In my 2003 Explorer 4.0, I run nothing but premium. I got 20-25 more miles per tank, and I tested it 4 times. 2 times were purely highway miles at same speed. (I tested the regular 87 octane first because at the time it was used to it). My first city tank of premium was almost worst than the 87. This may be because of my driving style (didn't pay much attention). After that, the other 3 tanks were 22, 23, and 26 more miles per tank if I remember right.
On the 3.0 ranger, I've done no such testing. I know that it feels like it runs better on Shell Premium. As mentioned it's probably the fuel additives, not the gas itself. OR the placebo effect.
Here is my logic guys.
At .05 more a gallon, a 21 gallon tank costs 1.05 more to fill up. Wouldn't you pay 1.05 to clean your injectors each tank?
At .10 a gallon, it's still 2.10 more per tank.. I'd still pay an extra 2.10 for a chance at better mileage, and to clean my injectors.
The price difference is minimal, and even if the benefit is minimal I see no reason NOT to treat the trucks to better gas.
Also, realistically, 87 and 91 octane are NOT THAT MUCH DIFFERENT octane levels. I'd be willing to bet it makes less than a 5% difference.