Just try to keep your foot out of it (not gonna be easy) even though these motors sound B*TCHIN' uncorked at the upper RPM range, not to mention if you add a programmer to the mix. On the highway is the only time I'm good, in town I'll delay the shift a few seconds to get a nice roar from the pipe and I rarely speed...now that doesn't mean I don't get up to the speed limit fairly quickly, I just don't go far over it. Lol.

 

Agreed, not quite anything like the sound of a tuned 6.0 turning over 4k rpms.... .

 

Equally, if not more important, is knowing how to correct your speedo. Please keep in mind that these speedos are calibrated by Ford, NOT NASA! It's very simple, you only need to find the rate of change. Lets say you went from 265/75-16s to 315/75-16s.
width: 265/25.4= 10.433 315/25.4=12.401
aspect ratio(x2):10.433x1.5=15.649 12.401x1.5=18.601
rim dia :15.649+16=31.649 18.601+16=34.601
rate of change :34.601/31.649=1.093
So, your speedo AND trip odometer are off by 9%. Simply stated: 1.09X60mph indicated is 66 on Johnny Law's radar. 1.09X450 miles on the trip odometer is 491.85 miles traveled.

So what gear ratio will correct this? 3.73X1.09=4.065 ~ 4.11 would be close.

This is about as close as any of us can hope to get without some crazy Myth Busters lab test. Why do I do it? I'm a sicko and like basic math.

 

Ever checked the speedo from Ford, it's off from the factory...

I set mine with my GPS and Autoenginuity, so my speedo is as close as possible (AE only let's you change tire sizes .25" at a time, so still hard to get dead on, but you can get reallll close).

Your math probably still works well for metric sizes, but for the tires with inch sizes... IDK. A 35" tire next to a 37" tire, and there is way more than a 2" difference. Air pressure will also play a role in the overall size of the tire. Best way to do it is to mark a spot on one of the tires (installed on your truck) and roll it forward and measure the distance on the pavement it takes for one full revolution.

 

You can play with diameter with air pressure, but you can NOT change circumference/revs per mile. "Ye ca'naw change the laws of physics, captain!"
I used the metric conversion for the goal of finding diameter. The conversion factor is the same for diameter as it is for circumference. By referencing rev per mile on tire rack, thats about the same error rate also.
Bluntly, that method is a tool to sniff out the 25mpg BS artists. That was half of my motovation for figuring it out.

 

the best that i got was 18 mpg but that was over a two day trip and it was all freeway driving and the best i get in town is 12 to 14