'71 Delta 88 Royale tires
 

I'm gettin a 1971 Oldsmobile Delta 88 Royale soon. This car has been in a barn since 1981 and is in immaculate condition. Granted it will need engine work, but that shouldn't be a problem. What I am having trouble with is since it has been off the road so long, the tires on it are not even the newer style coding system. If anyone knows what an HR 70 15 tire is in now days standards, please let me know. From what research I have done I am thinking it is a P235/75R15 but I can find nothing to confirm this.

I can't confirm that, but it sounds about right. You could always measure the tires it has now. The P in todays system means passenger car. The 235 is the tread width in millimeters, and the 75 is the sidewall height (also in millimeters) as a percentage of tread width (75% of 235 is 176.25). The final number 15 is the rim diameter.

'71 Delta 88 Tires.
 

The correct tire size would be 225/75R15.