I haven't had them long enough yet to really understand impact on mpg - but, mpg isn't awesome anyway compared to the RRS Diesel I traded ... so that wasn't my primary concern. But, I'll happily report back as I start to get through a couple of tanks of gas. I hadn't planned on changing out my GY Adventures but once the sidewall was cut, I went ahead and pulled the trigger. I went in to get the Duratecs, but they weren't readily available and the tech recommended the KO2s as being far more available over the odd ball sized Duratecs (in case of future tire issues / cuts / etc). I'm pleased with them so far and they are not noticeably louder - but I do have the roof rack which may cover up any differences in tire noise. I think they look awesome for what that's worth :-)