i dont think that thing is accurate, it says going from the 205-65-15 stockies to my 225/40/18's would only change it 1mph, there would be no way to calculate this, becuz......rims are different weights, depending on how much air pressure you got in your tires, and also on my local freeway we got a digital speedometer thing for the cops, ill look on my tach and it reads im going 60mph and on the sign it says 55, i think it is to get a idea of how much it is off but its not accurate....just my .02 cents
rims and tire weight doesnt have anything to do with the speedometer, because speed are measure through the total wheel diameter/circumference of the tire. so even if you are running flat you still have the same tire circumference.
the radar/laser that the cops use to track speed is base on wavelength or light in which they have a different approach on calculating speed.
on the otherhand, you can never tell if our speedemeter is 100% calibrated all the time. So when you change the face gauges never pull out the needle or else your gauge are screwed and you have to re-calibrate them again.
take the wection with of the tire 205 multiply by the section height (also called the aspect ratio) .65 this is because the height of the sidewall is 65% of the with.
205 x .65 = 133.25(height of one sidewall) x2 (sideall above and below the rim) = 266.5 mm of sidewall total. now take 266.5 and divide by 25.4 aqnd this converts the mm to inches. you have a total of 10.49 inches of sidewall. add that to the 15 inces of the rim and you have a total of 25.49 inches of overall height for your tire and wheel assemby.
do this for your current plus sized wheel and tire and see how much of a height difference there is....