GM everyone, I'm back with another dumb question. I have a GG 5.7L 2005. According to the manual I can put either 87 or 89 in the gas tank and drive with no problem. This is true, but here's the strange thing. When I put 87 inside the tank and hit the button to see how many miles I can go on this one tank of gas I find that I can go 264/280 miles. When I put 89 inside the tank, it reads 321/340. In addition to this, when I put 87 in the gas tank, when I push on the gas pedel it hear a loud noise in the back as if I had loud ice packs on the muffler. When I put 89 inside the tank the noise is down to a low noise. But I'm more interested in the first question which deals with the two different grades. Thanks

 

I am using 87 on 4.0L, but my fuel millage is poor. I get 12mpg. It is 96 GCL

 

It is normal for there to be a milege difference, thats the whole point in the grades of fuel. The higher the grade the cleaner/more efficent the engine will run. That explains the milege ? as for the noise I'd have to hear it. If its a slight difference it could be the engine is simply running smoother. But like I said I would have to hear it.

 

What would happen if I put 98 or the highest I could find, would it give me 375 or even 400 miles?

 

I do not know of what it is recommended fuel grade to put in, but if you put $20 super and $20 regular then the fuel millage will be almost equals as you get more regular fuel that super.

 

It's recommended to put either low or mid grade. Just wondering what would happen if I put high grade in it that's all.

 

There will be a point where higher grade will not make it any better, and may make it run a little warmer. Sea foam it!!!

Do a search on sea foam, either hear or youtube. It will burn out carbon, therfore lowering compression a bit and getting rid of hotspots in cylinders. Will result in less detonation sound (assuming that is what you are describing) and better milage on lower octane gas (in some cases)