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Using premium
I fill my tank every month (2.95 x 17.4gal = 51.33/month ) x12 = $615.96 / year x 10 years = 6,159.60
Using regular (2.55 x 17.4 = 44.37/month) x 12 =532.44 / year x 10 years = 5324.40
Difference of $83.52 a YEAR!
and
835.20 in 10 years
Difference is not even enough to buy tires for 10 years
C'MON MAN!
I fill up in average 5 times a month and gas here in Canada is more expensive. by your calculations I am paying more 83.52 x 5 = 417.60 per year = $4,176 in 10 years
which turns out to be paying 34.80 extra per month (or 6.96 per fill up) for the additional ponies and sense of security.
I still put Premium because I fear that something happen to my engine or it will be less refined as it gets older. I want to put regular but i have not seen any long term reviews on this because people keep putting other people down about this "its a luxury car you....."
I still don't think you have a strong argument. I would take an educated guess and say 98% of drivers driving Lexus' that require premium do so. You want proof of engines that were destroyed due to running 87. You won't hear about them because people don't run 87 lol.
Weird as the Lexus IS is considered a luxury car...you'd think most wouldn't quibble about a few bucks here and there. Honestly, $800 in 10 years is a pittance.