Now the warmer weather is here and I have been using the A/C I have noticed that my Tachometer reads low when the A/C is on. It is stable and moves with engine speed, but it is reading very low. It shows 2000rpm at 75mph where normally it should be close to 3000rpm. I haven't seen it flicker when the compressor switches in and out but when I floored it into kickdown (which I believe does disengage the compressor) the tach jumped back to normal until I backed off.

Any ideas anybody

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown