not really true! the V10's (new and old) have a 2000-2300 sweet spot. getting a V10's rpm's down at cruise isn't really going to help a 8000lb truck!

to compensate for tire diameter and rolling weight the 5.13/5.38s will be right where you want your crusing with 39's and up! now PSD's are different since you want to keep the rpm's under the max torque rpm! when i changed from 3.73's to 4.56's with stock tires i actually noticed a slight (let me repeat that SLIGHT) increase in mpg. but remember the fuel economy has been beat to death regaurding a truck that has the dimensions and areodynamics of a BRICK! but while on the subject..........the 4.56's help the engine and drivetrain work LESS to achive speed thus making it better on fuel not having to struggle to get up to speed.

but with any lower gear change (higher #'s) you will sacrifice some top end performance. if you could go 80 @ 22-2300rpm's with 3.73's, when you drop in some 4.56's you would be at apx 2700 rpm's @ 80mph. so you'd have to live with going around 65-70mph which would put you at apx 2300-2400rpm's wich is still in the "sweet spot" for the V10's

now going to a 5.38 with stock tires will put you way over what you want in a v10 (25-2600 in OD @ 60). yea you'd get there REAL quick but it would take away what you had for top end and live with crusing at 55mph.

it will also depend on the truck. some one with a reg cab long box and a 5spd might not need as low of a gear (due to less weight and less drivetrain resistance) as some one with a CC, auto, Long Bed dually (more dead weight to push, and more drivetrain resistance)!

unless you actually run them, or can get ahold of somone that has close to the same set up, it's hard to nail it! i tried a set of 4.56's (according to the computer calculators) in my race truck, it was good, but i went one step further and droped in 4.88's! you'd think that woun't have made that much difference, but at the track it was a completely different truck!!