CossieRich,

The accuracy problem I menationed related to low rpm,

How did you measure that ?
Was it the peak reading ?

Unless you can hold the pressure steady accurate for a period of time
you cant compare the values as both will update at different rates.

Do both read zero when the engine is switched off ?

If so, then its ok as you are implying a larger difference.

(0 psi is not actually 0 volts on the map sensor wire)