I need to get tires for my 1977 Cadillac and the first challenge is what does a "GR78 - 15" map to?
It has 215/75R15 tires on it. Tirerack.com seems to think I should use a 225/75R15:
SEARCH FOR TIRES BY VEHICLE
ORIGINAL EQUIPMENT SIZES FOR:
1977 CADILLAC DEVILLE
Original Equipment Wheel Diameter: 15"
This article, http://www.cadillacforums.com/forums/rwd-19xx-1984-deville-fleetwood-1985/245184-old-tire-size-conversion-newer-numbers.html explains that "One reason they went to the P-metric standard was the difference in sizing among the tire manufacturers. Some resouces claim the GR78 - 15 were actually somewhere in the middle, on average, between 215 75 - 15 & 225 75 - 15 sized tires."
So that could explain why I am seeing two different sizes. Further down in the article, someone posts a picture with two 1977 Cadillacs, one with 215s and one with 225s. At least from the picture, I can't see much of a difference.
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