There is a fuel load setting in your garage/setup window. i haven't messed around with this feature so im not sure if its what ur looking for.
There is also a program called NR2003 Editor that allows u change several things in ur game like championship schedules, AI strength, and acouple of other settings including fuel consuption, tire wear, etc. Im not sure what site i got it from but if u google which is how i found it im sure u will find it.
Im not sure this is what ur looking for but its worth a try.