Sorry guys but i trust you all. I changed my tires on my truck from 17" to 18". Called local dealer to find out what it would cost to recalibrate speedodometer and was told I do not have to. As long as I did not go over 20" everything is accurate. Is this true ?

 

I believe tirerack.com has a caluclator.

You input you tire size(s) and it will return revolutions / mile.
At that point you can see the difference (if any).

And decide from there.

Since, I believe 17" rims only came with a dually.
Can you tell us what truck you have?

If it's a dually, did they add spacers, etc?

 

You only have to worry about it if the overall tire diameter is different. If you changed wheels and your new tires are the same height as the old ones then your ok