yes your correct. the reality though is you'll use more fuel so you'll be adding more then 10 gallons. ideally when you put bigger tires you would adjust so your speedometer is correct. of course it depends on a few things like tire size, engine type, lift kit, etc. typically the more load you add to your engine, the worse your mileage will be.....more torque required to turn a bigger tire. instead of using 10 gallons, might be 11 or 12. now figure your mileage...189/11=17, 189/12=15.75...worse not better. vehicles like diesels which produce high torque might not be affected as much. just depends on how big of tire you go.