What I did, Is looked up the Number of turns per mile that our wheels are spec'd at. This can be found on the tire-rack website. So with the sport package the number of turns front/rear per mile is 777/785 for the bridgestone tires (this varies on tire size and make).
Next you take the tires you want to replace it with and the size/make. Look up the specs on turns per mile.
For example: my replacement Pole Position Bridgestones at 255/30/20 is 772 revs per mile and 305/25/20 801 revs per mile.
So my new 772 will read "slower" and my new 801 will read "faster" on the speedo. Some say keep it less than 3% difference. Now you take the ratio and multiple the percentage. So, my fronts are 0.6% slower and my rears are 2.03% faster. (Revs/turns per mile is found on the tire-rack website under specs after you look up your tire make/model/size)
Example: If you are calculating 2.6% faster and your speedo is reading 60mph. You are actually doing 58.5 mph!
This website will give you an idea BUT
you have to look up the tire specs (revs per mile) for each brand and make. It Is different!
Tire size calculator
If I said something wrong....sorry! and hopefully someone will correct me 