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wrwalke -> RE: 1978 tire question (6/7/2008 5:45:45 AM)
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The first number is the "fatness" of the tire, the bulge between the rim and the tread. Your front bulge is the same, so you should have no issues there. Your rear bulge is going to be 10mm wider than what you currently have. That is less than 1/2 inch. So if you have 1/4" clearance on both sides now, you shouldn''t have any problems. The second number is the "tallness" of the tire in relation to the bulge. 60 is 10 less than 70, so 10% of the bulge "shorter" from the rim out to the tread. your tire tread will be about 22/23mm closer to the rim, just under an inch. Closer is usually not an issue. :)
This combination will change your revolutions per mile though, so your speedo and odometer will not be the same.
rim to tread...
(225 * .70 / 25.4 ) == 6.2 inches
(225 * .60 / 25.4 ) == 5.3 inches
radius from center to tread.
6.2 + 15/2 = 13.7" radius from center to tread.
5.3 + 15/2 = 12.8" radius
circumference of tire (distance per rotation)
13.7 * 2 * 3.14 = 86.0 inches
12.8 * 2 * 3.14 = 80.3
So you''ll be showing about 7.5% higher on your speedo than you are showing now at the same speed, and your odometer will be counting about 7.5% more miles for the same trip.
bill.
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