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There are many calculators online but all assumes that the gas price will be the same for the two cars.
In the case that one car uses premium gas, what would be the equation to calculate the difference in monthly gas cost for two cars?
Keep in mind that "Premium" gas isn't named that way because it is "better" or automatically gets you higher mileage.
It just means it has a higher Octane Number. Which is an additive that lets some cars run hotter in the engine without causing a problem (knocking). Hotter (but not too hot), is better as it lets the engine pull that much more energy from the gasoline.
What it does with that extra energy depends on how the engine was designed and controlled. Nowadays, they have very sophisticated sensors and computer controls that allow for a range of operations. Maybe the engine utilizes the extra heat to get better mileage, maybe to get extra performance.
So it's conceivable that the higher priced, higher octane gas, also gets you higher mileage. Is it break even? That's even harder to say since your driving habits, weight of luggage, routes, weather, and even the direction of the wind, all also effect your mileage.
Or maybe it's how the designers made the car to go so fast, so quickly.
To answer the OP's specific question:
The best way to resolve this confusion is use the Sticker "EPA Fuel Economy Estimates, Annual Fuel Cost. That will be based on a LARGE number of variables, BUT, using the EPA tests means the calculations are standardized - so it IS valid for comparing them between two cars.
Bottom line: It's pretty much impossible to guess what your annual gas bill will be, but the above will give you a pretty good estimate of the difference between two cars.
I compare cents per mile. For example, fuel costs $2.40/gallon and vehicle gets 30 mpg, then it costs 2.40/30 so 8 cents per mile.
Another vehicle requires premium gasoline costing $2.80 and gets 20 mpg. So $2.80/20 equals 16 cents per mile.
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