Pls help...6 cyl or 8 cylinder Luxury autos ?

Pls help...6 cyl or 8 cylinder Luxury autos ?
My sister in California is thinking of buying a "used" Luxury car / sedan around 5 years old like a Mercedes or Lexus.
She is confused as to the gas mileage for 6 cylinder and 8 cylinder luxury cars and the related cost of gas.
I was told that for normal driving say 10,000 miles or so in a year, the cost difference wasn't much on a weekly basis even with these high gas prices.
Is the story any different with 6 cyl and 8 cyl SUVs ?
Pls educate us on this as we are NOT all knowledgeable
Thanks Anna
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anna wrote:

If you do a web search, it should not be hard to find the EPA ratings of the actual cars under consideration. While those aren't 100% accurate and a lot depends on how you drive, it will give you a far better idea of what the diff in gas cost will be than random online opinions. If you got about 3MPG diff in one vs another, it would be about $250 a yr.
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