Me thinks that should say:
The lowest cylinder should have at least 70% of the psi of the highest cylinder.

Or something to that effect.

 

& even that 30% variation would be very noticable in rough running.

I stick to 10 - 15% variable, as acceptable.

 

About a year ago, in the process of developing a better calibration for our '07, I took a lot of ECM data with HPTuners ECM Scanner.

I found that, given the way I drive the truck, on 87-oct fuel, knock retard was common. On 89-oct. it became rare and on 91-oct it was non-existent. I had my Wife, who drives the truck most of the time, switch to 89.

If the testing is done accurately, the climate is mld and the driving is never aggressive, there will likely be no fuel economy difference between running 87, 89 or 91.

If you drive your truck normally, which means accelerating onto the expressway/highway/freeway quick enough to be safe and when driving on two-lane highways having to occasionally pass other cars, you will see a fuel mileage decrease on 87-oct gas due to the retarded spark during acceleration under high load. Whether this is enough to negate the savings in using cheaper fuel depends on individual driving habit and gas price.

If you drive your truck hard, don't use 87-oct. Use at least 89-oct and if you want best performance use 91.

The reason the Owner's book says premium is recommended but not required is that the ECM's electronic spark control (ESC) feature has enough knock retard to automatically retard spark during detonation such that engine damage is prevented. Of course, retarded spark decreases performance, decreases MPG and increases coolant temperature. That's the trade-off of ESC-driven knock retard to prevent detonation.

 

OH BOY ... Looky What Our Wonderful Government Is Doing Now ....


SF