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I have done a simpler/similar exercice for the last 4 years.
02 Infiniti Q45 comes with 245/45/18. I have used 255/45/18 on the stock alloy rims.
The auto tran has overdrive and the rear raqtio is 2.76 stock.
I consistantly got 25-27mpg over the 23hwy epa avging 65mph
I am running about 1780 rpms at 60mph.
I have sacrificed some off the line with the wider/taller tires but the car did 0-60 in 5.9 sec already! I do about 65% highway and I like the 'fuller' tire. gives me a smooth ride even at 40psi.
The determining factor for me was the front strut lower base is 1/4 inch from the edge of the tire! I could not run any other size up with out going to air suspension.....and I'm not gonna VIP the ride. Also I did not want to dramaticly chang the ratios and relationship of the suspension parts. I began running this larger size at 60k. I am at 192,000 and am replacing the shocks and struts as normal maitanance this month.
Warning: Increase in size increases wieght. I have been told that the static weight increase, say 8lbs is multiplied by 4 when rolling. This DRAMATICLY increases the demands on the brakes and associated parts like the rotor. So I would start with fresh rotors and pads.
Good luck!!
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