What it boils down to is the cars performance aspects. Look at the user manual in your glovebox and see what it states. Some say run 87... some say run 87 or 89... some say run 87-93 ...some say to run premium fuel only 93 and some say unleaded fuel only. Some cars perform well on 87 and some don't. All depends on the car, size engine, what it was designed for. Remember there's economy cars which were probably designed for the use of 87. I've had cars in the past that performed well with 87 and I had cars in the past that simply hated it and ran like crap. So usually when it came down to my non-turbo cars I would put mid-grade 89.