Wrong web site...right catagory. I have to assume you mean the odometer when you say clock as alot of rigs use both. New tires will help fuel milage some but running correct preasure (checked OFTEN) helps more. To ask the mpg with relation to 500k vs 100k seems to indicate that you have alot to learn in regards to running a rig and alot more in dealing with revenue/overhead stats, P&L's and cost per mile analasys. There are alot more important variables that would impact your mpg other than your "clock" ie;engine size,last major maint, make and body style,rear gear ratio. Most recent previous owner could probably tell you exactly what you'll get for starters.
I've seen 500hp get 6.8 and 350hp get 5.3! Greatest impact variable is the driver. Best of luck out there.
Keep it between the lines and take care of each other.