Just to chime in. For my '97 5.7L Vortec Tahoe I found that the truck runs poorly and reduced MPG with 87 gas containing ethanol.
Using non ethanol 87 and it runs very good and averages 17MPG and this is with a 332,000 mile engine.
I recently tried 91 non ethanol and even though the truck "seemed" to have a little more "pep" the idle was rougher and it seemed to develop an intermittent hesitation at speed.
Switched back to 87 non ethanol and it runs great.
The truck did have a computer tune when it was new but I don't recall if it was for 87 or 91. My guess is it was for 87 since the truck was all stock.
As a side note I tried 91 non ethanol in my 97 K3500 7.4L vortec and it ran like crap. Back to 87 and it runs great.
Just my experience.
Using non ethanol 87 and it runs very good and averages 17MPG and this is with a 332,000 mile engine.
I recently tried 91 non ethanol and even though the truck "seemed" to have a little more "pep" the idle was rougher and it seemed to develop an intermittent hesitation at speed.
Switched back to 87 non ethanol and it runs great.
The truck did have a computer tune when it was new but I don't recall if it was for 87 or 91. My guess is it was for 87 since the truck was all stock.
As a side note I tried 91 non ethanol in my 97 K3500 7.4L vortec and it ran like crap. Back to 87 and it runs great.
Just my experience.