I put about 3400 miles on my 2005 SRT-10 before I added the 3.55 gears. I've been logging my mileage pretty religiously. I bought the car from Woodhouse Dodge in May, picked it up in person, and took the scenic route back to Virginia via the Black Hills, Mt Rushmore and the Badlands. I averaged 18.5 mpg on that 2600-mile trip, with the pure interstate segments at just over 20 mpg. The next 713 miles were a combination of highway, 2-lane roads, some traffic and minimal spirited driving on the stock gears, good for 14.5 mpg. I then had the 3.55's installed. Over the next 1450 miles, with the same type of combination driving as the previous 713 miles, I've averaged 14.9 mpg. (I've not been able to get a good mileage check for solely interstate driving since I've added the 3.55's.) So there's not been an apparent decrease in mileage with the 3.55's. The 3.55's make using 5th and 6th gear more viable, and that probably offsets the loss due to the gear ratio. Of course, your mileage may vary, especially if you're doing daily driving in any kind of traffic.
