The 58 volt upgrade is an option that adds 40 horsepower to the existing hp you currently have.  It is unproven whether your gas mileage will increase or not because of the upgrade.  Their has been no increase in failure rate with the upgrade.  The only drawback to the upgrade is the computer will always store the code P0611.  The code will not activate the CEL(check engine light), but will be present if the truck is scanned for codes.  P0611 code is set anytime  there is plus or minus 5 volts with 48 volts being the optimal voltage.  As with the regular repair, your ficm will be test run in our shop truck for at least 5 minutes to make sure your FICM is problem free and ready for installation into your truck.