Originally Posted by dirtsurferus
will it still run well if i use 87 with the mileage tune?
Not as good as the 91 tune and my experience with my truck indicates that it will pre-detonate a bit with 87, especially when warm outside or pulling a load up a hill.
I actually get better mileage with 89 vs. 87.
Keep in mind, I have no tuner on my truck and won't get a tune until after I build the motor.