Posted by Kevin Woods on March 06, 1999 at 11:43:02:
In Reply to: Re: What will happen when I change 235/75R15s for 215/75R15s? posted by TerryA on February 11, 1999 at 22:39:07:
Your calculations were great but your conclusion is exactly opposite what it should've been.
The higher the axel RPM's, the faster the computer (ECM) thinks the car/truck is travelling. As you noted, smaller diameter tires result in more RPM's at a given speed (60 MPH in your example). The on board computer is calabrated to show a given MPH pursuent to a given engine RPM in a given gear.
Bottom line; computer says HIGHER RPM's = Higher speed. But the computer has no way of know that you've put smaller diameter tires on the car. Again, with the smaller diameter tires you are in effect, lying to (or fooling) the computer which in turn will lie back to you buy showing a HIGHER than actual speed.
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