Unfortunatly, the tire size DOES matter. The overall diameter of the tire will affect the accuracy of the speedometer, as well as the final gearing. I'd also like to say that if you are going to test the speedo for operation, do it with a drill attached to the cable. Running the truck off the ground is a disaster waiting to happen. You can easily over speed the rotating parts, as there is no load on them. If you have a drill that has rpm settings, that will work just as well. You will want to re-check the accuracy by driving it with the tires that you plan to run, that's the best way to get an accurate reading. There is also a good calculator for this in the Links section, Randy's Ring and Pinion. I tried it myself, and it's very easy to use.
