i have a 2000 2500 automatic standard cab with 165,000 miles. i am currently looking at a second truck which is a 2002 3500 dually quad cab 6 speed. i ttest drove the newer truck (only 18,000 miles) and found that the rpm's at a given speed is about 120rpm's higher than the automatic. does anyone have input on this? can anyone advise on the downsides of a dually versus single wheel?