Ford: 96 Explorer, vehicle speed sensor, size tires

Question
Hi Ragu
I have two questions about my 96 explorer, w/112000 miles,V6,2WD,2DR. What would cause the speedometer to read 12 to 14 miles fast than actual speed? I have right size tires, nothing is modied that I can tell, I bought it 98,000 miles on it.
Also what might cause the A/C to smell musty, mostly only at idle, the drain does not seem plugged, is there any filters that can be cleaned or replaced. I tried spraying lysol in the vent in front of windshield.

Thank you for taking my question, and for help you can give me.
                         Mike

Answer
Mike  the best thing to do about the spedo. is hook up a scanner to it and read the VSS . (vehicle speed sensor)and see if it is the same as the dash spedo.if so then have some one drive long side you an see if your spedo reads the same as  theirs. as far as the ac what you need to do is turn the heat up all the way let the car run then mix bleach and water in a spray bottle with the vehicle running spray it in the outside air intake for about 3 min.then then wait 3 more min and shut every thing down. wait about an hour .you will be good to go .the reasons why you turn the heat on all the way and the blower motor up is because the plastic has small pores in it and it holds mildew .