change tire size VS change in speedmeter?

[uaesolidsnake]

Until now i saw only one method which tell how much you can change in tire size in the same rims which come from manufacture of the car to have no change in your speedmeter.

This is the site that have been tell from someone from this site for calculating the difference in tire size to tell you if its acceptable change to not have change in speedmeter:
http://www.1010tires.com/TireSizeCalculator.asp

But until know i am not sure if its accurate method or not for doing plus zero change in tire size so anyone here know other method which is more accurate than the method in the site which is above.

Other question for my own information why chnaging tire size change speedmeter reading and what are the solution to adjust it, Is it by putting accurate tire size or by adjusting something in the speedmeter?

[toyomoho]

The speedometer only responds to axle RPM. Depending on the cars setup, a sensor measures RPM at the differential or axle. It sends a pulse signal to the ECU, ABS ECU, etc or directly to the speedometer. The system is designed so the speedometer will provide an accurate reading over a given axle RPM range assuming a specific tire circumference.

Suppose you somehow installed small 8-inch diameter tires (with a much smaller circumference). For each revolution of the axle (and tire), the car will now move forward a much shorter distance. Yet the speedometer will read out as if the car had stock tires.

The concern is tire circumference, not diameter. Larger diameter rims with low profile tires might end up with about the same circumference as stock ones.

Not sure if there is an adjustment.

Measure the circumference of the tires you have now. Then measure the tires you are interested in. Determine the percentage increase or decrease in circumference. If a decrease the speedometer will indicate a faster MPH the actual speed, if an increase a slower MPH. The error will be about the amount of the percent change in circumference.

[uaesolidsnake]

After i read your reply i think that the link i added in the first post for the site which calculate the change in the speedometer after changing the tire size from the original tire size is accurate according to your reply.

Thank you for sharing your knowledeg.