Loosing 10 to 25 psi in a week from what are apparently large volume low pressure tires is quite a lot. What type of valves do you have? If Presta, be sure the valve nut is snugged down after filling the tires. If Schrader, be sure the valve core is tight.
Also, you may be loosing a fair bit of pressure if you use a tire gage. Every pressure reading lets air escape and bike tires don't have much volume to spare.