PRESUMING you want to tow 4-down: If the ignition switch is in the "off" position....no power to the computer...no milage registers on the odometer.
My previous toad, a Ford Contour had a stick shift. Nothing to it. Leave the key in the switch (off position), put the tranny in neutral...good to go. The key if left in so the steering wheel turns free.
With automatics it's different...more steps to take. My HHR...pull a 2 amp fuse. Key in switch, tranny in "N" and off y'go.
My Ford Ranger 4WD auto....had to buy a Ford only "Transfer Case Disconnect Kit". A multipstep proceedure, but it works.
In all cases, no milage regesterd while towing 4 down. The HHR and Ranger were both bought brand new. No warranty problems that I'm aware of.