Your calculation is still off becase your tires have a larger circumference than the OE.. your truck is actually getting about 17mpg because you're covering more ground than what your odometer is registering.
sorry, but this is confusing me. if im covering more ground than my odometer is registering, wouldnt that mean im getting more miles to the gallon? not saying your wrong JLDickmon, just trying to understand this. kinda feelin like an idiot here.
You are correct Clint. Increasing the rollout on your tires will lower the displayed mileage on your odometer traveled below what the distance you have actually traveled. Translating to: yes your mpg would actually be higher than your math shows you.
If you are still considering a tuner with a data display the SCT livewire IMO is far better than the edge and much easier to see.
Let me know if you need to find a dealer who might help you out on pricing or take your superchips micro tuner in on a trade.