Help with my truck..Please

That is a false statement and a stupid thing to say....60 mph at 1200 rpm will get better fuel mileage than the same EXACT truck doing 60 mph at 1400.
A piston going up 200 more times a minute (100 exhaust strokes & 100 power strokes) is a total of 600 sprays of the injectors each minute more on 1400 than 1200. In same truck, to think those 200 extra crank revolutions per minute have no fuel mpg effect is assenine...

 

I'd bet good money that less than 3 % CAT engines get better mpg ithan their counterparts in a similar final drive ratio'd truck under similar loads.

 

Wheelbase = measurement from center if front axle to center of rearmost axle. Center being located by looking at the wheel from the side, it's center of of the wheel diameter.

 

Gear ratios are talked about funny . The lower the number (2.64) the higher the ratio. 5.13 is a LOW ratio. You needing to run 65 at a lower RPM you need a higher ratio, but basically you have the highest ratio now, so you need a overdrive transmission, but that swap will be very costly, since you are starting with a auto shift( you are missing many parts) . Whoever speced this truck missed the mark or wanted to run at 60 MPH. Swapping tires may drop your cruising RPM (maybe ) 200 .

 

Eaton has a gearing calculator, to help you out with ideal speed.