You know, you can easily accomplish your goal for under a hundred bucks. Just buy four of these digital 400 lb. scales off Amazon at $22 a scale, then cut small wood platforms so that the platform is half on the scale and half on the pavement (less than $5 of wood per scale). Each wheel then sits on the platform and the reading is effectively half of the true weight of the wheel. If you "synch" out each scale beforehand by standing on it and holding a 50ish lb. weight in your hands then the delta between the scales (which is all you really care about) is going to be very, very small. Remembering that a gallon of gas weighs six pounds, I think this level of accuracy is just fine. And you can accomplish all of this for under a hundred bucks... and then give the scales to your friends as Christmas presents.
