The truck has a little over 500k miles. Seems to be well maintained. My question is, what kinda mileage should I expect? Has isx15 set at 450, the trans is a FAOM15810C-EA3 10 speed auto, and the rears are 2.64 ratio. This will be my first auto shift. Mixed emotions on that part. Trading in my 579 with paccar engine cause I’m tire of the issues. Mileage is my biggest concern. I got over 7 sometimes 8 mpg with the paccar. Hoping with a 2.64 gear it’s tall enough to sip fuel but still run decent on hills. I usually only run about 68 mph out of habit and normally gross about 62k.