Tire size specs have changed over the years, and radial tires were a new thing back then....

The 70 or 75 is the height of the tire, expressed in percent of tire width (so a 75 tire is taller than a 70).

You need to find a tire guy who is old enough and is knowledgeable enough to determine which current tire will have the same rolling diameter as the original spec tire -- this will give you correct gear ratio and speedo/odometer readings.

The tire size required may not spec out much like the original -- for instance, the "correct" tire for the 72 280 SE 4.5 is a 205/70, original tires are a 185/75 spec, but not radials, so they were taller for the same "height" -- radials squash down more that bias belt tires.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!