Here's my 2¢, take it for what it is worth:
'98-'11 Ranger wheel's have back space of 0-12mm depending on exactly which rims you have. Narrower rims have less backspacing - Ford was more concerned with keeping tire inside body work than anything else.
'99-'04 Mustang rims have 20-30mm backspacing again depending on exactly which rims you have. 20-12= 8mm ~ 5/16", but that has spindle cap right up against wheel cover (any mfg tolerances and your wheel covers won't stay on..), so the common spacer is 7/16".
'05-'08 Mustang have 39-50mm backspacing again depending on exactly which rims you have. So, with these rims, you would want to run adapter rather than spacers (39-12=27mm, over an inch).
I personally run 1" adapters on my Ranger with '95 Cobra rims (30mm backspacing) <245/45R17 tires> as I want the track at OEM width (it also balances the rear in my truck which has t-bird IRS).
Welcome Visitor! Please take a few seconds and 
