Does anyone know if it is possible to have interference cause it to at times be off quite a lot ??

Could be 15mph slow .

Magnetic field ?? It is possible I might have created a electromagnet by a loop of excess wire

No clutch slipping

I know anything is possible.

 

Electronically the speedometer is a pulse counter, X pulses per meter/foot. What can confuse it is low or varried voltage (it is a "comparator" device) at the dash cluster as well as a sync issue with the drive shaft.

I've got both of these problems. My grounds are questionable and the pinion angles are crap due to the lift blocks.

What I see is at speed underload its pretty normalized, but at lower speeds thru the little towns the tailshaft speed has a bit of surge which is read by the speedometer also over bumps. The turbo (86) is cable driven with the pulse counter in the digital dash.

So again. If the shaft has "oscillations" the pulse counter will indicate the fluctuation. If you have odd voltage swings it will also confuse an pluse counter. If you have an intermittent signal from the electrical type sender on the transmission also.

 

Yeah in theory a loop of wire could cancel out or alter the pulses. They are generally not that high of frequency, but if it's bad to have loops in networking cables so stands to reason a poorly insulated wire wrapped into a bundle could be a problem. It's not really a high voltage, high current, or high frequency so I don't expect its the wire loops. Easy enough to stretch the wire out for testing?

 

Loops are not ideal but I would think unless it carries very high current it has to have many turns in order to have any effect.
Curious what kind of electronic speedometer you have, and how is it hooked up. Your stock speedo broken?
I can hear my speedo cable making buzzing noise so I'm afraid I may have to go that route, too someday. (Tried comparing with GPS Speedometer, an Android app and it looks pretty close, considering that I change my tires from stock 225's to 31-Inchers)

 

Maybe I used the wrong word It is the electric rather then a cable

I got pulled over for going @20 miles under the speed limit

Strange a little Boro in Carbon county has some really Hi Tech equipment From the electronics I bet they could send Crit Coms to Sec Def