I don't think any of the plumb bob, bubble level, 4x4 inclinometers, or other gravity based sensing systems will work since they can't distinguish between the gravitational pull of the earth and the "gravitational" pull from changes in the car's velocity. Even changing to a thicker liquid is not going to have any effect in trying to avoid detecting the continuous gradual decceleration of coasting on a flat road.
Silveredwings' gyroscopic pitch indicator would work, but is not inexpensive, the cheapest device of this type I could find, the E-GYRO, was $500.
Lug Nut's string "torque-o-meter" or zpiloto's vacuum gauge would be more likely to work since they could detect the engine's "null point", where it is not either driving forward or holding back on the wheels. I'll investigate my car's engine compartment to see how I can implement one or the other of these two suggestions.
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