The verdict is, 89 provides a definite and consistent increase over 87 of on average about 6%. For me that means 1.1l/100 km, or 0.86 Imperial MPG. 89 is between 6-7% more money than 87, which means I’m paying about 1% more per tank, or between 60-80 cents. Small price to pay for a better running engine and more power. Also, more potential for tuning—see the link in the comments of my old post.
91, so far, seems a dead loss. I’m seeing the same numbers on 89 and 91. I’m putting one more tank through, and then unless I see some major changes I’m sticking to 89. Whether the engine could make use of it if it was tuned for it, I don’t know.
Bear in mind these numbers are all on winter gas. I have a very small sample of similar conditions on summer gas as I drove a lot more highway on random tanks and I didn’t start using 89 until the fall when mileage was already starting to drop off. The last post’s numbers were based off 2 tanks of 89 from last winter. 1 tank got me 3.5% better, the other got 7. Driving conditions/city-highway mileage/behaviour were about the same on each.
I’m planning on getting the truck tuned as soon as the winter gas gets put away, so expect one more follow-up post.