The first screen that comes up is what we will call the main screen.  This will list "Tire A" and "Tire B" with their inputs.  "Tire A" should represent the current tire size on the car.  "Tire B" represents the new tire size.

To change a value, click the down arrow next to the number.  Go ahead and select different values.  They don't have to be the same as in the example shown to the left.  Note that when you select a new number, the calculations are made immediately.  Please be patient between changing multiple values.

Below both tires is the circumference length and percentage value of difference.  There are two buttons in the bottom right of the screen.

As shown to the left, select the "Explain!" button.  This should help you understand what the percentage values mean.  For example; if you're speedometer says 60mph, and the percentage value of the new tires are 3.045% faster, then your actual speed is 60 x 1.03045 = 61.83mph.  If you were .294% slower, then your actual speed would be 60 x .96955 = 58.17mph.  The formula looks like this:

(speedometer speed) x (1 + or - xxx) = actual speed

Where xxx is the percent value which is added (faster) or subtracted (slower) depending on the sizes you input. 

There are two pages to the explanation, depending on your view point.  The first page is on the left, and second page to the right.  Use the "Say what?" button to see the 2nd page.  The "<" button moves back to the 1st page and the "Ok" button will close the window.

Once you have selected your sizes and have your result, there is an easy way to save those numbers.  By clicking the "To Memo" button as shown below, all of the information is then copied to the memo pad.  Below you can see an example of what info is copied.  Once in the memo pad you can hot-sync them to your PC or Mac.  That way you have a record of the data stored.

The screen on the left shows the "To Memo" button being selected. 

Once selected, the screen on the right shows the information save to memo pad was successful.

The "Options" pull down menu allows you to choose two more buttons.  While the application is running in the main screen, using the menu button of the palm pilot, a pull down menu appears.  "About Tire Size Calc" will show you the about screen window shown on the right.  This will tell you which version of the program is running.

A vehicle's weight will flatten a small area of contact between the tire and the road.  This changes the actual effective circumference when on the car.  The tire industry uses a standard percentage factor.  When you choose to include the squash factor, your % change won't be affected, but the actual circumference numbers will.  Since most people aren't concerned with the actual circumference value, it might not matter to you and hence the squash factor could be ignored.  Under the Options pull down menu, you will find the Preferences button.