Ok... would using premium grade over regular grade improve a 1998 GMC 5.7's towing power and mileage. I had heard that by using premium it allows the computer to advance the timing more and thus increase the motor performance. Is this true??

 

It does advance the spark timing yes.
What you could do is put 11:1 high compression pinstons in
and you could put in adjustable cam sprockets in to take full
advantage of the high comp pistons and the truck will pull like never before.

if you use 11:1 pistons you will still be able to use supreme octane, but you
will ONLY be able to use supreme. But you will net a ton of more power.

However if you don't get the pistons I would just stick with regular gas and put in
Chevrons Techron formula (if you don't use chevron) and your engine will run
better.

However for something that was not meant to run on premium gas,
I don't think you'd be able to notice much power gain with only 2 or 3 extra octane gas.

 

I have an '04 Silverado with the 5.3L V8. It is designed to run on unleaded regular (87) gasoline, and does so with aplomb. However, under hard acceleration it does sometimes ping or knock.

Every so often, I will throw in some mid-grade (89) fuel and it won't knock; but it costs more to buy (about 12 to 15 cents more per gallon) and my fuel mileage doesn't seem to improve at all.

Even with regular, my truck takes more than $80 to fill the tank. No way will I be able to afford a full tank of premium. Yikes!

 

To fill my tank....

87 octane at current pricing will cost me $133 @ 35 gallons
92 octane at current pricing will cost me $141 @ 35 gallons

I fill up every time with the 92 octane and spend one less day eating out at fast food for each fill up.

I think it is worth it for running the 92 octane ...I here no pinging when I climb hills (that is what happens when I use the 87 octane)