I've driven 3.90 cars. They are great if you like buzzing along at 55mph, sucking gas and running hot. At 70 you are up about 3800-4000 rpm.....so 50-55 is much better to cruise at. Takes all day to get somewhere, and many tanks of fuel. 3.23 gears allow for effortless 70mph cruising, right on the cusp of the car's powerband, not in the middle of it. Real fun can be had with even taller rear gears, but you need an auto or a 5 speed for them..... nothing like running neck and neck at 100MPH against a 3.90 car with a 2.56 car, and having the 2.56 car upshift into third gear and leap forward, pulling a 20-30mph lead on the 3.90 car with ease, while he's sitting there at about 107mph, floating his valves. I recently bought an old Chevy that was supposed to have a 3.70 out back, but someone had slipped in a set of 3.36 gears. I was delighted!!
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Jeff 
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