I used the 1950 cortex on my F150 2006 5.4 KR and I did the 87 octan performance. Its seems to run fine and all, my gas mileage increased by 2 mpg which is cool but when I give it the gas to pass or just to speed up I seemed to get a jolt in the rearend. Is this normal? I did the measure of the tires, and I have a 373 rearend and I slected it too... if this is not normal any ideas? Thanks for the help.