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Why does everyone need a website to calculate this stuff? You'd think it was some secret :-).
You have 205/60-15 tires, so the overall diameter is a combination of the wheel diameter and two sidewall heights (one on each side of the wheel) as you would measure across the tire. I'll work in mm, although you could convert the other way, working in inches as well.
1) The wheel diameter is 15" x 25.4 mm/inch, or 381 mm.
2) Because the aspect ratio (0.60, from the "60-series" designation of the tire) is the ratio of sidewall height to cross-sectional width (in this case, 205 mm), by algebraic manipulation, we can get the sidewall height by taking the product of cross-sectional width and the aspect ratio, or 205 mm x 0.60, or 123 mm.
3) Total diameter is the wheel plus two sidewalls, or 381 + (2 x 123), or 627 mm.
Your original tire was probably* 185/70-14, and we'll go from that point.
[ * if it was soemething else, such as 175-14, I'll let you deal with the changes to these calculations.]
1) Wheel diameter is 14 x 25.4, or 355.6 mm.
2) Sidewall height is 185 x .70, or 129.5 mm.
3) Total diameter is 355.6 + (2 x 129.5) =614.6 mm.
Your newer tires are 12.4 mm (half an inch) taller than the originals (assuming that aforementioned tire size), or almost exactly 2% larger.
[note that when you figure comparisons such as how much larger or smaller, the change is always compared to the original amount, not the new amount, so it's (12.4/614.6) = .020 = 2%]
That means that your speedometer will be indicating 50 mph when you're actually going 51 mph (just as it would be indicating 100 mph when you're actually going 102 mph, the 2% being easier to understand here).
And your odometer will likewise underreport the distances you've actually driven by the same proportion, or, e.g., it will indicate 1,000 miles when you've really driven 1,020 miles.
Hope this helped you.
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