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Discussion Starter #1
I have a 2007 Titan that came with 265-70's on 18" rims. By my calculation that is a total tire height of 32.606".

I have an opportunity to buy a set of 17" rims for an older Titan. If I run 285-70's on 17" rims I calculate that to be 32.708".

If I do this, shouldn't my speedometer still be correct? I just want to know if I should invest in the wheels.
 

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Because of manufacturing tolerances, side wall design, and tire inflation, they are going to vary by up to an inch or so. Go measure your tires ... doubt if they are exactly 32.606", even without the trucks weight on them.

Even the spec sheets which are far more accurate for a specific brand and model of a tire than tire calculators are, state that the sizes listed are an average when measured on a test rim of a specific width.

BFGoodrich AT KO spec sheet said:
1. The dimensions shown are average tire design values for tires measured on specified measuring rim widths. Some tires may vary from this value by +/- 3% of the section height (affecting overall diameter), and +/- 4% of the section width.
2. Section width varies approximately 0.2" (5mm) for every 0.5" change in rim width.
 

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Discussion Starter #5
Thanks guys. Using the calculators - that's about as close as it is going to get.
 
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