Manufacuturers should add a tire size program to the ecu

[Young Roids]

They should add a program to the ecu that allows you to input the tire size that you are running and it would adjust the speedometer to the correct speed for that size. They could also give you the reccommended air pressure for that tire size.

 

[Mike240SX]

Air pressure is on the side of the tire, you know?

 

[paylew]

Air pressure is on the side of the tire, you know?

lol, true. It would be cool if the ECU did that automatically, but if you are only going an inch or so bigger does it make a big difference? I thought it wasn't that big until you are going from 15" or 16" to like 22".

 

[Wastegate13]

lol, true. It would be cool if the ECU did that automatically, but if you are only going an inch or so bigger does it make a big difference? I thought it wasn't that big until you are going from 15" or 16" to like 22".

Any change in tire diameter will change what the speedometer reads.

 

[Young Roids]

Air pressure is on the side of the tire, you know?

That is the max psi the tire can run the psi based on the cars weight is a different number all together. Most tires max psi is abround 55psi but for the stock tire size mazda recommends 34psi ofr the front tires and 32psi for the rear tires. This is an important number because if you just crank the psi up to the max psi it will wear out the center of the tire tread sooner. And if you underinflate them you will wear out the tread on the shoulders sooner. When finding the optimal psi for your tires it depends on the vehicle weight AND the size of tire you are running. The optimal psi for the ms3 will be different for the stock size(215/45/18) vs. say 235/40/18.

 

[bdiet0001]

Any change in tire diameter will change what the speedometer reads.

Here is a link that will show you the difference from the OEM wheel to the new wheel size. It will also tell you how much your speedometer will be off with the new wheel size. Very informative, check it out! http://www.miata.net/garage/tirecalc.html