I don't know all the technical reasons why bikes get the mileage they get but here is my personal experience with my 07 YZ 450: If I am on the gas in the desert I can ride for just over two hours straight. If I am not on the gas I can go just under 3 hours. If I am at the dunes climbing hills I can go an hour and 45 minutes. I weight 235 w/out gear. My bike has stock gear ratios and a DR.D full exhaust. I am an average "B" rider on my average day. On a fast day I am a good "B" rider. My bike is well jetted. If your buddy can go all day on a stock tank on a CRF450 I say his bike is running extremely lean, he is not on the gas (think old, old, scared lady), or he is not really riding the full day (ride, chat, ride a little, eat, hit a jump for the ladies, crash for the ladies, talk to buddies about crash, sleep under a rock to let the pain subside from said crash, talk to buddies about heading back to truck, ride back to truck, do some gravel pit wheelies for ladies, etc.) If he rides hard and can go a full day on a stock tank there is a discrepancy with something.