I don't see the relevance of gravity. 

The earth's attraction, the gravity force F = m*g,  is almost
compensated by the reaction of the earth's surface, the resultant of
both is the centripetal force F = m*w^2*R, where  w is the angular
velocity ( = 2*pi/(24*60*60) ) and R is the earth's radius. If you do
the math the centripetal force is almost zero (actually 0.0000337
m/s^2), and that is what an accelerometer should measure along the
axis perpendicular to the earth's surface. No matter if it's downhill
or 'uphill' as long as it is steady.

By far the acceleration/deceleration of the car relative to the
earth's surface/road is what an accelerometer will measure.


Oscar

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