Hey RobbyR, I did ask this question to my local "honey wagon" man a while back... Here in SC I am not sure of the building code but I have yet to see a septic less than 16" in the ground. when asked I was told I was good to drive over the system with an average size car. (google search of an average size car is about 4000lbs.) And to worry more about the baffle tubes/pipes(4" pvc) than the tank or in the question you asked the lid...? seems to make sense to me, my lid is precast pretty stout, and I am sure I could break the pvc pipes before that lid would break. So to the tractors... Of the tractors I was interested in... the '63 4000 is was most power for the least amount of weight and in chassis not much bigger than a 600 series tractor. The 3000 and 3600 weigh more and in some cases more than an average size car with about the same power as the 4000 I bought. (I found this info on tractordata.com) This information I found/came up with, could possibly be inaccurate so don't take it as the cold hard fact. It is good enough for me though.