When testing collars to find which ones have the best range, there are a few important rules to follow:
1) Always compare collars at the same place at the same time and hanging up in the same way. Once you know which collar is putting out the strongest signal, then you can try placing them in adverse places like in a ditch or behind some obstruction to see the differences there.
2) Hang the collars so the antenna is vertical. Since the signal radiates broadside from the antenna in a donut pattern, you'll be able to drive off in any direction there are good roads and get an equal signal. Your strongest signal will be when the yagi antenna on the receiver is also lined up with the collar antenna, as shown in this image, because the signal is polarized.
3) Drive off in whatever direction is accesible for you, and stop at regular intervals to check the signal strength of each collar. You will want to continue out until the faint signal from only one collar can be heard. Remember that sometimes, due to an obstacle such as hills or a gradual elevation change, you may be in a radio "shadow" where all the signals are suddenly lost. You may find that you can continue even further out and to a higher spot and pick up the signals again. This is an important thing to remember: if you've lost the signal, get high fast. A FireTower is a great spot that will suddenly give you miles of new range. If you see a radio tower in the vicinity, it's probably the highest spot for miles around.