My truck is a 2003 5.7 4X4 quad cab currently with 75k miles. i have had it about 6 months and it had 67k miles at the time. For the first 4 1/2 months i ran it on 87 octane and was getting 12-13 mpg. After reading info on this site, I switched to 89 octane and have been running it for about 6 weeks and in that time i have run 6 0r 7 tank fulls of gas averaging 9.5 to 10.5 mpg. No other changes have been made. Once during this time i ran a tank of 87 (due to funds) and it seemed like the mpg increased a little. Now, my driving habits are not more aggressive with the 89 octane. I switched to avoid issues with engine damage, but with 74k on the truck already, i am guessing it may have been run on 87 most of its life? The truck runs great on either gas. i am a bit confused. is this normal? The truck does have an AEM intake and gibson exhaust if that matters.

 

how are you measuring the MPG by miles driven or just looking at the Ovehead console,,,, i know that for some reason since I change to 22's and havent reprogram the PCM to the tires specs my overhead display is for by 3 MPG

 

Get an SC tuner. i ran 89 octane on the 87 program with good results, and now i run 91 octane on the towing/performance setting. made the biggest difference in mpg's for me.

 

Also try getting your gas from somewhere else if you consistantly get it from one place.

 

Can you run tow tune with 89?

 

SC does not recommend it. It actually states on the programmer before you do the changes to your tune to use 91 or higher.


DK

 

I recently switched from 87 to 89 and my MPG went up about 2 MPGs

 

The only way to get accurate mileage information is the tried and true pencil and paper method. Plus, you MUST fill the tank absolutely full -- pump cut-off is not accurate enough. My truck gets a smidge under 14 on my commutes and almost 18 on the highway (if I keep it at the limit). Who, me?

 

I wish I could get a good average for all three, I just cant seem to keep my foot out of it.

 

some clarification, i get the mpg via pencil and paper. I only use the overhead to kind of moderate my driving habits, but I doubt its accurracy. I have been getting the gas (both octanes) at the same station. I didnt switch to increase mpg, I did it in fear of doing some sort of damage with the 87 octane. I am thining about the tuner idea, but i dont need more hp (I know i'm getting old). so i dont know. I just couldnt figure out the worse mpg, I figured it would be the same at least.