Tachometer just started not registering correctly. At idle up to about 40 mph shows no rpm's. Will show rpm's on acceleration. Driving on highway rpm registers at 1100 to 1500. Speedometer is also off by 5 mph. I know this by my GPS and speedometer used to match exactly. Both problems started same time about three weeks ago. No recent repairs. Last maintenance was an oil change 2000 miles ago. About 2 months ago. Any ideas?
Speedometer readings would seldom match exactly with GPS as tire wear and inflation pressure etc would affect the actual reading and a variation of about 3 % is expected.
As to rpm not registering correctly and the speedometer suddenly going out of specs, it can mean a fault with the electrical circuit of the meter cluster. A connection could be loose or has poor continuity, high resistance in the ground circuit.