Reply to "'Feel Good' Speedo"

By the way......

When a speedometer is off by the same amount at any speed, that is called a zero error.

When a speedometer is off by the same percentage at any speed, that is an error in span, or multiplication.

It is quite common for the error of a speedometer to be a combination of both zero and span.

It is also common for auto manufacturers to err on the side of the law, in other words, any error in reading should be on the high side.

Because of the way the needle of the speedometer is coupled to the drive mechanism, a mechanical speedometer is inherently worthless at low speeds. That is why the minimum reading is usually 10, 15, even 20 mph.

To check a speedo, it needs to be compared to a known reliable standard (your GPS in this case) at 3 different speeds, mid-scale, and either side of mid-scale. Since mid-scale on the Pantera speedometer is 100 mph, it would need to be checked at say 50 mph, 100 mph & 150 mph.

Any error that can't be explained by zero and span, a non-linear error, indicates a problem with the instrument beyond simply calibration error.

Most problems with speedometers in older cars can be remedied by cleaning & re-greasing the speedo cable. Don't use a thick grease, like that for wheel bearings, use a synthetic general purpose grease.

cowboy from hell
Instrument Society of America
certified control systems technician, level III