G'day,
My Veglia (electronic) speedo runs at 85 kph when you are doing 100, similar right throughout range. After half an hour or so it will start to read correctly. Early in my ownership of the car (1997) I had to replace the sensor at the gearbox because the speedo would not work at all for the first 10 mins of any journey. I believe that those sensors are no longer available so I hope it is not on the way out.
Any ideas or solutions welcome.
Thanks
If it is like the 33 electronic speedo, then it has a phonic wheel on the output shaft in the gearbox, the sensor counts the number of times the phonic wheels go around and the number of blips causes the speedo to show your speed.
On my 33, the phonic wheel would slip and not rotate with the output shaft and cause the speedo to read slow, if i remember rightly, a new tighter phonic wheel fixed the problem, or you could use the old phonic wheel, and loctite it to the output shaft or use some teflon tape to make it tighter on the shaft, but it may be electrical, as it is an 80's Alfa Romeo, Colin.
The sender in my GTV6 had bad solder joints where the metal tabs in the plug connect to the circuit board inside the unit. This caused the speedo to work intermittently. Don't know why your speedo would go slow but it sounds like a heat fault and the most likely place for those is a bad/cold solder joint. If it was me I'd open the unit (carefully cut the metal band), touch up all the solder joints inside it, reseal the unit using the cut metal band and a hose clamp and see how it goes.
http://www.alfabb.com/bb/forums/alfetta-gtv6-1972-1986/219001-speedo-sender.html