I have a VDO 0-3000 rpm tachometer sensing the alternator. When I start from cold it under reads by about 50% for 15 minutes or so and then it is fine. Initially I thought it was due to the alternator belt slipping while the batteries charged from the starting, but I have tightened the belt and used some anti-slip spray and still no change. No problems when restarting once it has sorted itself. Any suggestions?