Reddi......it's all in the math! You can do it manually or I developed a crappy little spreadsheet that I use.
Here is the gist of it. The tire/rim combo that came on your truck have a circumference of X. As long as the tire/rim combo that you put on the truck has the same circumference (X) you don't have to change anything.
If ya want a copy of the spreadsheet (cause all that math can be a bit boaring!) I can send you one via e-mail....just e-mail me and and remind me that you want the tire size spreadsheet. I'll share!
And anyone else that wants the spreadsheet just e-mail me....or if ya want me to post an example of the math I'd be glad to do it...unlike the written word, math and me are on a first name basis!!
reddi - the height changes as the width changes.....ex a 285/45/16 has almost the same diameter of a 235/55/16....because the series tire is in ration with the width....and keeping the same diameter will keep the speedo the same.....but even so a inch difference is like 1mph for every 50 mph your doing.......check out s-series.org they have some good calc....maybe the admin should look into the calcs for this page....
OK guys I just looked in the mirror. And on my chin it said insert foot. Sorry to doubt you rhenry I was drinking at the time and Coors had taken over the keypad. Thanks for the great info and advice here all!