Turn off you AC and get smaller tires.
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What are trying to do here get exemplary milage or save money? If the later give up and buy a different car. Nobody canceled second law of thermodynamic. Paraphrasing: you can't get something for nothing. Theoretically RP saves you 1 mpg, which I doubt. Lets say your milage was 18, now 19 mpg. It translates into saving about 260 gallons of gas over next 100'000 miles. At current rate of $2.00/gallon = $520. Now suppose you do oil change every 5000 miles = 20 oil changes. 5 quarts each = 100 quarts of oil. I imagine you pay premium of $4-5 per quart over dino oil. So total you end up spending additional $400-500. Savings: $20-120 over say 7 years. You may extend you oil change interval but still it's peanuts comparing to total cost of ownership. Don't bother.
Don't get me wrong, I'm not against synthetics. I have them everywhere and get 15-16mpg. I'm not using them for money saving reason.