I drive a 2002 CTD 3500. I have a 4.10 differential. I am thinking about putting on some American Force 19.5" direct bolt on wheels and commercial grade tires. Is there a reference I can use to determine the new gear ratio after changng wheel/tire size?

 

The rims will not matter. Tell us what size tires you are putting on and then some one can help.

 

Right on! The rim size means nothing, other than the absolutely smallest diameter you can have! Look online at the tires you are considering, and you will see a whole list of specifications including rev's per mile, weight, diameter. trewad depth, etc. All of these will be listed for a particular rim size eg 6.00 or 6.50 width. If you go with a 225/70R19.5 tire, you will be pretty close to stock. If you move up the say a 9R19.5, you will be quite a bit taller.

 

Got it. I will clarify the question. Is there any reference material that will allow someone to look up the effective gear ratio based on a known change in tire circumference? I read some time back about such a document. What I would like to be able to do is predict my final gear ratio before I buy new rims and tires so I will know ahead of time of what I can expect. Make sense?

 

I found one on a 4x4 site that worked off of inputted sizes and ratios. Do a google search for Tire Size Claculators. Several out there work well.

 

There are tire size calculators at the Rickson site where they sell the 19.5 kits.

http://ricksontruck.com/speedometer.html