Peterson averages 4.8 ypc to Grant's 4.3 ypc over the last 3 years. The difference is due to Peterson's number of long gainers. This is from Advanced NFL Stats:
It's the open field breakaway ability that separates the guys with big YPC stats from the other RBs. Of Tomlinson's runs, 1.5% were for 30 yards or more. Bettis' 30+ yd gains comprised only 0.46% of his carries. The other RBs and the league average are as follows:
NFL 0.91%
Lewis 0.88%
Westbrook 0.93%
Peterson 2.20%
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Last year Grant had 3 runs over 30 yards, and Peterson had 5. All running backs in the NFL have a very similar graph of yardage gained per carry, but a few have the ability to break long gainers--like Peterson and like Barry Sanders was. Peterson's liability is his fumbling as everyone agrees. But I would take that risk in return for his big play ability.
On the surface, I would disagree with your opinion that the Yikes' offensive line has been better (both lines are pretty average I would say), but I'd be interested in any stats that you have to back your opinion up.
Advanced NFL Stats (
Advanced NFL Stats) is a great website that I think you would enjoy if you haven't already checked it out.