weel , from calculating the speed difference on my speedo and all the radar things in my area ( the ones that tell yo how fast your going ) 31" tires will make my speedo right on .
But I'll make my own 1" block at work with all the scrap 1" aluminum that we have , and get the 31" for those 15" and I'm all set with mods on my truck
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Sorry that is not on topic , but i would probly investigate what is your actual speed . and determen if you will need to regear the speedo with 31" tires .
BTW it just happen to be that 31" will be right on . I was going to go with 30" actually .
