I did not notice any MPG difference. I wouldn't expect the 20" replicas to be lighter than stock because they are bigger than the 17" wheels they replaced, but I am not sure. I do know that the OEM SRT wheels are lighter, but way more expensive (full retail
= $7302 for 4 of the early model style -- $1825.50 a wheel!!!).
Because the 20" replicas have a larger diameter than the 17" stock, I would assume that you are going a few miles faster than your speedometer shows on the dash, but I have not tested.