If you are talking about a golf cart, that is not the way it works, at least on my Yamaha. On my Yamaha, and I assume this is true for most if not all golf carts, there is a sensor on the front suspension that measures tire rotations. When the speedometer/odometer is setup, the tire diameter is entered. With the tire rotation rate and the built in timer, the speed is a trivial computation. There is no need to know the engine RPMs.
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Originally Posted by burky
Cookie - FYI the speedometer on any vehicle knows nothing about the tire diameter. It either assumes the factory default circumference or allows you to enter a new one in a Setup screen. The speedometer is essentially measuring the RPM of the engine, then calculating the tire rotation in RPM by converting engine speed to tire RPM using the transmission/drive train gear ratios.
Thus if the speedometer is calibrated to think you are going 20 mph at an engine RPM of 4850 with a certain sized tire, and you put a bigger diameter tire on the vehicle, you will then be actually going faster over the ground at that 4850 RPM but the speedometer (not knowing you have changed the tire size) will not be changed but will still be reading "20 mph" at the 4850 RPM engine speed even though you are really traveling faster than that with the new bigger tires.
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