of 89 = 12.2MPG

I'm looking at changing transmission fluid and filters. Also, I'm about to change the rear brakes.

I'm not sure if it'll make a big difference in MPG, but it's kinda fun thinking through this.

 

have you changed your spark plugs lately?

 

Agreed Corey, I also have the 5.9 and i get about 8.5 and 10-11 if I drive really light, but then it takes me 5 minutes to get going from a stoplight.